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The current X86 NOP padding uses one long NOP followed by the remainder in
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
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@ -315,18 +315,18 @@ bool X86AsmBackend::writeNopData(uint64_t Count, MCObjectWriter *OW) const {
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return true;
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}
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// Write an optimal sequence for the first 15 bytes.
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const uint64_t OptimalCount = (Count < 16) ? Count : 15;
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const uint64_t Prefixes = OptimalCount <= 10 ? 0 : OptimalCount - 10;
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for (uint64_t i = 0, e = Prefixes; i != e; i++)
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// 15 is the longest single nop instruction. Emit as many 15-byte nops as
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// needed, then emit a nop of the remaining length.
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do {
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const uint8_t ThisNopLength = (uint8_t) std::min(Count, (uint64_t) 15);
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const uint8_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength - 10;
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for (uint8_t i = 0; i < Prefixes; i++)
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OW->Write8(0x66);
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const uint64_t Rest = OptimalCount - Prefixes;
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for (uint64_t i = 0, e = Rest; i != e; i++)
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const uint8_t Rest = ThisNopLength - Prefixes;
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for (uint8_t i = 0; i < Rest; i++)
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OW->Write8(Nops[Rest - 1][i]);
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// Finish with single byte nops.
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for (uint64_t i = OptimalCount, e = Count; i != e; ++i)
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OW->Write8(0x90);
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Count -= ThisNopLength;
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} while (Count != 0);
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return true;
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}
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27
test/MC/X86/AlignedBundling/long-nop-pad.s
Normal file
27
test/MC/X86/AlignedBundling/long-nop-pad.s
Normal file
@ -0,0 +1,27 @@
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# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - \
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# RUN: | llvm-objdump -disassemble -no-show-raw-insn - | FileCheck %s
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# Test that long nops are generated for padding where possible.
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.text
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foo:
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.bundle_align_mode 5
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# This callq instruction is 5 bytes long
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.bundle_lock align_to_end
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callq bar
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.bundle_unlock
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# To align this group to a bundle end, we need a 15-byte NOP and a 12-byte NOP.
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# CHECK: 0: nop
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# CHECK-NEXT: f: nop
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# CHECK-NEXT: 1b: callq
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# This push instruction is 1 byte long
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.bundle_lock align_to_end
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push %rax
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.bundle_unlock
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# To align this group to a bundle end, we need two 15-byte NOPs, and a 1-byte.
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# CHECK: 20: nop
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# CHECK-NEXT: 2f: nop
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# CHECK-NEXT: 3e: nop
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# CHECK-NEXT: 3f: pushq
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