[InstCombine] fold negate disguised as select+mul
Name: negate if true
%sel = select i1 %cond, i32 -1, i32 1
%r = mul i32 %sel, %x
=>
%m = sub i32 0, %x
%r = select i1 %cond, i32 %m, i32 %x
Name: negate if false
%sel = select i1 %cond, i32 1, i32 -1
%r = mul i32 %sel, %x
=>
%m = sub i32 0, %x
%r = select i1 %cond, i32 %x, i32 %m
https://rise4fun.com/Alive/Nlh
llvm-svn: 373230
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 7f152aa..a301d9e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -213,6 +213,25 @@
if (Instruction *FoldedMul = foldBinOpIntoSelectOrPhi(I))
return FoldedMul;
+ // TODO: This is a specific form of a much more general pattern.
+ // We could detect a select with any binop identity constant, or we
+ // could use SimplifyBinOp to see if either arm of the select reduces.
+ // But that needs to be done carefully and/or while removing potential
+ // reverse canonicalizations as in InstCombiner::foldSelectIntoOp().
+ // mul (select Cond, 1, -1), Op1 --> select Cond, Op1, -Op1
+ // mul (select Cond, -1, 1), Op1 --> select Cond, -Op1, Op1
+ // mul Op0, (select Cond, 1, -1) --> select Cond, Op0, -Op0
+ // mul Op0, (select Cond, -1, 1) --> select Cond, -Op0, Op0
+ Value *Cond;
+ if (match(Op0, m_OneUse(m_Select(m_Value(Cond), m_One(), m_AllOnes()))))
+ return SelectInst::Create(Cond, Op1, Builder.CreateNeg(Op1));
+ if (match(Op0, m_OneUse(m_Select(m_Value(Cond), m_AllOnes(), m_One()))))
+ return SelectInst::Create(Cond, Builder.CreateNeg(Op1), Op1);
+ if (match(Op1, m_OneUse(m_Select(m_Value(Cond), m_One(), m_AllOnes()))))
+ return SelectInst::Create(Cond, Op0, Builder.CreateNeg(Op0));
+ if (match(Op1, m_OneUse(m_Select(m_Value(Cond), m_AllOnes(), m_One()))))
+ return SelectInst::Create(Cond, Builder.CreateNeg(Op0), Op0);
+
// Simplify mul instructions with a constant RHS.
if (isa<Constant>(Op1)) {
// Canonicalize (X+C1)*CI -> X*CI+C1*CI.