[llvm-branch-commits] [llvm] [InstCombine] Fold exp2(uitofp iN x) to ldexp when N equals the width of int (PR #227380)

[Eric Ross via llvm-branch-commits]

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@@ -2633,28 +2633,43 @@ Value *LibCallSimplifier::optimizeExp2(CallInst *CI, IRBuilderBase &B) {
 
   // exp2(sitofp(x)) -> ldexp(1.0, sext(x))  if sizeof(x) <= IntSize
   // exp2(uitofp(x)) -> ldexp(1.0, zext(x))  if sizeof(x) < IntSize
+  // exp2(uitofp(x)) -> ldexp(1.0, umin(x, C)) if sizeof(x) == IntSize
   Value *Op = CI->getArgOperand(0);
-  if ((isa<SIToFPInst>(Op) || isa<UIToFPInst>(Op)) &&
-      (UseIntrinsic ||
-       hasFloatFn(M, TLI, Ty, LibFunc_ldexp, LibFunc_ldexpf, LibFunc_ldexpl))) {
-    if (Value *Exp = getIntToFPVal(Op, B, TLI->getIntSize())) {
-      Constant *One = ConstantFP::get(Ty, 1.0);
+  if (!isa<SIToFPInst, UIToFPInst>(Op) ||
+      (!UseIntrinsic &&
+       !hasFloatFn(M, TLI, Ty, LibFunc_ldexp, LibFunc_ldexpf, LibFunc_ldexpl)))
+    return Ret;
 
-      if (UseIntrinsic) {
-        return copyFlags(*CI, B.CreateIntrinsic(Intrinsic::ldexp,
-                                                {Ty, Exp->getType()},
-                                                {One, Exp}, CI));
-      }
+  unsigned IntSize = TLI->getIntSize();
+  Value *Exp = getIntToFPVal(Op, B, IntSize);
+  Value *UIntOp = nullptr;
+  // A uitofp source as wide as int may exceed INT_MAX, so clamp it for ldexp.
+  if (!Exp && UseIntrinsic && match(Op, m_UIToFP(m_Value(UIntOp))) &&
+      UIntOp->getType()->getScalarSizeInBits() == IntSize) {
+    const fltSemantics &Sem = Ty->getScalarType()->getFltSemantics();
+    // exp2(x) and ldexp(1.0, Clamp) both overflow for every x >= Clamp.
+    int Clamp = llvm::ilogb(APFloat::getLargest(Sem)) + 1;
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ZERICO2005 wrote:

> Isn't it equal to `APFloat::semanticsMaxExponent + 1`?

Yes except for `semFloat8E8M0FNU` since it can only represent exact powers of two.

https://github.com/llvm/llvm-project/pull/227380