[llvm] [InstCombine] Fix miscompile in negation of select (PR #89698)

[via llvm-commits]

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@@ -8042,17 +8042,28 @@ static SelectPatternResult matchMinMax(CmpInst::Predicate Pred,
   return {SPF_UNKNOWN, SPNB_NA, false};
 }
 
-bool llvm::isKnownNegation(const Value *X, const Value *Y, bool NeedNSW) {
+bool llvm::isKnownNegation(const Value *X, const Value *Y, bool NeedNSW,
+                           bool AllowPoison) {
   assert(X && Y && "Invalid operand");
 
-  // X = sub (0, Y) || X = sub nsw (0, Y)
-  if ((!NeedNSW && match(X, m_Sub(m_ZeroInt(), m_Specific(Y)))) ||
-      (NeedNSW && match(X, m_NSWNeg(m_Specific(Y)))))
+  auto IsNegationOf = [&](const Value *X, const Value *Y) {
+    if (!match(X, m_Neg(m_Specific(Y))))
+      return false;
+
+    auto *BO = cast<BinaryOperator>(X);
+    if (NeedNSW && !BO->hasNoSignedWrap())
+      return false;
+
+    auto *Zero = cast<Constant>(BO->getOperand(0));
+    if (!AllowPoison && !Zero->isNullValue())
+      return false;
+
     return true;
+  };
 
+  // X = sub (0, Y) || X = sub nsw (0, Y)
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goldsteinn wrote:

Think 2x comments is unnecessarily verbose, people will get it.

https://github.com/llvm/llvm-project/pull/89698