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MC: Introduce the ABS8 symbol modifier.

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@ABS8 can be applied to symbols which appear as immediate operands to
instructions that have a 8-bit immediate form for that operand. It causes
the assembler to use the 8-bit form and an 8-bit relocation (e.g. R_386_8
or R_X86_64_8) for the symbol.

Differential Revision: https://reviews.llvm.org/D28688

llvm-svn: 293667
This commit is contained in:
Peter Collingbourne 2017-01-31 18:28:44 +00:00
parent 4c9c29a7b1
commit aa7dd32bfa
6 changed files with 40 additions and 0 deletions
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22
docs/Extensions.rst
View File

@ -207,6 +207,28 @@ in the assembler to differentiate the sections.
Target Specific Behaviour
=========================
X86
---
Relocations
^^^^^^^^^^^
``@ABS8`` can be applied to symbols which appear as immediate operands to
instructions that have an 8-bit immediate form for that operand. It causes
the assembler to use the 8-bit form and an 8-bit relocation (e.g. ``R_386_8``
or ``R_X86_64_8``) for the symbol.
For example:
.. code-block:: gas
cmpq $foo@ABS8, %rdi
This causes the assembler to select the form of the 64-bit ``cmpq`` instruction
that takes an 8-bit immediate operand that is sign extended to 64 bits, as
opposed to ``cmpq $foo, %rdi`` which takes a 32-bit immediate operand. This
is also not the same as ``cmpb $foo, %dil``, which is an 8-bit comparison.
Windows on ARM
--------------

2
include/llvm/MC/MCExpr.h
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@ -194,6 +194,8 @@ public:
VK_SIZE, // symbol@SIZE
VK_WEAKREF, // The link between the symbols in .weakref foo, bar
VK_X86_ABS8,
VK_ARM_NONE,
VK_ARM_GOT_PREL,
VK_ARM_TARGET1,

4
lib/MC/MCAssembler.cpp
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@ -747,6 +747,10 @@ bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
MCValue Target;
uint64_t Value;
bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
if (Target.getSymA() &&
Target.getSymA()->getKind() == MCSymbolRefExpr::VK_X86_ABS8 &&
Fixup.getKind() == FK_Data_1)
return false;
return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
Layout);
}

2
lib/MC/MCExpr.cpp
View File

@ -208,6 +208,7 @@ StringRef MCSymbolRefExpr::getVariantKindName(VariantKind Kind) {
case VK_SECREL: return "SECREL32";
case VK_SIZE: return "SIZE";
case VK_WEAKREF: return "WEAKREF";
case VK_X86_ABS8: return "ABS8";
case VK_ARM_NONE: return "none";
case VK_ARM_GOT_PREL: return "GOT_PREL";
case VK_ARM_TARGET1: return "target1";
@ -317,6 +318,7 @@ MCSymbolRefExpr::getVariantKindForName(StringRef Name) {
.Case("imgrel", VK_COFF_IMGREL32)
.Case("secrel32", VK_SECREL)
.Case("size", VK_SIZE)
.Case("abs8", VK_X86_ABS8)
.Case("l", VK_PPC_LO)
.Case("h", VK_PPC_HI)
.Case("ha", VK_PPC_HA)

2
lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
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@ -96,6 +96,7 @@ static unsigned getRelocType64(MCContext &Ctx, SMLoc Loc,
default:
llvm_unreachable("Unimplemented");
case MCSymbolRefExpr::VK_None:
case MCSymbolRefExpr::VK_X86_ABS8:
switch (Type) {
case RT64_64:
return IsPCRel ? ELF::R_X86_64_PC64 : ELF::R_X86_64_64;
@ -219,6 +220,7 @@ static unsigned getRelocType32(MCContext &Ctx,
default:
llvm_unreachable("Unimplemented");
case MCSymbolRefExpr::VK_None:
case MCSymbolRefExpr::VK_X86_ABS8:
switch (Type) {
case RT32_32:
return IsPCRel ? ELF::R_386_PC32 : ELF::R_386_32;

8
test/MC/X86/abs8.s Normal file
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@ -0,0 +1,8 @@
// RUN: llvm-mc -filetype=obj %s -o - -triple i686-pc-linux | llvm-objdump -d -r - | FileCheck --check-prefix=32 %s
// RUN: llvm-mc -filetype=obj %s -o - -triple x86_64-pc-linux | llvm-objdump -d -r - | FileCheck --check-prefix=64 %s
// 32: 0: 83 ff 00 cmpl $0, %edi
// 32: 00000002: R_386_8 foo
// 64: 0: 83 ff 00 cmpl $0, %edi
// 64: 0000000000000002: R_X86_64_8 foo+0
cmp $foo@ABS8, %edi