[llvm] [InstCombine] Fold fmod to frem if we know it does not set errno. (PR #107912)

[Matt Arsenault via llvm-commits]

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@@ -2796,6 +2796,25 @@ Value *LibCallSimplifier::optimizeSqrt(CallInst *CI, IRBuilderBase &B) {
   return copyFlags(*CI, FabsCall);
 }
 
+Value *LibCallSimplifier::optimizeFMod(CallInst *CI, IRBuilderBase &B) {
+  SimplifyQuery SQ(DL, TLI, DT, AC, CI, true, true, DC);
+
+  // fmod(x,y) can set errno if y == 0 or x == +/-inf.
+  KnownFPClass Known0 = computeKnownFPClass(CI->getOperand(0), fcInf,
+                                            /*Depth=*/0, SQ);
+  if (Known0.isKnownNeverInfinity()) {
+    KnownFPClass Known1 = computeKnownFPClass(CI->getOperand(1), fcZero,
+                                              /*Depth=*/0, SQ);
+    if (Known1.isKnownNeverZero()) {
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arsenm wrote:

This is wrong if DAZ is enabled, this needs to account of the denormal mode. Can you add a negative test with "denormal-fp-math"="preserve-sign,preserve-sign" and/or ="dynamic,dynamic"? This should be checking isKnownNeverLogicalZero 

https://github.com/llvm/llvm-project/pull/107912