[all-commits] [llvm/llvm-project] 31c8e1: [MC][ARM] Emit R_ARM_BASE_PREL for _GLOBAL_OFFSET_...

[Peter Smith via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 31c8e118961913341e4f5c066bbdcf9b3d2109c7
      https://github.com/llvm/llvm-project/commit/31c8e118961913341e4f5c066bbdcf9b3d2109c7
  Author: Peter Smith <peter.smith at arm.com>
  Date:   2020-04-14 (Tue, 14 Apr 2020)

  Changed paths:
    M llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
    M llvm/test/MC/ARM/symbol-variants.s

  Log Message:
  -----------
  [MC][ARM] Emit R_ARM_BASE_PREL for _GLOBAL_OFFSET_TABLE_ expressions

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