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Add comment about potential partial register stall.
llvm-svn: 91220
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@ -1081,6 +1081,11 @@ X86InstrInfo::convertToThreeAddressWithLEA(unsigned MIOpc,
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// Build and insert into an implicit UNDEF value. This is OK because
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// well be shifting and then extracting the lower 16-bits.
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// This has the potential to cause partial stall. e.g.
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// movw (%rbp,%rcx,2), %dx
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// leal -65(%rdx), %esi
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// But testing has shown this *does* help performance (at least on modern
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// x86 machines).
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BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::IMPLICIT_DEF), leaInReg);
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MachineInstr *InsMI =
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BuildMI(*MFI, MBBI, MI->getDebugLoc(), get(X86::INSERT_SUBREG),leaInReg)
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