Fangrui Song 5be7f2a943 [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
relocations.

However, AArch64 encodes the GOT-generating intent in MCValue::RefKind
instead of MCSymbolRef::Kind (see commit
0999cbd0b9 (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

.data
// .globl x, y  would suppress STT_SECTION conversion
x:
.zero 4
y:
.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

2023-08-29 11:07:12 -07:00

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