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1094 lines
42 KiB
Plaintext
1094 lines
42 KiB
Plaintext
@c Copyright (C) 1991-2015 Free Software Foundation, Inc.
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@c This is part of the GAS manual.
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@c For copying conditions, see the file as.texinfo.
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@ifset GENERIC
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@page
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@node MIPS-Dependent
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@chapter MIPS Dependent Features
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@end ifset
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@ifclear GENERIC
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@node Machine Dependencies
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@chapter MIPS Dependent Features
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@end ifclear
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@cindex MIPS processor
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@sc{gnu} @code{@value{AS}} for MIPS architectures supports several
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different MIPS processors, and MIPS ISA levels I through V, MIPS32,
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and MIPS64. For information about the MIPS instruction set, see
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@cite{MIPS RISC Architecture}, by Kane and Heindrich (Prentice-Hall).
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For an overview of MIPS assembly conventions, see ``Appendix D:
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Assembly Language Programming'' in the same work.
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@menu
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* MIPS Options:: Assembler options
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* MIPS Macros:: High-level assembly macros
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* MIPS Symbol Sizes:: Directives to override the size of symbols
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* MIPS Small Data:: Controlling the use of small data accesses
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* MIPS ISA:: Directives to override the ISA level
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* MIPS assembly options:: Directives to control code generation
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* MIPS autoextend:: Directives for extending MIPS 16 bit instructions
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* MIPS insn:: Directive to mark data as an instruction
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* MIPS FP ABIs:: Marking which FP ABI is in use
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* MIPS NaN Encodings:: Directives to record which NaN encoding is being used
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* MIPS Option Stack:: Directives to save and restore options
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* MIPS ASE Instruction Generation Overrides:: Directives to control
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generation of MIPS ASE instructions
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* MIPS Floating-Point:: Directives to override floating-point options
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* MIPS Syntax:: MIPS specific syntactical considerations
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@end menu
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@node MIPS Options
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@section Assembler options
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The MIPS configurations of @sc{gnu} @code{@value{AS}} support these
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special options:
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@table @code
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@cindex @code{-G} option (MIPS)
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@item -G @var{num}
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Set the ``small data'' limit to @var{n} bytes. The default limit is 8 bytes.
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@xref{MIPS Small Data,, Controlling the use of small data accesses}.
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@cindex @code{-EB} option (MIPS)
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@cindex @code{-EL} option (MIPS)
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@cindex MIPS big-endian output
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@cindex MIPS little-endian output
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@cindex big-endian output, MIPS
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@cindex little-endian output, MIPS
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@item -EB
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@itemx -EL
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Any MIPS configuration of @code{@value{AS}} can select big-endian or
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little-endian output at run time (unlike the other @sc{gnu} development
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tools, which must be configured for one or the other). Use @samp{-EB}
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to select big-endian output, and @samp{-EL} for little-endian.
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@item -KPIC
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@cindex PIC selection, MIPS
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@cindex @option{-KPIC} option, MIPS
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Generate SVR4-style PIC. This option tells the assembler to generate
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SVR4-style position-independent macro expansions. It also tells the
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assembler to mark the output file as PIC.
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@item -mvxworks-pic
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@cindex @option{-mvxworks-pic} option, MIPS
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Generate VxWorks PIC. This option tells the assembler to generate
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VxWorks-style position-independent macro expansions.
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@cindex MIPS architecture options
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@item -mips1
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@itemx -mips2
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@itemx -mips3
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@itemx -mips4
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@itemx -mips5
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@itemx -mips32
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@itemx -mips32r2
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@itemx -mips32r3
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@itemx -mips32r5
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@itemx -mips32r6
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@itemx -mips64
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@itemx -mips64r2
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@itemx -mips64r3
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@itemx -mips64r5
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@itemx -mips64r6
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Generate code for a particular MIPS Instruction Set Architecture level.
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@samp{-mips1} corresponds to the R2000 and R3000 processors,
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@samp{-mips2} to the R6000 processor, @samp{-mips3} to the
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R4000 processor, and @samp{-mips4} to the R8000 and R10000 processors.
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@samp{-mips5}, @samp{-mips32}, @samp{-mips32r2}, @samp{-mips32r3},
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@samp{-mips32r5}, @samp{-mips32r6}, @samp{-mips64}, @samp{-mips64r2},
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@samp{-mips64r3}, @samp{-mips64r5}, and @samp{-mips64r6} correspond to
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generic MIPS V, MIPS32, MIPS32 Release 2, MIPS32 Release 3, MIPS32
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Release 5, MIPS32 Release 6, MIPS64, and MIPS64 Release 2, MIPS64
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Release 3, MIPS64 Release 5, and MIPS64 Release 6 ISA processors,
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respectively. You can also switch instruction sets during the assembly;
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see @ref{MIPS ISA, Directives to override the ISA level}.
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@item -mgp32
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@itemx -mfp32
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Some macros have different expansions for 32-bit and 64-bit registers.
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The register sizes are normally inferred from the ISA and ABI, but these
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flags force a certain group of registers to be treated as 32 bits wide at
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all times. @samp{-mgp32} controls the size of general-purpose registers
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and @samp{-mfp32} controls the size of floating-point registers.
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The @code{.set gp=32} and @code{.set fp=32} directives allow the size
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of registers to be changed for parts of an object. The default value is
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restored by @code{.set gp=default} and @code{.set fp=default}.
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On some MIPS variants there is a 32-bit mode flag; when this flag is
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set, 64-bit instructions generate a trap. Also, some 32-bit OSes only
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save the 32-bit registers on a context switch, so it is essential never
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to use the 64-bit registers.
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@item -mgp64
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@itemx -mfp64
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Assume that 64-bit registers are available. This is provided in the
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interests of symmetry with @samp{-mgp32} and @samp{-mfp32}.
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The @code{.set gp=64} and @code{.set fp=64} directives allow the size
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of registers to be changed for parts of an object. The default value is
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restored by @code{.set gp=default} and @code{.set fp=default}.
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@item -mfpxx
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Make no assumptions about whether 32-bit or 64-bit floating-point
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registers are available. This is provided to support having modules
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compatible with either @samp{-mfp32} or @samp{-mfp64}. This option can
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only be used with MIPS II and above.
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The @code{.set fp=xx} directive allows a part of an object to be marked
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as not making assumptions about 32-bit or 64-bit FP registers. The
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default value is restored by @code{.set fp=default}.
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@item -modd-spreg
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@itemx -mno-odd-spreg
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Enable use of floating-point operations on odd-numbered single-precision
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registers when supported by the ISA. @samp{-mfpxx} implies
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@samp{-mno-odd-spreg}, otherwise the default is @samp{-modd-spreg}
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@item -mips16
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@itemx -no-mips16
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Generate code for the MIPS 16 processor. This is equivalent to putting
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@code{.set mips16} at the start of the assembly file. @samp{-no-mips16}
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turns off this option.
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@item -mmicromips
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@itemx -mno-micromips
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Generate code for the microMIPS processor. This is equivalent to putting
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@code{.set micromips} at the start of the assembly file. @samp{-mno-micromips}
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turns off this option. This is equivalent to putting @code{.set nomicromips}
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at the start of the assembly file.
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@item -msmartmips
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@itemx -mno-smartmips
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Enables the SmartMIPS extensions to the MIPS32 instruction set, which
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provides a number of new instructions which target smartcard and
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cryptographic applications. This is equivalent to putting
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@code{.set smartmips} at the start of the assembly file.
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@samp{-mno-smartmips} turns off this option.
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@item -mips3d
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@itemx -no-mips3d
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Generate code for the MIPS-3D Application Specific Extension.
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This tells the assembler to accept MIPS-3D instructions.
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@samp{-no-mips3d} turns off this option.
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@item -mdmx
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@itemx -no-mdmx
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Generate code for the MDMX Application Specific Extension.
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This tells the assembler to accept MDMX instructions.
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@samp{-no-mdmx} turns off this option.
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@item -mdsp
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@itemx -mno-dsp
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Generate code for the DSP Release 1 Application Specific Extension.
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This tells the assembler to accept DSP Release 1 instructions.
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@samp{-mno-dsp} turns off this option.
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@item -mdspr2
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@itemx -mno-dspr2
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Generate code for the DSP Release 2 Application Specific Extension.
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This option implies -mdsp.
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This tells the assembler to accept DSP Release 2 instructions.
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@samp{-mno-dspr2} turns off this option.
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@item -mmt
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@itemx -mno-mt
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Generate code for the MT Application Specific Extension.
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This tells the assembler to accept MT instructions.
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@samp{-mno-mt} turns off this option.
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@item -mmcu
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@itemx -mno-mcu
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Generate code for the MCU Application Specific Extension.
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This tells the assembler to accept MCU instructions.
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@samp{-mno-mcu} turns off this option.
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@item -mmsa
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@itemx -mno-msa
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Generate code for the MIPS SIMD Architecture Extension.
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This tells the assembler to accept MSA instructions.
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@samp{-mno-msa} turns off this option.
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@item -mxpa
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@itemx -mno-xpa
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Generate code for the MIPS eXtended Physical Address (XPA) Extension.
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This tells the assembler to accept XPA instructions.
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@samp{-mno-xpa} turns off this option.
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@item -mvirt
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@itemx -mno-virt
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Generate code for the Virtualization Application Specific Extension.
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This tells the assembler to accept Virtualization instructions.
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@samp{-mno-virt} turns off this option.
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@item -minsn32
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@itemx -mno-insn32
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Only use 32-bit instruction encodings when generating code for the
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microMIPS processor. This option inhibits the use of any 16-bit
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instructions. This is equivalent to putting @code{.set insn32} at
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the start of the assembly file. @samp{-mno-insn32} turns off this
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option. This is equivalent to putting @code{.set noinsn32} at the
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start of the assembly file. By default @samp{-mno-insn32} is
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selected, allowing all instructions to be used.
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@item -mfix7000
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@itemx -mno-fix7000
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Cause nops to be inserted if the read of the destination register
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of an mfhi or mflo instruction occurs in the following two instructions.
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@item -mfix-rm7000
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@itemx -mno-fix-rm7000
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Cause nops to be inserted if a dmult or dmultu instruction is
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followed by a load instruction.
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@item -mfix-loongson2f-jump
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@itemx -mno-fix-loongson2f-jump
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Eliminate instruction fetch from outside 256M region to work around the
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Loongson2F @samp{jump} instructions. Without it, under extreme cases,
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the kernel may crash. The issue has been solved in latest processor
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batches, but this fix has no side effect to them.
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@item -mfix-loongson2f-nop
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@itemx -mno-fix-loongson2f-nop
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Replace nops by @code{or at,at,zero} to work around the Loongson2F
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@samp{nop} errata. Without it, under extreme cases, the CPU might
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deadlock. The issue has been solved in later Loongson2F batches, but
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this fix has no side effect to them.
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@item -mfix-vr4120
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@itemx -mno-fix-vr4120
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Insert nops to work around certain VR4120 errata. This option is
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intended to be used on GCC-generated code: it is not designed to catch
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all problems in hand-written assembler code.
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@item -mfix-vr4130
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@itemx -mno-fix-vr4130
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Insert nops to work around the VR4130 @samp{mflo}/@samp{mfhi} errata.
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@item -mfix-24k
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@itemx -mno-fix-24k
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Insert nops to work around the 24K @samp{eret}/@samp{deret} errata.
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@item -mfix-cn63xxp1
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@itemx -mno-fix-cn63xxp1
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Replace @code{pref} hints 0 - 4 and 6 - 24 with hint 28 to work around
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certain CN63XXP1 errata.
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@item -m4010
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@itemx -no-m4010
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Generate code for the LSI R4010 chip. This tells the assembler to
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accept the R4010-specific instructions (@samp{addciu}, @samp{ffc},
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etc.), and to not schedule @samp{nop} instructions around accesses to
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the @samp{HI} and @samp{LO} registers. @samp{-no-m4010} turns off this
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option.
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@item -m4650
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@itemx -no-m4650
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Generate code for the MIPS R4650 chip. This tells the assembler to accept
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the @samp{mad} and @samp{madu} instruction, and to not schedule @samp{nop}
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instructions around accesses to the @samp{HI} and @samp{LO} registers.
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@samp{-no-m4650} turns off this option.
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@item -m3900
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@itemx -no-m3900
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@itemx -m4100
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@itemx -no-m4100
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For each option @samp{-m@var{nnnn}}, generate code for the MIPS
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R@var{nnnn} chip. This tells the assembler to accept instructions
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specific to that chip, and to schedule for that chip's hazards.
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@item -march=@var{cpu}
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Generate code for a particular MIPS CPU. It is exactly equivalent to
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@samp{-m@var{cpu}}, except that there are more value of @var{cpu}
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understood. Valid @var{cpu} value are:
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@quotation
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2000,
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3000,
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3900,
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4000,
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4010,
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4100,
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4111,
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vr4120,
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vr4130,
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vr4181,
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4300,
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4400,
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4600,
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4650,
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5000,
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rm5200,
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rm5230,
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rm5231,
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rm5261,
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rm5721,
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vr5400,
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vr5500,
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6000,
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rm7000,
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8000,
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rm9000,
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10000,
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12000,
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14000,
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16000,
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4kc,
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4km,
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4kp,
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4ksc,
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4kec,
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4kem,
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4kep,
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4ksd,
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m4k,
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m4kp,
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m14k,
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m14kc,
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m14ke,
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m14kec,
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24kc,
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24kf2_1,
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24kf,
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24kf1_1,
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24kec,
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24kef2_1,
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24kef,
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24kef1_1,
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34kc,
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34kf2_1,
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34kf,
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34kf1_1,
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34kn,
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74kc,
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74kf2_1,
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74kf,
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74kf1_1,
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74kf3_2,
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1004kc,
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1004kf2_1,
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1004kf,
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1004kf1_1,
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p5600,
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5kc,
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5kf,
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20kc,
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25kf,
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sb1,
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sb1a,
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loongson2e,
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loongson2f,
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loongson3a,
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octeon,
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octeon+,
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octeon2,
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octeon3,
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xlr,
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xlp
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@end quotation
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For compatibility reasons, @samp{@var{n}x} and @samp{@var{b}fx} are
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accepted as synonyms for @samp{@var{n}f1_1}. These values are
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deprecated.
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@item -mtune=@var{cpu}
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Schedule and tune for a particular MIPS CPU. Valid @var{cpu} values are
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identical to @samp{-march=@var{cpu}}.
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@item -mabi=@var{abi}
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Record which ABI the source code uses. The recognized arguments
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are: @samp{32}, @samp{n32}, @samp{o64}, @samp{64} and @samp{eabi}.
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@item -msym32
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@itemx -mno-sym32
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@cindex -msym32
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@cindex -mno-sym32
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Equivalent to adding @code{.set sym32} or @code{.set nosym32} to
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the beginning of the assembler input. @xref{MIPS Symbol Sizes}.
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@cindex @code{-nocpp} ignored (MIPS)
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@item -nocpp
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This option is ignored. It is accepted for command-line compatibility with
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other assemblers, which use it to turn off C style preprocessing. With
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@sc{gnu} @code{@value{AS}}, there is no need for @samp{-nocpp}, because the
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@sc{gnu} assembler itself never runs the C preprocessor.
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@item -msoft-float
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@itemx -mhard-float
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Disable or enable floating-point instructions. Note that by default
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floating-point instructions are always allowed even with CPU targets
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that don't have support for these instructions.
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@item -msingle-float
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@itemx -mdouble-float
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Disable or enable double-precision floating-point operations. Note
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that by default double-precision floating-point operations are always
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allowed even with CPU targets that don't have support for these
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operations.
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@item --construct-floats
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@itemx --no-construct-floats
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The @code{--no-construct-floats} option disables the construction of
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double width floating point constants by loading the two halves of the
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value into the two single width floating point registers that make up
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the double width register. This feature is useful if the processor
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support the FR bit in its status register, and this bit is known (by
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the programmer) to be set. This bit prevents the aliasing of the double
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width register by the single width registers.
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By default @code{--construct-floats} is selected, allowing construction
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of these floating point constants.
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@item --relax-branch
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@itemx --no-relax-branch
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The @samp{--relax-branch} option enables the relaxation of out-of-range
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branches. Any branches whose target cannot be reached directly are
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converted to a small instruction sequence including an inverse-condition
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branch to the physically next instruction, and a jump to the original
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target is inserted between the two instructions. In PIC code the jump
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will involve further instructions for address calculation.
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The @code{BC1ANY2F}, @code{BC1ANY2T}, @code{BC1ANY4F}, @code{BC1ANY4T},
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@code{BPOSGE32} and @code{BPOSGE64} instructions are excluded from
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relaxation, because they have no complementing counterparts. They could
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be relaxed with the use of a longer sequence involving another branch,
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however this has not been implemented and if their target turns out of
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reach, they produce an error even if branch relaxation is enabled.
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Also no MIPS16 branches are ever relaxed.
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By default @samp{--no-relax-branch} is selected, causing any out-of-range
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branches to produce an error.
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@cindex @option{-mnan=} command line option, MIPS
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@item -mnan=@var{encoding}
|
|
This option indicates whether the source code uses the IEEE 2008
|
|
NaN encoding (@option{-mnan=2008}) or the original MIPS encoding
|
|
(@option{-mnan=legacy}). It is equivalent to adding a @code{.nan}
|
|
directive to the beginning of the source file. @xref{MIPS NaN Encodings}.
|
|
|
|
@option{-mnan=legacy} is the default if no @option{-mnan} option or
|
|
@code{.nan} directive is used.
|
|
|
|
@item --trap
|
|
@itemx --no-break
|
|
@c FIXME! (1) reflect these options (next item too) in option summaries;
|
|
@c (2) stop teasing, say _which_ instructions expanded _how_.
|
|
@code{@value{AS}} automatically macro expands certain division and
|
|
multiplication instructions to check for overflow and division by zero. This
|
|
option causes @code{@value{AS}} to generate code to take a trap exception
|
|
rather than a break exception when an error is detected. The trap instructions
|
|
are only supported at Instruction Set Architecture level 2 and higher.
|
|
|
|
@item --break
|
|
@itemx --no-trap
|
|
Generate code to take a break exception rather than a trap exception when an
|
|
error is detected. This is the default.
|
|
|
|
@item -mpdr
|
|
@itemx -mno-pdr
|
|
Control generation of @code{.pdr} sections. Off by default on IRIX, on
|
|
elsewhere.
|
|
|
|
@item -mshared
|
|
@itemx -mno-shared
|
|
When generating code using the Unix calling conventions (selected by
|
|
@samp{-KPIC} or @samp{-mcall_shared}), gas will normally generate code
|
|
which can go into a shared library. The @samp{-mno-shared} option
|
|
tells gas to generate code which uses the calling convention, but can
|
|
not go into a shared library. The resulting code is slightly more
|
|
efficient. This option only affects the handling of the
|
|
@samp{.cpload} and @samp{.cpsetup} pseudo-ops.
|
|
@end table
|
|
|
|
@node MIPS Macros
|
|
@section High-level assembly macros
|
|
|
|
MIPS assemblers have traditionally provided a wider range of
|
|
instructions than the MIPS architecture itself. These extra
|
|
instructions are usually referred to as ``macro'' instructions
|
|
@footnote{The term ``macro'' is somewhat overloaded here, since
|
|
these macros have no relation to those defined by @code{.macro},
|
|
@pxref{Macro,, @code{.macro}}.}.
|
|
|
|
Some MIPS macro instructions extend an underlying architectural instruction
|
|
while others are entirely new. An example of the former type is @code{and},
|
|
which allows the third operand to be either a register or an arbitrary
|
|
immediate value. Examples of the latter type include @code{bgt}, which
|
|
branches to the third operand when the first operand is greater than
|
|
the second operand, and @code{ulh}, which implements an unaligned
|
|
2-byte load.
|
|
|
|
One of the most common extensions provided by macros is to expand
|
|
memory offsets to the full address range (32 or 64 bits) and to allow
|
|
symbolic offsets such as @samp{my_data + 4} to be used in place of
|
|
integer constants. For example, the architectural instruction
|
|
@code{lbu} allows only a signed 16-bit offset, whereas the macro
|
|
@code{lbu} allows code such as @samp{lbu $4,array+32769($5)}.
|
|
The implementation of these symbolic offsets depends on several factors,
|
|
such as whether the assembler is generating SVR4-style PIC (selected by
|
|
@option{-KPIC}, @pxref{MIPS Options,, Assembler options}), the size of symbols
|
|
(@pxref{MIPS Symbol Sizes,, Directives to override the size of symbols}),
|
|
and the small data limit (@pxref{MIPS Small Data,, Controlling the use
|
|
of small data accesses}).
|
|
|
|
@kindex @code{.set macro}
|
|
@kindex @code{.set nomacro}
|
|
Sometimes it is undesirable to have one assembly instruction expand
|
|
to several machine instructions. The directive @code{.set nomacro}
|
|
tells the assembler to warn when this happens. @code{.set macro}
|
|
restores the default behavior.
|
|
|
|
@cindex @code{at} register, MIPS
|
|
@kindex @code{.set at=@var{reg}}
|
|
Some macro instructions need a temporary register to store intermediate
|
|
results. This register is usually @code{$1}, also known as @code{$at},
|
|
but it can be changed to any core register @var{reg} using
|
|
@code{.set at=@var{reg}}. Note that @code{$at} always refers
|
|
to @code{$1} regardless of which register is being used as the
|
|
temporary register.
|
|
|
|
@kindex @code{.set at}
|
|
@kindex @code{.set noat}
|
|
Implicit uses of the temporary register in macros could interfere with
|
|
explicit uses in the assembly code. The assembler therefore warns
|
|
whenever it sees an explicit use of the temporary register. The directive
|
|
@code{.set noat} silences this warning while @code{.set at} restores
|
|
the default behavior. It is safe to use @code{.set noat} while
|
|
@code{.set nomacro} is in effect since single-instruction macros
|
|
never need a temporary register.
|
|
|
|
Note that while the @sc{gnu} assembler provides these macros for compatibility,
|
|
it does not make any attempt to optimize them with the surrounding code.
|
|
|
|
@node MIPS Symbol Sizes
|
|
@section Directives to override the size of symbols
|
|
|
|
@kindex @code{.set sym32}
|
|
@kindex @code{.set nosym32}
|
|
The n64 ABI allows symbols to have any 64-bit value. Although this
|
|
provides a great deal of flexibility, it means that some macros have
|
|
much longer expansions than their 32-bit counterparts. For example,
|
|
the non-PIC expansion of @samp{dla $4,sym} is usually:
|
|
|
|
@smallexample
|
|
lui $4,%highest(sym)
|
|
lui $1,%hi(sym)
|
|
daddiu $4,$4,%higher(sym)
|
|
daddiu $1,$1,%lo(sym)
|
|
dsll32 $4,$4,0
|
|
daddu $4,$4,$1
|
|
@end smallexample
|
|
|
|
whereas the 32-bit expansion is simply:
|
|
|
|
@smallexample
|
|
lui $4,%hi(sym)
|
|
daddiu $4,$4,%lo(sym)
|
|
@end smallexample
|
|
|
|
n64 code is sometimes constructed in such a way that all symbolic
|
|
constants are known to have 32-bit values, and in such cases, it's
|
|
preferable to use the 32-bit expansion instead of the 64-bit
|
|
expansion.
|
|
|
|
You can use the @code{.set sym32} directive to tell the assembler
|
|
that, from this point on, all expressions of the form
|
|
@samp{@var{symbol}} or @samp{@var{symbol} + @var{offset}}
|
|
have 32-bit values. For example:
|
|
|
|
@smallexample
|
|
.set sym32
|
|
dla $4,sym
|
|
lw $4,sym+16
|
|
sw $4,sym+0x8000($4)
|
|
@end smallexample
|
|
|
|
will cause the assembler to treat @samp{sym}, @code{sym+16} and
|
|
@code{sym+0x8000} as 32-bit values. The handling of non-symbolic
|
|
addresses is not affected.
|
|
|
|
The directive @code{.set nosym32} ends a @code{.set sym32} block and
|
|
reverts to the normal behavior. It is also possible to change the
|
|
symbol size using the command-line options @option{-msym32} and
|
|
@option{-mno-sym32}.
|
|
|
|
These options and directives are always accepted, but at present,
|
|
they have no effect for anything other than n64.
|
|
|
|
@node MIPS Small Data
|
|
@section Controlling the use of small data accesses
|
|
|
|
@c This section deliberately glosses over the possibility of using -G
|
|
@c in SVR4-style PIC, as could be done on IRIX. We don't support that.
|
|
@cindex small data, MIPS
|
|
@cindex @code{gp} register, MIPS
|
|
It often takes several instructions to load the address of a symbol.
|
|
For example, when @samp{addr} is a 32-bit symbol, the non-PIC expansion
|
|
of @samp{dla $4,addr} is usually:
|
|
|
|
@smallexample
|
|
lui $4,%hi(addr)
|
|
daddiu $4,$4,%lo(addr)
|
|
@end smallexample
|
|
|
|
The sequence is much longer when @samp{addr} is a 64-bit symbol.
|
|
@xref{MIPS Symbol Sizes,, Directives to override the size of symbols}.
|
|
|
|
In order to cut down on this overhead, most embedded MIPS systems
|
|
set aside a 64-kilobyte ``small data'' area and guarantee that all
|
|
data of size @var{n} and smaller will be placed in that area.
|
|
The limit @var{n} is passed to both the assembler and the linker
|
|
using the command-line option @option{-G @var{n}}, @pxref{MIPS Options,,
|
|
Assembler options}. Note that the same value of @var{n} must be used
|
|
when linking and when assembling all input files to the link; any
|
|
inconsistency could cause a relocation overflow error.
|
|
|
|
The size of an object in the @code{.bss} section is set by the
|
|
@code{.comm} or @code{.lcomm} directive that defines it. The size of
|
|
an external object may be set with the @code{.extern} directive. For
|
|
example, @samp{.extern sym,4} declares that the object at @code{sym}
|
|
is 4 bytes in length, while leaving @code{sym} otherwise undefined.
|
|
|
|
When no @option{-G} option is given, the default limit is 8 bytes.
|
|
The option @option{-G 0} prevents any data from being automatically
|
|
classified as small.
|
|
|
|
It is also possible to mark specific objects as small by putting them
|
|
in the special sections @code{.sdata} and @code{.sbss}, which are
|
|
``small'' counterparts of @code{.data} and @code{.bss} respectively.
|
|
The toolchain will treat such data as small regardless of the
|
|
@option{-G} setting.
|
|
|
|
On startup, systems that support a small data area are expected to
|
|
initialize register @code{$28}, also known as @code{$gp}, in such a
|
|
way that small data can be accessed using a 16-bit offset from that
|
|
register. For example, when @samp{addr} is small data,
|
|
the @samp{dla $4,addr} instruction above is equivalent to:
|
|
|
|
@smallexample
|
|
daddiu $4,$28,%gp_rel(addr)
|
|
@end smallexample
|
|
|
|
Small data is not supported for SVR4-style PIC.
|
|
|
|
@node MIPS ISA
|
|
@section Directives to override the ISA level
|
|
|
|
@cindex MIPS ISA override
|
|
@kindex @code{.set mips@var{n}}
|
|
@sc{gnu} @code{@value{AS}} supports an additional directive to change
|
|
the MIPS Instruction Set Architecture level on the fly: @code{.set
|
|
mips@var{n}}. @var{n} should be a number from 0 to 5, or 32, 32r2, 32r3,
|
|
32r5, 32r6, 64, 64r2, 64r3, 64r5 or 64r6.
|
|
The values other than 0 make the assembler accept instructions
|
|
for the corresponding ISA level, from that point on in the
|
|
assembly. @code{.set mips@var{n}} affects not only which instructions
|
|
are permitted, but also how certain macros are expanded. @code{.set
|
|
mips0} restores the ISA level to its original level: either the
|
|
level you selected with command line options, or the default for your
|
|
configuration. You can use this feature to permit specific MIPS III
|
|
instructions while assembling in 32 bit mode. Use this directive with
|
|
care!
|
|
|
|
@cindex MIPS CPU override
|
|
@kindex @code{.set arch=@var{cpu}}
|
|
The @code{.set arch=@var{cpu}} directive provides even finer control.
|
|
It changes the effective CPU target and allows the assembler to use
|
|
instructions specific to a particular CPU. All CPUs supported by the
|
|
@samp{-march} command line option are also selectable by this directive.
|
|
The original value is restored by @code{.set arch=default}.
|
|
|
|
The directive @code{.set mips16} puts the assembler into MIPS 16 mode,
|
|
in which it will assemble instructions for the MIPS 16 processor. Use
|
|
@code{.set nomips16} to return to normal 32 bit mode.
|
|
|
|
Traditional MIPS assemblers do not support this directive.
|
|
|
|
The directive @code{.set micromips} puts the assembler into microMIPS mode,
|
|
in which it will assemble instructions for the microMIPS processor. Use
|
|
@code{.set nomicromips} to return to normal 32 bit mode.
|
|
|
|
Traditional MIPS assemblers do not support this directive.
|
|
|
|
@node MIPS assembly options
|
|
@section Directives to control code generation
|
|
|
|
@cindex MIPS directives to override command line options
|
|
@kindex @code{.module}
|
|
The @code{.module} directive allows command line options to be set directly
|
|
from assembly. The format of the directive matches the @code{.set}
|
|
directive but only those options which are relevant to a whole module are
|
|
supported. The effect of a @code{.module} directive is the same as the
|
|
corresponding command line option. Where @code{.set} directives support
|
|
returning to a default then the @code{.module} directives do not as they
|
|
define the defaults.
|
|
|
|
These module-level directives must appear first in assembly.
|
|
|
|
Traditional MIPS assemblers do not support this directive.
|
|
|
|
@cindex MIPS 32-bit microMIPS instruction generation override
|
|
@kindex @code{.set insn32}
|
|
@kindex @code{.set noinsn32}
|
|
The directive @code{.set insn32} makes the assembler only use 32-bit
|
|
instruction encodings when generating code for the microMIPS processor.
|
|
This directive inhibits the use of any 16-bit instructions from that
|
|
point on in the assembly. The @code{.set noinsn32} directive allows
|
|
16-bit instructions to be accepted.
|
|
|
|
Traditional MIPS assemblers do not support this directive.
|
|
|
|
@node MIPS autoextend
|
|
@section Directives for extending MIPS 16 bit instructions
|
|
|
|
@kindex @code{.set autoextend}
|
|
@kindex @code{.set noautoextend}
|
|
By default, MIPS 16 instructions are automatically extended to 32 bits
|
|
when necessary. The directive @code{.set noautoextend} will turn this
|
|
off. When @code{.set noautoextend} is in effect, any 32 bit instruction
|
|
must be explicitly extended with the @code{.e} modifier (e.g.,
|
|
@code{li.e $4,1000}). The directive @code{.set autoextend} may be used
|
|
to once again automatically extend instructions when necessary.
|
|
|
|
This directive is only meaningful when in MIPS 16 mode. Traditional
|
|
MIPS assemblers do not support this directive.
|
|
|
|
@node MIPS insn
|
|
@section Directive to mark data as an instruction
|
|
|
|
@kindex @code{.insn}
|
|
The @code{.insn} directive tells @code{@value{AS}} that the following
|
|
data is actually instructions. This makes a difference in MIPS 16 and
|
|
microMIPS modes: when loading the address of a label which precedes
|
|
instructions, @code{@value{AS}} automatically adds 1 to the value, so
|
|
that jumping to the loaded address will do the right thing.
|
|
|
|
@kindex @code{.global}
|
|
The @code{.global} and @code{.globl} directives supported by
|
|
@code{@value{AS}} will by default mark the symbol as pointing to a
|
|
region of data not code. This means that, for example, any
|
|
instructions following such a symbol will not be disassembled by
|
|
@code{objdump} as it will regard them as data. To change this
|
|
behavior an optional section name can be placed after the symbol name
|
|
in the @code{.global} directive. If this section exists and is known
|
|
to be a code section, then the symbol will be marked as pointing at
|
|
code not data. Ie the syntax for the directive is:
|
|
|
|
@code{.global @var{symbol}[ @var{section}][, @var{symbol}[ @var{section}]] ...},
|
|
|
|
Here is a short example:
|
|
|
|
@example
|
|
.global foo .text, bar, baz .data
|
|
foo:
|
|
nop
|
|
bar:
|
|
.word 0x0
|
|
baz:
|
|
.word 0x1
|
|
|
|
@end example
|
|
|
|
@node MIPS FP ABIs
|
|
@section Directives to control the FP ABI
|
|
@menu
|
|
* MIPS FP ABI History:: History of FP ABIs
|
|
* MIPS FP ABI Variants:: Supported FP ABIs
|
|
* MIPS FP ABI Selection:: Automatic selection of FP ABI
|
|
* MIPS FP ABI Compatibility:: Linking different FP ABI variants
|
|
@end menu
|
|
|
|
@node MIPS FP ABI History
|
|
@subsection History of FP ABIs
|
|
@cindex @code{.gnu_attribute 4, @var{n}} directive, MIPS
|
|
@cindex @code{.gnu_attribute Tag_GNU_MIPS_ABI_FP, @var{n}} directive, MIPS
|
|
The MIPS ABIs support a variety of different floating-point extensions
|
|
where calling-convention and register sizes vary for floating-point data.
|
|
The extensions exist to support a wide variety of optional architecture
|
|
features. The resulting ABI variants are generally incompatible with each
|
|
other and must be tracked carefully.
|
|
|
|
Traditionally the use of an explicit @code{.gnu_attribute 4, @var{n}}
|
|
directive is used to indicate which ABI is in use by a specific module.
|
|
It was then left to the user to ensure that command line options and the
|
|
selected ABI were compatible with some potential for inconsistencies.
|
|
|
|
@node MIPS FP ABI Variants
|
|
@subsection Supported FP ABIs
|
|
The supported floating-point ABI variants are:
|
|
|
|
@table @code
|
|
@item 0 - No floating-point
|
|
This variant is used to indicate that floating-point is not used within
|
|
the module at all and therefore has no impact on the ABI. This is the
|
|
default.
|
|
|
|
@item 1 - Double-precision
|
|
This variant indicates that double-precision support is used. For 64-bit
|
|
ABIs this means that 64-bit wide floating-point registers are required.
|
|
For 32-bit ABIs this means that 32-bit wide floating-point registers are
|
|
required and double-precision operations use pairs of registers.
|
|
|
|
@item 2 - Single-precision
|
|
This variant indicates that single-precision support is used. Double
|
|
precision operations will be supported via soft-float routines.
|
|
|
|
@item 3 - Soft-float
|
|
This variant indicates that although floating-point support is used all
|
|
operations are emulated in software. This means the ABI is modified to
|
|
pass all floating-point data in general-purpose registers.
|
|
|
|
@item 4 - Deprecated
|
|
This variant existed as an initial attempt at supporting 64-bit wide
|
|
floating-point registers for O32 ABI on a MIPS32r2 CPU. This has been
|
|
superseded by 5, 6 and 7.
|
|
|
|
@item 5 - Double-precision 32-bit CPU, 32-bit or 64-bit FPU
|
|
This variant is used by 32-bit ABIs to indicate that the floating-point
|
|
code in the module has been designed to operate correctly with either
|
|
32-bit wide or 64-bit wide floating-point registers. Double-precision
|
|
support is used. Only O32 currently supports this variant and requires
|
|
a minimum architecture of MIPS II.
|
|
|
|
@item 6 - Double-precision 32-bit FPU, 64-bit FPU
|
|
This variant is used by 32-bit ABIs to indicate that the floating-point
|
|
code in the module requires 64-bit wide floating-point registers.
|
|
Double-precision support is used. Only O32 currently supports this
|
|
variant and requires a minimum architecture of MIPS32r2.
|
|
|
|
@item 7 - Double-precision compat 32-bit FPU, 64-bit FPU
|
|
This variant is used by 32-bit ABIs to indicate that the floating-point
|
|
code in the module requires 64-bit wide floating-point registers.
|
|
Double-precision support is used. This differs from the previous ABI
|
|
as it restricts use of odd-numbered single-precision registers. Only
|
|
O32 currently supports this variant and requires a minimum architecture
|
|
of MIPS32r2.
|
|
@end table
|
|
|
|
@node MIPS FP ABI Selection
|
|
@subsection Automatic selection of FP ABI
|
|
@cindex @code{.module fp=@var{nn}} directive, MIPS
|
|
In order to simplify and add safety to the process of selecting the
|
|
correct floating-point ABI, the assembler will automatically infer the
|
|
correct @code{.gnu_attribute 4, @var{n}} directive based on command line
|
|
options and @code{.module} overrides. Where an explicit
|
|
@code{.gnu_attribute 4, @var{n}} directive has been seen then a warning
|
|
will be raised if it does not match an inferred setting.
|
|
|
|
The floating-point ABI is inferred as follows. If @samp{-msoft-float}
|
|
has been used the module will be marked as soft-float. If
|
|
@samp{-msingle-float} has been used then the module will be marked as
|
|
single-precision. The remaining ABIs are then selected based
|
|
on the FP register width. Double-precision is selected if the width
|
|
of GP and FP registers match and the special double-precision variants
|
|
for 32-bit ABIs are then selected depending on @samp{-mfpxx},
|
|
@samp{-mfp64} and @samp{-mno-odd-spreg}.
|
|
|
|
@node MIPS FP ABI Compatibility
|
|
@subsection Linking different FP ABI variants
|
|
Modules using the default FP ABI (no floating-point) can be linked with
|
|
any other (singular) FP ABI variant.
|
|
|
|
Special compatibility support exists for O32 with the four
|
|
double-precision FP ABI variants. The @samp{-mfpxx} FP ABI is specifically
|
|
designed to be compatible with the standard double-precision ABI and the
|
|
@samp{-mfp64} FP ABIs. This makes it desirable for O32 modules to be
|
|
built as @samp{-mfpxx} to ensure the maximum compatibility with other
|
|
modules produced for more specific needs. The only FP ABIs which cannot
|
|
be linked together are the standard double-precision ABI and the full
|
|
@samp{-mfp64} ABI with @samp{-modd-spreg}.
|
|
|
|
@node MIPS NaN Encodings
|
|
@section Directives to record which NaN encoding is being used
|
|
|
|
@cindex MIPS IEEE 754 NaN data encoding selection
|
|
@cindex @code{.nan} directive, MIPS
|
|
The IEEE 754 floating-point standard defines two types of not-a-number
|
|
(NaN) data: ``signalling'' NaNs and ``quiet'' NaNs. The original version
|
|
of the standard did not specify how these two types should be
|
|
distinguished. Most implementations followed the i387 model, in which
|
|
the first bit of the significand is set for quiet NaNs and clear for
|
|
signalling NaNs. However, the original MIPS implementation assigned the
|
|
opposite meaning to the bit, so that it was set for signalling NaNs and
|
|
clear for quiet NaNs.
|
|
|
|
The 2008 revision of the standard formally suggested the i387 choice
|
|
and as from Sep 2012 the current release of the MIPS architecture
|
|
therefore optionally supports that form. Code that uses one NaN encoding
|
|
would usually be incompatible with code that uses the other NaN encoding,
|
|
so MIPS ELF objects have a flag (@code{EF_MIPS_NAN2008}) to record which
|
|
encoding is being used.
|
|
|
|
Assembly files can use the @code{.nan} directive to select between the
|
|
two encodings. @samp{.nan 2008} says that the assembly file uses the
|
|
IEEE 754-2008 encoding while @samp{.nan legacy} says that the file uses
|
|
the original MIPS encoding. If several @code{.nan} directives are given,
|
|
the final setting is the one that is used.
|
|
|
|
The command-line options @option{-mnan=legacy} and @option{-mnan=2008}
|
|
can be used instead of @samp{.nan legacy} and @samp{.nan 2008}
|
|
respectively. However, any @code{.nan} directive overrides the
|
|
command-line setting.
|
|
|
|
@samp{.nan legacy} is the default if no @code{.nan} directive or
|
|
@option{-mnan} option is given.
|
|
|
|
Note that @sc{gnu} @code{@value{AS}} does not produce NaNs itself and
|
|
therefore these directives do not affect code generation. They simply
|
|
control the setting of the @code{EF_MIPS_NAN2008} flag.
|
|
|
|
Traditional MIPS assemblers do not support these directives.
|
|
|
|
@node MIPS Option Stack
|
|
@section Directives to save and restore options
|
|
|
|
@cindex MIPS option stack
|
|
@kindex @code{.set push}
|
|
@kindex @code{.set pop}
|
|
The directives @code{.set push} and @code{.set pop} may be used to save
|
|
and restore the current settings for all the options which are
|
|
controlled by @code{.set}. The @code{.set push} directive saves the
|
|
current settings on a stack. The @code{.set pop} directive pops the
|
|
stack and restores the settings.
|
|
|
|
These directives can be useful inside an macro which must change an
|
|
option such as the ISA level or instruction reordering but does not want
|
|
to change the state of the code which invoked the macro.
|
|
|
|
Traditional MIPS assemblers do not support these directives.
|
|
|
|
@node MIPS ASE Instruction Generation Overrides
|
|
@section Directives to control generation of MIPS ASE instructions
|
|
|
|
@cindex MIPS MIPS-3D instruction generation override
|
|
@kindex @code{.set mips3d}
|
|
@kindex @code{.set nomips3d}
|
|
The directive @code{.set mips3d} makes the assembler accept instructions
|
|
from the MIPS-3D Application Specific Extension from that point on
|
|
in the assembly. The @code{.set nomips3d} directive prevents MIPS-3D
|
|
instructions from being accepted.
|
|
|
|
@cindex SmartMIPS instruction generation override
|
|
@kindex @code{.set smartmips}
|
|
@kindex @code{.set nosmartmips}
|
|
The directive @code{.set smartmips} makes the assembler accept
|
|
instructions from the SmartMIPS Application Specific Extension to the
|
|
MIPS32 ISA from that point on in the assembly. The
|
|
@code{.set nosmartmips} directive prevents SmartMIPS instructions from
|
|
being accepted.
|
|
|
|
@cindex MIPS MDMX instruction generation override
|
|
@kindex @code{.set mdmx}
|
|
@kindex @code{.set nomdmx}
|
|
The directive @code{.set mdmx} makes the assembler accept instructions
|
|
from the MDMX Application Specific Extension from that point on
|
|
in the assembly. The @code{.set nomdmx} directive prevents MDMX
|
|
instructions from being accepted.
|
|
|
|
@cindex MIPS DSP Release 1 instruction generation override
|
|
@kindex @code{.set dsp}
|
|
@kindex @code{.set nodsp}
|
|
The directive @code{.set dsp} makes the assembler accept instructions
|
|
from the DSP Release 1 Application Specific Extension from that point
|
|
on in the assembly. The @code{.set nodsp} directive prevents DSP
|
|
Release 1 instructions from being accepted.
|
|
|
|
@cindex MIPS DSP Release 2 instruction generation override
|
|
@kindex @code{.set dspr2}
|
|
@kindex @code{.set nodspr2}
|
|
The directive @code{.set dspr2} makes the assembler accept instructions
|
|
from the DSP Release 2 Application Specific Extension from that point
|
|
on in the assembly. This directive implies @code{.set dsp}. The
|
|
@code{.set nodspr2} directive prevents DSP Release 2 instructions from
|
|
being accepted.
|
|
|
|
@cindex MIPS MT instruction generation override
|
|
@kindex @code{.set mt}
|
|
@kindex @code{.set nomt}
|
|
The directive @code{.set mt} makes the assembler accept instructions
|
|
from the MT Application Specific Extension from that point on
|
|
in the assembly. The @code{.set nomt} directive prevents MT
|
|
instructions from being accepted.
|
|
|
|
@cindex MIPS MCU instruction generation override
|
|
@kindex @code{.set mcu}
|
|
@kindex @code{.set nomcu}
|
|
The directive @code{.set mcu} makes the assembler accept instructions
|
|
from the MCU Application Specific Extension from that point on
|
|
in the assembly. The @code{.set nomcu} directive prevents MCU
|
|
instructions from being accepted.
|
|
|
|
@cindex MIPS SIMD Architecture instruction generation override
|
|
@kindex @code{.set msa}
|
|
@kindex @code{.set nomsa}
|
|
The directive @code{.set msa} makes the assembler accept instructions
|
|
from the MIPS SIMD Architecture Extension from that point on
|
|
in the assembly. The @code{.set nomsa} directive prevents MSA
|
|
instructions from being accepted.
|
|
|
|
@cindex Virtualization instruction generation override
|
|
@kindex @code{.set virt}
|
|
@kindex @code{.set novirt}
|
|
The directive @code{.set virt} makes the assembler accept instructions
|
|
from the Virtualization Application Specific Extension from that point
|
|
on in the assembly. The @code{.set novirt} directive prevents Virtualization
|
|
instructions from being accepted.
|
|
|
|
@cindex MIPS eXtended Physical Address (XPA) instruction generation override
|
|
@kindex @code{.set xpa}
|
|
@kindex @code{.set noxpa}
|
|
The directive @code{.set xpa} makes the assembler accept instructions
|
|
from the XPA Extension from that point on in the assembly. The
|
|
@code{.set noxpa} directive prevents XPA instructions from being accepted.
|
|
|
|
Traditional MIPS assemblers do not support these directives.
|
|
|
|
@node MIPS Floating-Point
|
|
@section Directives to override floating-point options
|
|
|
|
@cindex Disable floating-point instructions
|
|
@kindex @code{.set softfloat}
|
|
@kindex @code{.set hardfloat}
|
|
The directives @code{.set softfloat} and @code{.set hardfloat} provide
|
|
finer control of disabling and enabling float-point instructions.
|
|
These directives always override the default (that hard-float
|
|
instructions are accepted) or the command-line options
|
|
(@samp{-msoft-float} and @samp{-mhard-float}).
|
|
|
|
@cindex Disable single-precision floating-point operations
|
|
@kindex @code{.set singlefloat}
|
|
@kindex @code{.set doublefloat}
|
|
The directives @code{.set singlefloat} and @code{.set doublefloat}
|
|
provide finer control of disabling and enabling double-precision
|
|
float-point operations. These directives always override the default
|
|
(that double-precision operations are accepted) or the command-line
|
|
options (@samp{-msingle-float} and @samp{-mdouble-float}).
|
|
|
|
Traditional MIPS assemblers do not support these directives.
|
|
|
|
@node MIPS Syntax
|
|
@section Syntactical considerations for the MIPS assembler
|
|
@menu
|
|
* MIPS-Chars:: Special Characters
|
|
@end menu
|
|
|
|
@node MIPS-Chars
|
|
@subsection Special Characters
|
|
|
|
@cindex line comment character, MIPS
|
|
@cindex MIPS line comment character
|
|
The presence of a @samp{#} on a line indicates the start of a comment
|
|
that extends to the end of the current line.
|
|
|
|
If a @samp{#} appears as the first character of a line, the whole line
|
|
is treated as a comment, but in this case the line can also be a
|
|
logical line number directive (@pxref{Comments}) or a
|
|
preprocessor control command (@pxref{Preprocessing}).
|
|
|
|
@cindex line separator, MIPS
|
|
@cindex statement separator, MIPS
|
|
@cindex MIPS line separator
|
|
The @samp{;} character can be used to separate statements on the same
|
|
line.
|