[all-commits] [llvm/llvm-project] 5be7f2: [MC, AArch64] Suppress local symbol to STT_SECTION ...

Fangrui Song via All-commits all-commits at lists.llvm.org
Tue Aug 29 11:07:26 PDT 2023

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 5be7f2a94371b2a5c129309e2bcca9254ec8dc42
  Author: Fangrui Song <i at maskray.me>
  Date:   2023-08-29 (Tue, 29 Aug 2023)

  Changed paths:
    M llvm/include/llvm/MC/MCELFObjectWriter.h
    M llvm/lib/MC/ELFObjectWriter.cpp
    M llvm/lib/MC/MCELFObjectTargetWriter.cpp
    M llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFObjectWriter.cpp
    M llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
    M llvm/lib/Target/Lanai/MCTargetDesc/LanaiELFObjectWriter.cpp
    M llvm/lib/Target/Mips/MCTargetDesc/MipsELFObjectWriter.cpp
    M llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFObjectWriter.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
    M llvm/lib/Target/Sparc/MCTargetDesc/SparcELFObjectWriter.cpp
    M llvm/lib/Target/VE/MCTargetDesc/VEELFObjectWriter.cpp
    M llvm/lib/Target/Xtensa/MCTargetDesc/XtensaELFObjectWriter.cpp
    M llvm/test/MC/AArch64/arm64-elf-relocs.s

  Log Message:
  [MC,AArch64] Suppress local symbol to STT_SECTION conversion for GOT relocations

Assemblers change certain relocations referencing a local symbol to
reference the section symbol instead. This conversion is disabled for
many conditions (`shouldRelocateWithSymbol`), e.g. TLS symbol, for most
targets (including AArch32, x86, PowerPC, and RISC-V) GOT-generating

However, AArch64 encodes the GOT-generating intent in MCValue::RefKind
instead of MCSymbolRef::Kind (see commit
0999cbd0b9ed8aa893cce10d681dec6d54b200ad (2014)), therefore not affected
by the code `case MCSymbolRefExpr::VK_GOT:`. As GNU ld and ld.lld
create GOT entries based on the symbol, ignoring addend, the two ldr
instructions will share the same GOT entry, which is not expected:
ldr     x1, [x1, :got_lo12:x]  // converted to .data+0
ldr     x1, [x1, :got_lo12:y]  // converted to .data+4

// .globl x, y  would suppress STT_SECTION conversion
.zero 4
.long 42

This patch changes AArch64 to suppress local symbol to STT_SECTION
conversion for GOT relocations, matching most other targets. x and y
will use different GOT entries, which IMO is the most sensable behavior.

With this change, the ABI decision on https://github.com/ARM-software/abi-aa/issues/217
will only affect relocations explicitly referencing STT_SECTION symbols, e.g.
ldr     x1, [x1, :got_lo12:(.data+0)]
ldr     x1, [x1, :got_lo12:(.data+4)]
// I consider this unreasonable uses

IMO all reasonable use cases are unaffected.

Link: https://github.com/llvm/llvm-project/issues/63418
GNU assembler PR: https://sourceware.org/bugzilla/show_bug.cgi?id=30788

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D158577

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