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Add comment about potential partial register stall.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91220 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2009-12-12 18:55:26 +00:00
parent 15b0d97068
commit 04ab19cb14
1 changed files with 5 additions and 0 deletions
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5
lib/Target/X86/X86InstrInfo.cpp
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@ -1081,6 +1081,11 @@ X86InstrInfo::convertToThreeAddressWithLEA(unsigned MIOpc,
// Build and insert into an implicit UNDEF value. This is OK because
// well be shifting and then extracting the lower 16-bits.
// This has the potential to cause partial stall. e.g.
// movw (%rbp,%rcx,2), %dx
// leal -65(%rdx), %esi
// But testing has shown this *does* help performance (at least on modern
// x86 machines).
BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::IMPLICIT_DEF), leaInReg);
MachineInstr *InsMI =
BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::INSERT_SUBREG),leaInReg)