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274 lines
8.0 KiB
Plaintext
274 lines
8.0 KiB
Plaintext
=pod
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=head1 NAME
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llvm-ld - LLVM linker
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=head1 SYNOPSIS
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B<llvm-ld> <options> <files>
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=head1 DESCRIPTION
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The B<llvm-ld> tool takes a set of LLVM bytecode files and links them
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together into a single LLVM bytecode file. The output bytecode file can be
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another bytecode file or an executable bytecode program. Using additional
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options, B<llvm-ld> is able to produce native code executables.
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The B<llvm-ld> tool is the main linker for LLVM. It is used to link together
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the output of LLVM front-end compilers and run "link time" optimizations (mostly
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the inter-procedural kind).
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The B<llvm-ld> tools attemps to mimic the interface provided by the default
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system linker so that it can act as a I<drop-in> replacement.
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=head2 Search Order
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When looking for objects specified on the command line, B<llvm-ld> will search
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for the object first in the current directory and then in the directory
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specified by the B<LLVM_LIB_SEARCH_PATH> environment variable. If it cannot
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find the object, it fails.
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When looking for a library specified with the B<-l> option, B<llvm-ld> first
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attempts to load a file with that name from the current directory. If that
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fails, it looks for libI<library>.bc, libI<library>.a, or libI<library>.I<shared
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library extension>, in that order, in each directory added to the library search
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path with the B<-L> option. These directories are searched in the order they
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are specified. If the library cannot be located, then B<llvm-ld> looks in the
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directory specified by the B<LLVM_LIB_SEARCH_PATH> environment variable. If it
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does not find a library there, it fails.
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The I<shared library extension> may be I<.so>, I<.dyld>, I<.dll>, or something
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different, depending upon the system.
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The B<-L> option is global. It does not matter where it is specified in the
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list of command line arguments; the directory is simply added to the search path
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and is applied to all libraries, preceding or succeeding, in the command line.
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=head2 Link order
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All object and bytecode files are linked first in the order they were
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specified on the command line. All library files are linked next.
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Some libraries may not be linked into the object program; see below.
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=head2 Library Linkage
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Object files and static bytecode objects are always linked into the output
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file. Library archives (.a files) load only the objects within the archive
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that define symbols needed by the output file. Hence, libraries should be
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listed after the object files and libraries which need them; otherwise, the
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library may not be linked in, and the dependent library will not have its
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undefined symbols defined.
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=head2 Native code generation
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The B<llvm-ld> program has limited support for native code generation, when
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using the B<-native> or B<-native-cbe> options. Native code generation is
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perfomed by converting the linked bytecode into native assembly (.s) or C code
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and running the system compiler (typically gcc) on the result.
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=head1 OPTIONS
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=head2 General Options
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=over
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=item B<-help>
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Print a summary of command line options.
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=item B<-v>
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Specifies verbose mode. In this mode the linker will print additional
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information about the actions it takes, programs it executes, etc.
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=item B<-stats>
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Print statistics.
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=item B<-time-passes>
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Record the amount of time needed for each pass and print it to standard
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error.
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=back
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=head2 Input/Output Options
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=over
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=item B<-o> F<filename>
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This overrides the default output file and specifies the name of the file that
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should be generated by the linker. By default, B<llvm-ld> generates a file named
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F<a.out> for compatibility with B<ld>. The output will be written to
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F<filename>.
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=item B<-l>F<name>
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This option specifies the F<name> of a library to search when resolving symbols
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for the program. Only the base name should be specified as F<name>, without a
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F<lib> prefix or any suffix.
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=item B<-L>F<Path>
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This option tells B<llvm-ld> to look in F<Path> to find any library subsequently
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specified with the B<-l> option. The paths will be searched in the order in
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which they are specified on the command line. If the library is still not found,
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a small set of system specific directories will also be searched. Note that
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libraries specified with the B<-l> option that occur I<before> any B<-L> options
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will not search the paths given by the B<-L> options following it.
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=item B<-link-as-library>
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Link the bytecode files together as a library, not an executable. In this mode,
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undefined symbols will be permitted.
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=item B<-r>
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An alias for -link-as-library.
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=item B<-march=>C<target>
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Specifies the kind of machine for which code or assembly should be generated.
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=item B<-native>
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Generate a native machine code executable.
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When generating native executables, B<llvm-ld> first checks for a bytecode
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version of the library and links it in, if necessary. If the library is
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missing, B<llvm-ld> skips it. Then, B<llvm-ld> links in the same
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libraries as native code.
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In this way, B<llvm-ld> should be able to link in optimized bytecode
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subsets of common libraries and then link in any part of the library that
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hasn't been converted to bytecode.
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=item B<-native-cbe>
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Generate a native machine code executable with the LLVM C backend.
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This option is identical to the B<-native> option, but uses the
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C backend to generate code for the program instead of an LLVM native
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code generator.
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=item B<-disable-compression>
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Do not compress bytecode files.
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=back
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=head2 Optimization Options
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=over
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=item B<-O0>
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An alias for the -O1 option.
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=item B<-O1>
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Optimize for linking speed, not execution speed. The optimizer will attempt to
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reduce the size of the linked program to reduce I/O but will not otherwise
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perform any link-time optimizations.
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=item B<-O2>
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Perform only the minimal or required set of scalar optimizations.
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=item B<-03>
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An alias for the -O2 option.
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=item B<-04>
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Perform the standard link time inter-procedural optimizations. This will
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attempt to optimize the program taking the entire program into consideration.
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=item B<-O5>
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Perform aggressive link time optimizations. This is the same as -O4 but works
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more aggressively to optimize the program.
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=item B<-disable-inlining>
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Do not run the inlining pass. Functions will not be inlined into other
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functions.
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=item B<-disable-opt>
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Completely disable optimization. The various B<-On> options will be ignored and
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no link time optimization passes will be run.
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=item B<-disable-internalize>
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Do not mark all symbols as internal.
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=item B<-verify-each>
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Run the verification pass after each of the passes to verify intermediate
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results.
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=item B<-strip-all>
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Strip all debug and symbol information from the executable to make it smaller.
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=item B<-strip-debug>
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Strip all debug information from the executable to make it smaller.
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=item B<-s>
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An alias for B<-strip-all>.
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=item B<-S>
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An alias for B<-strip-debug>.
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=item B<-export-dynamic>
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An alias for B<-disable-internalize>
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=item B<-load> F<module>
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Load an optimization module, F<module>, which is expected to be a dynamic
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library that provides the function name C<RunOptimizations>. This function will
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be passed the PassManager, and the optimization level (values 0-5 based on the
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B<-On> option). This function may add passes to the PassManager that should be
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run. This feature allows the optimization passes of B<llvm-ld> to be extended.
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=item B<-post-link-opt>F<Path>
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Run post-link optimization program. After linking is completed a bytecode file
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will be generated. It will be passed to the program specified by F<Path> as the
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first argument. The second argument to the program will be the name of a
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temporary file into which the program should place its optimized output. For
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example, the "no-op optimization" would be a simple shell script:
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#!/bin/bash
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cp $1 $2
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=back
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=head1 EXIT STATUS
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If B<llvm-ld> succeeds, it will exit with 0 return code. If an error occurs,
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it will exit with a non-zero return code.
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=head1 ENVIRONMENT
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The C<LLVM_LIB_SEARCH_PATH> environment variable is used to find bytecode
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libraries. Any paths specified in this variable will be searched after the C<-L>
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options.
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=head1 SEE ALSO
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L<llvm-link|llvm-link>
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=head1 AUTHORS
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Maintained by the LLVM Team (L<http://llvm.org>).
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=cut
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