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generalize a transformation even more: we don't care whether the 

input the the mul is a zext from bool, just that it is all zeros
other than the low bit.  This fixes some phase ordering issues
that would cause us to miss some xforms in mul.ll when the worklist
is visited differently.

llvm-svn: 83794
This commit is contained in:
Chris Lattner 2009-10-11 21:29:45 +00:00
parent 2295f31810
commit fd27f8a5b3
2 changed files with 17 additions and 12 deletions
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20
llvm/lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -2781,20 +2781,18 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
// If one of the operands of the multiply is a cast from a boolean value, then
// we know the bool is either zero or one, so this is a 'masking' multiply.
// See if we can simplify things based on how the boolean was originally
// formed.
{
// X * Y (where Y is 0 or 1) -> X & (0-Y)
if (!isa<VectorType>(I.getType())) {
// -2 is "-1 << 1" so it is all bits set except the low one.
APInt Negative2(I.getType()->getPrimitiveSizeInBits(), -2, true);
Value *BoolCast = 0, *OtherOp = 0;
if (ZExtInst *CI = dyn_cast<ZExtInst>(Op0))
if (CI->getOperand(0)->getType() == Type::getInt1Ty(*Context))
BoolCast = CI, OtherOp = I.getOperand(1);
if (!BoolCast)
if (ZExtInst *CI = dyn_cast<ZExtInst>(I.getOperand(1)))
if (CI->getOperand(0)->getType() == Type::getInt1Ty(*Context))
BoolCast = CI, OtherOp = Op0;
if (MaskedValueIsZero(Op0, Negative2))
BoolCast = Op0, OtherOp = I.getOperand(1);
else if (MaskedValueIsZero(I.getOperand(1), Negative2))
BoolCast = I.getOperand(1), OtherOp = Op0;
if (BoolCast) {
// X * Y (where Y is 0 or 1) -> X & (0-Y)
Value *V = Builder->CreateSub(Constant::getNullValue(I.getType()),
BoolCast, "tmp");
return BinaryOperator::CreateAnd(V, OtherOp);

7
llvm/test/Transforms/InstCombine/mul.ll
View File

@ -105,5 +105,12 @@ define i32 @test16(i32 %b, i1 %c) {
ret i32 %e
}
; X * Y (when Y is 0 or 1) --> x & (0-Y)
define i32 @test17(i32 %a, i32 %b) {
%a.lobit = lshr i32 %a, 31
%e = mul i32 %a.lobit, %b
ret i32 %e
}