SimplifyLibCalls: Don't require ldexp to emit intrinsic in pow combine (#95277)

Do not require a libm ldexp libcall to emit the ldexp intrinsic when
transforming pow(2, x) -> ldexp(1, x)

This enables the half intrinsic case to fold.

Author: arsenm

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -2092,7 +2092,11 @@ Value *LibCallSimplifier::replacePowWithExp(CallInst *Pow, IRBuilderBase &B) {
   // pow(2.0, itofp(x)) -> ldexp(1.0, x)
   if ((UseIntrinsic || !Ty->isVectorTy()) && match(Base, m_SpecificFP(2.0)) &&
       (isa<SIToFPInst>(Expo) || isa<UIToFPInst>(Expo)) &&
-      hasFloatFn(M, TLI, Ty, LibFunc_ldexp, LibFunc_ldexpf, LibFunc_ldexpl)) {
+      (UseIntrinsic ||
+       hasFloatFn(M, TLI, Ty, LibFunc_ldexp, LibFunc_ldexpf, LibFunc_ldexpl))) {
+
+    // TODO: Shouldn't really need to depend on getIntToFPVal for intrinsic. Can
+    // just directly use the original integer type.
     if (Value *ExpoI = getIntToFPVal(Expo, B, TLI->getIntSize())) {
       Constant *One = ConstantFP::get(Ty, 1.0);
 

llvm/test/Transforms/InstCombine/pow-to-ldexp.ll

@@ -116,8 +116,7 @@ define double @pow_sitofp_f64_const_base_2(i32 %x) {
 define half @pow_sitofp_f16_const_base_2(i32 %x) {
 ; CHECK-LABEL: define half @pow_sitofp_f16_const_base_2(
 ; CHECK-SAME: i32 [[X:%.*]]) {
-; CHECK-NEXT:    [[ITOFP:%.*]] = sitofp i32 [[X]] to half
-; CHECK-NEXT:    [[POW:%.*]] = tail call half @llvm.pow.f16(half 0xH4000, half [[ITOFP]])
+; CHECK-NEXT:    [[POW:%.*]] = tail call half @llvm.ldexp.f16.i32(half 0xH3C00, i32 [[X]])
 ; CHECK-NEXT:    ret half [[POW]]
 ;
   %itofp = sitofp i32 %x to half