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[X86] Add NEG to isUseDefConvertible.

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We can use the C flag from NEG to detect that the input was zero.

Really we could probably use the Z flag too. But C matches what
we'd do for usubo 0, X.

Haven't found a test case for this due to the usubo formation
in CGP. But I verified if I comment out the CGP code this
transformation catches some of the same cases.

llvm-svn: 365929
This commit is contained in:
Craig Topper 2019-07-12 17:52:17 +00:00
parent 7f2a0e6b10
commit a262a2636b
1 changed files with 5 additions and 0 deletions
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5
lib/Target/X86/X86InstrInfo.cpp
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@ -3374,6 +3374,11 @@ inline static bool isDefConvertible(const MachineInstr &MI, bool &NoSignFlag) {
static X86::CondCode isUseDefConvertible(const MachineInstr &MI) {
switch (MI.getOpcode()) {
default: return X86::COND_INVALID;
case X86::NEG8r:
case X86::NEG16r:
case X86::NEG32r:
case X86::NEG64r:
return X86::COND_AE;
case X86::LZCNT16rr:
case X86::LZCNT32rr:
case X86::LZCNT64rr: