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MC: Fix NOP insertion between fused instructions that breaks macro fusion #155784

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Merged
merged 1 commit into from
Aug 28, 2025
Merged

MC: Fix NOP insertion between fused instructions that breaks macro fusion #155784

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11 changes: 10 additions & 1 deletion llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -485,7 +485,16 @@ void X86AsmBackend::emitInstructionBegin(MCObjectStreamer &OS,
if (!CanPadInst)
return;

if (PendingBA && PendingBA->getNext() == OS.getCurrentFragment()) {
if (PendingBA) {
auto *NextFragment = PendingBA->getNext();
assert(NextFragment && "NextFragment should not be null");
if (NextFragment == OS.getCurrentFragment())
return;
// We eagerly create an empty fragment when inserting a fragment
// with a variable-size tail.
if (NextFragment->getNext() == OS.getCurrentFragment())
return;

// Macro fusion actually happens and there is no other fragment inserted
// after the previous instruction.
//
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20 changes: 19 additions & 1 deletion llvm/test/MC/X86/align-branch-fused.s
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Original file line number Diff line number Diff line change
@@ -1,7 +1,10 @@
# RUN: llvm-mc -filetype=obj -triple x86_64 --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc %s | llvm-objdump -d --no-show-raw-insn - | FileCheck %s
# RUN: rm -rf %t && split-file %s %t && cd %t
# RUN: llvm-mc -filetype=obj -triple x86_64 --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc foo.s | llvm-objdump -d --no-show-raw-insn - | FileCheck foo.s
# RUN: llvm-mc -filetype=obj -triple x86_64 --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc -x86-pad-max-prefix-size=1 bar.s | llvm-objdump -d --no-show-raw-insn - | FileCheck bar.s

# Exercise cases where fused instructions need to be aligned.

#--- foo.s
.text
.globl foo
foo:
Expand Down Expand Up @@ -40,3 +43,18 @@ foo:
cmp %rax, %rbp
jne foo
int3

# Exercise the case where fused instructions need to be aligned,
# ensuring fusion is not broken by a NOP

#--- bar.s
.text
.globl bar
bar:
.nops 27
# CHECK: 20: testq %rcx, %rcx
# CHECK: 23: je
testq %rcx, %rcx
je .EXIT
.EXIT:
ret