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MCObjectStreamer: Refine absoluteSymbolDiff condition

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The function is called to test the fast path - when Lo/Hi are within the
same fragment. This is unsafe - Lo/Hi at the begin and end of a
relaxable fragment should not evaluate to a constant. However, we don't
have tests that exercise the code path.

Nevertheless, make the check safer and remove the now unnecessary
isRISCV check (from https://reviews.llvm.org/D103539).
This commit is contained in:
Fangrui Song 2025-04-12 19:02:07 -07:00
parent b0acbbee88
commit 39662922e1
1 changed files with 14 additions and 12 deletions
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26
llvm/lib/MC/MCObjectStreamer.cpp
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@ -94,8 +94,12 @@ void MCObjectStreamer::resolvePendingFixups() {
static std::optional<uint64_t> absoluteSymbolDiff(const MCSymbol *Hi,
const MCSymbol *Lo) {
assert(Hi && Lo);
if (!Hi->getFragment() || Hi->getFragment() != Lo->getFragment() ||
Hi->isVariable() || Lo->isVariable())
if (Lo == Hi)
return 0;
if (Hi->isVariable() || Lo->isVariable())
return std::nullopt;
auto *LoF = dyn_cast_or_null<MCDataFragment>(Lo->getFragment());
if (!LoF || Hi->getFragment() != LoF || LoF->isLinkerRelaxable())
return std::nullopt;
return Hi->getOffset() - Lo->getOffset();
@ -104,20 +108,18 @@ static std::optional<uint64_t> absoluteSymbolDiff(const MCSymbol *Hi,
void MCObjectStreamer::emitAbsoluteSymbolDiff(const MCSymbol *Hi,
const MCSymbol *Lo,
unsigned Size) {
if (!getAssembler().getContext().getTargetTriple().isRISCV())
if (std::optional<uint64_t> Diff = absoluteSymbolDiff(Hi, Lo))
return emitIntValue(*Diff, Size);
MCStreamer::emitAbsoluteSymbolDiff(Hi, Lo, Size);
if (std::optional<uint64_t> Diff = absoluteSymbolDiff(Hi, Lo))
emitIntValue(*Diff, Size);
else
MCStreamer::emitAbsoluteSymbolDiff(Hi, Lo, Size);
}
void MCObjectStreamer::emitAbsoluteSymbolDiffAsULEB128(const MCSymbol *Hi,
const MCSymbol *Lo) {
if (!getAssembler().getContext().getTargetTriple().isRISCV())
if (std::optional<uint64_t> Diff = absoluteSymbolDiff(Hi, Lo)) {
emitULEB128IntValue(*Diff);
return;
}
MCStreamer::emitAbsoluteSymbolDiffAsULEB128(Hi, Lo);
if (std::optional<uint64_t> Diff = absoluteSymbolDiff(Hi, Lo))
emitULEB128IntValue(*Diff);
else
MCStreamer::emitAbsoluteSymbolDiffAsULEB128(Hi, Lo);
}
void MCObjectStreamer::reset() {