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The current X86 NOP padding uses one long NOP followed by the remainder in

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one-byte NOPs.  If the processor actually executes those NOPs, as it sometimes
does with aligned bundling, this can have a performance impact.  From my
micro-benchmarks run on my one machine, a 15-byte NOP followed by twelve
one-byte NOPs is about 20% worse than a 15 followed by a 12.  This patch
changes NOP emission to emit as many 15-byte (the maximum) as possible followed
by at most one shorter NOP.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176464 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Sehr 2013-03-05 00:02:23 +00:00
parent 880e8c0ad4
commit 6c4265a541
2 changed files with 39 additions and 12 deletions
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20
lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
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@ -315,18 +315,18 @@ bool X86AsmBackend::writeNopData(uint64_t Count, MCObjectWriter *OW) const {
return true; return true;
} }
// Write an optimal sequence for the first 15 bytes. // 15 is the longest single nop instruction. Emit as many 15-byte nops as
const uint64_t OptimalCount = (Count < 16) ? Count : 15; // needed, then emit a nop of the remaining length.
const uint64_t Prefixes = OptimalCount <= 10 ? 0 : OptimalCount - 10; do {
for (uint64_t i = 0, e = Prefixes; i != e; i++) const uint8_t ThisNopLength = (uint8_t) std::min(Count, (uint64_t) 15);
const uint8_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength - 10;
for (uint8_t i = 0; i < Prefixes; i++)
OW->Write8(0x66); OW->Write8(0x66);
const uint64_t Rest = OptimalCount - Prefixes; const uint8_t Rest = ThisNopLength - Prefixes;
for (uint64_t i = 0, e = Rest; i != e; i++) for (uint8_t i = 0; i < Rest; i++)
OW->Write8(Nops[Rest - 1][i]); OW->Write8(Nops[Rest - 1][i]);
Count -= ThisNopLength;
// Finish with single byte nops. } while (Count != 0);
for (uint64_t i = OptimalCount, e = Count; i != e; ++i)
OW->Write8(0x90);
return true; return true;
} }

27
test/MC/X86/AlignedBundling/long-nop-pad.s Normal file
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@ -0,0 +1,27 @@
# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - \
# RUN: | llvm-objdump -disassemble -no-show-raw-insn - | FileCheck %s
# Test that long nops are generated for padding where possible.
.text
foo:
.bundle_align_mode 5
# This callq instruction is 5 bytes long
.bundle_lock align_to_end
callq bar
.bundle_unlock
# To align this group to a bundle end, we need a 15-byte NOP and a 12-byte NOP.
# CHECK: 0: nop
# CHECK-NEXT: f: nop
# CHECK-NEXT: 1b: callq
# This push instruction is 1 byte long
.bundle_lock align_to_end
push %rax
.bundle_unlock
# To align this group to a bundle end, we need two 15-byte NOPs, and a 1-byte.
# CHECK: 20: nop
# CHECK-NEXT: 2f: nop
# CHECK-NEXT: 3e: nop
# CHECK-NEXT: 3f: pushq