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[MC][ARM] Emit R_ARM_BASE_PREL for _GLOBAL_OFFSET_TABLE_ expressions

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The _GLOBAL_OFFSET_TABLE_ in SysVr4 ELF is conventionally the base of the
.got or .got.prel sections. Expressions such as _GLOBAL_OFFSET_TABLE_
- (.L1 +8) are used in assembler code to calculate offsets into the .got.
At present MC outputs a R_ARM_REL32 with respect to the
_GLOBAL_OFFSET_TABLE_ symbol, whereas gas outputs a R_ARM_BASE_PREL
relocation with respect to the _GLOBAL_OFFSET_TABLE_ symbol. While both are
correct the R_ARM_REL32 depends on the value of the _GLOBAL_OFFSET_TABLE_
symbol, wheras te R_ARM_BASE_PREL relocation is idependent of the symbol.
The R_ARM_BASE_PREL is therefore slightly more robust to linker's that may
not follow the conventional placement of _GLOBAL_OFFSET_TABLE_; for example
LLD for some time defined _GLOBAL_OFFSET_TABLE_ to 0.

Differential Revision: https://reviews.llvm.org/D46319
This commit is contained in:
Peter Smith 2020-04-13 08:55:07 +01:00
parent e0dbd02513
commit 31c8e11896
2 changed files with 20 additions and 5 deletions
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9
llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
View File

@ -93,8 +93,15 @@ unsigned ARMELFObjectWriter::GetRelocTypeInner(const MCValue &Target,
switch (Modifier) {
default:
llvm_unreachable("Unsupported Modifier");
case MCSymbolRefExpr::VK_None:
case MCSymbolRefExpr::VK_None: {
if (const MCSymbolRefExpr *SymRef = Target.getSymA()) {
// For GNU AS compatibility expressions such as
// _GLOBAL_OFFSET_TABLE_ - label emit a R_ARM_BASE_PREL relocation.
if (SymRef->getSymbol().getName() == "_GLOBAL_OFFSET_TABLE_")
return ELF::R_ARM_BASE_PREL;
}
return ELF::R_ARM_REL32;
}
case MCSymbolRefExpr::VK_GOTTPOFF:
return ELF::R_ARM_TLS_IE32;
case MCSymbolRefExpr::VK_ARM_GOT_PREL:

16
llvm/test/MC/ARM/symbol-variants.s
View File

@ -96,8 +96,16 @@ bl f05(plt)
@CHECK: 70 R_ARM_TLS_LDM32 f28
@CHECK: 74 R_ARM_TLS_LDM32 f29
@ relative
.word f30 - (.Lsym+8)
@CHECK: 78 R_ARM_REL32 f30
@ _GLOBAL_OFFSET_TABLE_ relative
.word _GLOBAL_OFFSET_TABLE_ - (.Lsym+8)
@CHECK: 7c R_ARM_BASE_PREL _GLOBAL_OFFSET_TABLE_
@ got_prel
.word f30(GOT_PREL) + (. - .Lsym)
ldr r3, =f31(GOT_PREL)
@ CHECK: 78 R_ARM_GOT_PREL f30
@ CHECK: 80 R_ARM_GOT_PREL f31
.word f31(GOT_PREL) + (. - .Lsym)
ldr r3, =f32(GOT_PREL)
@CHECK: 80 R_ARM_GOT_PREL f31
@CHECK: 88 R_ARM_GOT_PREL f32