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

[Matt Arsenault via llvm-commits]

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@@ -3109,6 +3109,50 @@ Value *LibCallSimplifier::optimizeRemquo(CallInst *CI, IRBuilderBase &B) {
   return ConstantFP::get(CI->getType(), Rem);
 }
 
+/// Constant folds fdim
+Value *LibCallSimplifier::optimizeFdim(CallInst *CI, IRBuilderBase &B) {
+  // Cannot perform the fold unless the call has attribute memory(none)
+  if (!CI->doesNotAccessMemory())
+    return nullptr;
+
+  // TODO : Handle undef values
+  // propagate poison if any
+  if (isa<PoisonValue>(CI->getArgOperand(0)))
+    return CI->getArgOperand(0);
+  if (isa<PoisonValue>(CI->getArgOperand(1)))
+    return CI->getArgOperand(1);
+
+  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())
+    return ConstantFP::get(CI->getType(), X->makeQuiet());
+  if (Y->isNaN())
+    return ConstantFP::get(CI->getType(), Y->makeQuiet());
+
+  // if X - Y overflows, it will set the errno, so we avoid the fold
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arsenm wrote:

You excluded writing errno above with the doesNotAccessMemory check. Either don't need the status check, or can try to handle the writing errno known-no-overflow case 

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