[InstCombine] Sink sub into hands of select if one hand becomes zero (PR44426)
This decreases use count of %Op1, makes one hand of select to be 0,
and possibly exposes further folding potential.
Name: sub (select %Cond, %Op1, %FalseVal), %Op1 -> select %Cond, 0, (sub %FalseVal, %Op1)
%Op1 = %TrueVal
%o = select i1 %Cond, i8 %Op1, i8 %FalseVal
%r = sub i8 %o, %Op1
=>
%n = sub i8 %FalseVal, %Op1
%r = select i1 %Cond, i8 0, i8 %n
Name: sub (select %Cond, %TrueVal, %Op1), %Op1 -> select %Cond, (sub %TrueVal, %Op1), 0
%Op1 = %FalseVal
%o = select i1 %Cond, i8 %TrueVal, i8 %Op1
%r = sub i8 %o, %Op1
=>
%n = sub i8 %TrueVal, %Op1
%r = select i1 %Cond, i8 %n, i8 0
https://rise4fun.com/Alive/avL
https://bugs.llvm.org/show_bug.cgi?id=44426
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 9676c6a..05d13ae 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1898,6 +1898,33 @@
}
}
+ {
+ // If we are subtracting from select with one hand matching the value
+ // we are subtracting, sink subtraction into hands of select:
+ // sub (select %Cond, %TrueVal, %FalseVal), %Op1
+ // ->
+ // select %Cond, (sub %TrueVal, %Op1), (sub %FalseVal, %Op1)
+ // This will result in select between new subtraction and 0.
+ Value *Cond, *TrueVal, *FalseVal;
+ if (match(Op0, m_OneUse(m_Select(m_Value(Cond), m_Value(TrueVal),
+ m_Value(FalseVal)))) &&
+ (Op1 == TrueVal || Op1 == FalseVal)) {
+ // While it is really tempting to just create two subtractions and let
+ // InstCombine fold one of those to 0, it isn't possible to do so
+ // because of worklist visitation order. So ugly it is.
+ bool SubtractingTrueVal = Op1 == TrueVal;
+ Value *NewSub =
+ Builder.CreateSub(SubtractingTrueVal ? FalseVal : TrueVal, Op1);
+ Constant *Zero = Constant::getNullValue(I.getType());
+ SelectInst *NewSel =
+ SelectInst::Create(Cond, SubtractingTrueVal ? Zero : NewSub,
+ SubtractingTrueVal ? NewSub : Zero);
+ // Preserve prof metadata if any.
+ NewSel->copyMetadata(cast<Instruction>(*Op0));
+ return NewSel;
+ }
+ }
+
if (Op1->hasOneUse()) {
Value *X = nullptr, *Y = nullptr, *Z = nullptr;
Constant *C = nullptr;