[llvm] [InstCombine] Lower ilogb to bit operations for normal values (PR #218044)

[via llvm-commits]

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@@ -3239,6 +3239,59 @@ Value *LibCallSimplifier::optimizeFdim(CallInst *CI, IRBuilderBase &B) {
   return ConstantFP::get(CI->getType(), Difference);
 }
 
+Value *LibCallSimplifier::optimizeILogB(CallInst *CI, IRBuilderBase &B) {
+  // Prevent the optimization from re-triggering and causing the fixpoint error.
+  if (CI->use_empty())
+    return nullptr;
+
+  Value *X = CI->getArgOperand(0);
+  Type *FTy = X->getType();
+
+  // Restricted to standard IEEE floats, doubles and fp128
+  if (!FTy->isFloatTy() && !FTy->isDoubleTy() && !FTy->isFP128Ty())
+    return nullptr;
+
+  SimplifyQuery SQ(DL, TLI, DT, AC, CI,
+                   /*UseInstrInfo*/ true, /*CanUseUndef*/ true, DC);
+  KnownFPClass Known = computeKnownFPClass(X, fcAllFlags, SQ);
+
+  // This optimization works on normal floating point arguments. In the future,
+  // if computing known bits for floating point numbers is possible, then the
+  // optimization can work on subnormals with the MSB of the mantissa set.
+  bool IsNormal =
+      Known.isKnownNever(~fcNormal) && Known.KnownFPClasses != fcNone;
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tadeuszjt wrote:

Changed in the latest push

https://github.com/llvm/llvm-project/pull/218044