[InstCombine] Canonicalize math-y conditional negation into a `select`

https://alive2.llvm.org/ce/z/vPs-gZ

This is a larger pattern than would seem necessary, with minimal being:
* `and` https://alive2.llvm.org/ce/z/q9-MqK
* `or`  https://alive2.llvm.org/ce/z/AUUEMZ
* `xor` https://alive2.llvm.org/ce/z/dm3Ume
.. so for all others, we canonicalize away from math to `select`,
but there we canonicalize in the opposite direction.

Fixes https://github.com/llvm/llvm-project/issues/59791

2 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 47f8c14..a4b14ad 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -1356,6 +1356,27 @@ Instruction *InstCombinerImpl::foldLogicOfIsFPClass(BinaryOperator &BO,
   return nullptr;
 }
 
+/// Look for the pattern that conditionally negates a value via math operations:
+///   cond.splat = sext i1 cond
+///   sub = add cond.splat, x
+///   xor = xor sub, cond.splat
+/// and rewrite it to do the same, but via logical operations:
+///   value.neg = sub 0, value
+///   cond = select i1 neg, value.neg, value
+Instruction *InstCombinerImpl::canonicalizeConditionalNegationViaMathToSelect(
+    BinaryOperator &I) {
+  assert(I.getOpcode() == BinaryOperator::Xor && "Only for xor!");
+  Value *Cond, *X;
+  // As per complexity ordering, `xor` is not commutative here.
+  if (!match(&I, m_c_BinOp(m_OneUse(m_Value()), m_Value())) ||
+      !match(I.getOperand(1), m_SExt(m_Value(Cond))) ||
+      !Cond->getType()->isIntOrIntVectorTy(1) ||
+      !match(I.getOperand(0), m_c_Add(m_SExt(m_Deferred(Cond)), m_Value(X))))
+    return nullptr;
+  return SelectInst::Create(Cond, Builder.CreateNeg(X, X->getName() + ".neg"),
+                            X);
+}
+
 /// This a limited reassociation for a special case (see above) where we are
 /// checking if two values are either both NAN (unordered) or not-NAN (ordered).
 /// This could be handled more generally in '-reassociation', but it seems like
@@ -4237,5 +4258,8 @@ Instruction *InstCombinerImpl::visitXor(BinaryOperator &I) {
   if (Instruction *Folded = foldLogicOfIsFPClass(I, Op0, Op1))
     return Folded;
 
+  if (Instruction *Folded = canonicalizeConditionalNegationViaMathToSelect(I))
+    return Folded;
+
   return nullptr;
 }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index f700cdb..bfbc31e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -365,6 +365,9 @@ private:
   Instruction *foldLogicOfIsFPClass(BinaryOperator &Operator, Value *LHS,
                                     Value *RHS);
 
+  Instruction *
+  canonicalizeConditionalNegationViaMathToSelect(BinaryOperator &i);
+
   Value *foldAndOrOfICmpsOfAndWithPow2(ICmpInst *LHS, ICmpInst *RHS,
                                        Instruction *CxtI, bool IsAnd,
                                        bool IsLogical = false);