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[BOLT][AArch64] Remove nops in functions with defined control flow #124705

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Merged
merged 1 commit into from
Jan 28, 2025
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[BOLT][AArch64] Remove nops in functions with defined control flow #124705

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5 changes: 4 additions & 1 deletion bolt/lib/Core/BinaryFunction.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -795,7 +795,6 @@ BinaryFunction::processIndirectBranch(MCInst &Instruction, unsigned Size,

auto Begin = Instructions.begin();
if (BC.isAArch64()) {
PreserveNops = BC.HasRelocations;
// Start at the last label as an approximation of the current basic block.
// This is a heuristic, since the full set of labels have yet to be
// determined
Expand Down Expand Up @@ -2300,6 +2299,10 @@ Error BinaryFunction::buildCFG(MCPlusBuilder::AllocatorIdTy AllocatorId) {
BC.errs() << "BOLT-WARNING: failed to post-process indirect branches for "
<< *this << '\n';
}

if (BC.isAArch64())
PreserveNops = BC.HasRelocations;

// In relocation mode we want to keep processing the function but avoid
// optimizing it.
setSimple(false);
Expand Down
28 changes: 28 additions & 0 deletions bolt/test/AArch64/remove-nops.s
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@@ -0,0 +1,28 @@
## Verify that llvm-bolt removes nop instructions from functions with indirect
## branches that have defined control flow.

# REQUIRES: system-linux

# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o
# RUN: %clang %cflags --target=aarch64-unknown-linux %t.o -o %t.exe -Wl,-q
# RUN: llvm-bolt %t.exe -o %t.bolt --print-normalized 2>&1 | FileCheck %s
# RUN: llvm-objdump -d --disassemble-symbols=_start %t.bolt \
# RUN: | FileCheck %s --check-prefix=CHECK-OBJDUMP

# CHECK-OBJDUMP-LABEL: _start
# CHECK-OBJDUMP-NOT: nop

.section .text
.align 4
.globl _start
.type _start, %function
_start:
# CHECK-LABEL: Binary Function "_start"
nop
# CHECK-NOT: nop
br x0
# CHECK: br x0 # TAILCALL
.size _start, .-_start

## Force relocation mode.
.reloc 0, R_AARCH64_NONE
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