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[MC] Relaxable Fragments Can be Linker Relaxable #150096

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[MC] Relaxable Fragments Can be Linker Relaxable #150096

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[MC] Relaxable Fragments Can be Linker Relaxable
lenary Jul 22, 2025
214d5f8
clang-format
lenary Jul 22, 2025
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8 changes: 8 additions & 0 deletions llvm/lib/MC/MCAssembler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -777,6 +777,14 @@ bool MCAssembler::relaxInstruction(MCFragment &F) {
getEmitter().encodeInstruction(Relaxed, Data, Fixups, *F.getSubtargetInfo());
F.setVarContents(Data);
F.setVarFixups(Fixups);

for (const auto &Fixup : Fixups) {
if (!Fixup.isLinkerRelaxable())
continue;
F.setLinkerRelaxable();
F.getParent()->setLinkerRelaxable();
}
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This does not cope with the other direction correctly, but I'm not sure we can.

The other direction is when a linker relaxable fixup is replaced by a non-linker-relaxable fixup in relaxation, which might need the fragment to be marked as not linker relaxable. I'm not sure this will ever happen though.


return true;
}

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6 changes: 5 additions & 1 deletion llvm/lib/MC/MCExpr.cpp
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Expand Up @@ -269,7 +269,7 @@ bool MCExpr::evaluateAsAbsolute(int64_t &Res, const MCAssembler *Asm,
return IsRelocatable && Value.isAbsolute() && Value.getSpecifier() == 0;
}

/// Helper method for \see EvaluateSymbolAdd().
/// Helper method for \see evaluateSymbolicAdd().
static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
bool InSet, const MCSymbol *&A,
const MCSymbol *&B,
Expand Down Expand Up @@ -361,6 +361,10 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
if (BBeforeRelax && AAfterRelax)
return;
}
if (F->getKind() == MCFragment::FT_Relaxable && Asm->hasFinalLayout() &&
F->isLinkerRelaxable())
// FIXME: More accurate calculation of AAfterRelax/BBeforeRelax?
return;
if (&*F == FA) {
// If FA and FB belong to the same subsection, the loop will find FA and
// we can resolve the difference.
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2 changes: 2 additions & 0 deletions llvm/lib/MC/MCFragment.cpp
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Expand Up @@ -68,6 +68,8 @@ LLVM_DUMP_METHOD void MCFragment::dump() const {
OS << "\n Fixup @" << F.getOffset() << " Value:";
F.getValue()->print(OS, nullptr);
OS << " Kind:" << F.getKind();
if (F.isLinkerRelaxable())
OS << " LinkerRelaxable";
}
};

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2 changes: 2 additions & 0 deletions llvm/lib/MC/MCSection.cpp
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Expand Up @@ -41,6 +41,8 @@ LLVM_DUMP_METHOD void MCSection::dump(
raw_ostream &OS = errs();

OS << "MCSection Name:" << getName();
if (isLinkerRelaxable())
OS << " LinkerRelaxable";
for (auto &F : *this) {
OS << '\n';
F.dump();
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18 changes: 18 additions & 0 deletions llvm/test/MC/RISCV/linker-relaxation-pr150071.s
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@@ -0,0 +1,18 @@
# RUN: llvm-mc --triple=riscv32 -mattr=+relax,+experimental-xqcilb \
# RUN: %s -filetype=obj -o - -riscv-add-build-attributes \
# RUN: | llvm-objdump -dr - \
# RUN: | FileCheck %s

.global foo

bar:
jal x1, foo
# CHECK: qc.e.jal 0x0 <bar>
# CHECK-NEXT: R_RISCV_VENDOR QUALCOMM
# CHECK-NEXT: R_RISCV_CUSTOM195 foo
# CHECK-NEXT: R_RISCV_RELAX *ABS*
bne a0, a1, bar
# CHECK-NEXT: bne a0, a1, 0x6 <bar+0x6>
# CHECK-NEXT: R_RISCV_BRANCH bar
ret
# CHECK-NEXT: ret
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