[llvm] [SimplifyLibCalls] fdim constant fold (PR #109235)

[Matt Arsenault via llvm-commits]

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@@ -3109,6 +3109,32 @@ Value *LibCallSimplifier::optimizeRemquo(CallInst *CI, IRBuilderBase &B) {
   return ConstantFP::get(CI->getType(), Rem);
 }
 
+/// Constant folds fdim
+Value *LibCallSimplifier::optimizeFdim(CallInst *CI, IRBuilderBase &B) {
+  const APFloat *X, *Y;
+  // Check if both values are constants
+  if (!match(CI->getArgOperand(0), m_APFloat(X)) ||
+      !match(CI->getArgOperand(1), m_APFloat(Y)))
+    return nullptr;
+  // If either argument is NaN, NaN is returned
+  if (X->isNaN() || Y->isNaN())
+    return ConstantFP::getQNaN(CI->getType());
+
+  IRBuilderBase::FastMathFlagGuard Guard(B);
+  FastMathFlags FMF = CI->getFastMathFlags();
+  // set no-NaN fast-math-flag as we already checked for NaN for both operands
+  FMF.setNoNaNs();
+  // set no-signed-zeroes as fdim will never return -0.0
+  FMF.setNoSignedZeros();
+  B.setFastMathFlags(FMF);
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arsenm wrote:

You can only preserve the fast math flags that were present on the original call 

https://github.com/llvm/llvm-project/pull/109235