Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1462,9 +1462,22 @@
   if (Value *Sqrt = replacePowWithSqrt(Pow, B))
     return Sqrt;
 
+  if (!Pow->isFast())
+    return Shrunk;
+
+  // powf(x, sitofp(n)) -> powi(x, n)
+  if (auto *IntToFP = dyn_cast<SIToFPInst>(Expo)) {
+     // TODO: better transformations if x is also (constant) integer?
+     Value* IntVal = IntToFP->getOperand(0);
+     Value *Args[] = {Base, IntVal};
+     Module *M = Pow->getModule();
+     Function *F = Intrinsic::getDeclaration(M, Intrinsic::powi, Base->getType());
+     return B.CreateCall(F, Args);
+  }
+
   // pow(x, n) -> x * x * x * ...
   const APFloat *ExpoF;
-  if (Pow->isFast() && match(Expo, m_APFloat(ExpoF))) {
+  if (match(Expo, m_APFloat(ExpoF))) {
     // We limit to a max of 7 multiplications, thus the maximum exponent is 32.
     // If the exponent is an integer+0.5 we generate a call to sqrt and an
     // additional fmul.
Index: test/Transforms/InstCombine/pow_fp_int.ll
===================================================================
--- test/Transforms/InstCombine/pow_fp_int.ll
+++ test/Transforms/InstCombine/pow_fp_int.ll
@@ -0,0 +1,97 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+define double @pow_sitofp_const_base_fast(i32 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call fast float @llvm.powi.f32(float 7.000000e+00, i32 [[X:%.*]])
+; CHECK-NEXT:    [[RES:%.*]] = fpext float [[TMP1]] to double
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %subfp = sitofp i32 %x to float
+  %powi = tail call fast float @llvm.pow.f32(float 7.000000e+00, float %subfp)
+  %res = fpext float %powi to double
+  ret double %res
+}
+
+define double @pow_sitofp_float_base_fast(float %base, i32 %x) {
+; CHECK-LABEL: @pow_sitofp_float_base_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call fast float @llvm.powi.f32(float [[BASE:%.*]], i32 [[X:%.*]])
+; CHECK-NEXT:    [[RES:%.*]] = fpext float [[TMP1]] to double
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %subfp = sitofp i32 %x to float
+  %powi = tail call fast float @llvm.pow.f32(float %base, float %subfp)
+  %res = fpext float %powi to double
+  ret double %res
+}
+
+define double @pow_sitofp_double_base_fast(double %base, i32 %x) {
+; CHECK-LABEL: @pow_sitofp_double_base_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call fast double @llvm.powi.f64(double [[BASE:%.*]], i32 [[X:%.*]])
+; CHECK-NEXT:    ret double [[TMP1]]
+;
+  %subfp = sitofp i32 %x to double
+  %res = tail call fast double @llvm.pow.f64(double %base, double %subfp)
+  ret double %res
+}
+
+define float @pow_sitofp_const_base_exp_i64_fast(i64 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_exp_i64_fast(
+; CHECK-NEXT:    [[SUBFP:%.*]] = uitofp i64 [[X:%.*]] to float
+; CHECK-NEXT:    [[RES:%.*]] = tail call fast float @llvm.pow.f32(float 7.000000e+00, float [[SUBFP]])
+; CHECK-NEXT:    ret float [[RES]]
+;
+  %subfp = uitofp i64 %x to float
+  %res = tail call fast float @llvm.pow.f32(float 7.000000e+00, float %subfp)
+  ret float %res
+}
+
+define double @pow_sitofp_const_base_no_fast(i32 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_no_fast(
+; CHECK-NEXT:    [[SUBFP:%.*]] = sitofp i32 [[X:%.*]] to float
+; CHECK-NEXT:    [[POWI:%.*]] = tail call float @llvm.pow.f32(float 7.000000e+00, float [[SUBFP]])
+; CHECK-NEXT:    [[RES:%.*]] = fpext float [[POWI]] to double
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %subfp = sitofp i32 %x to float
+  %powi = tail call float @llvm.pow.f32(float 7.000000e+00, float %subfp)
+  %res = fpext float %powi to double
+  ret double %res
+}
+
+define double @pow_sitofp_float_base_no_fast(float %base, i32 %x) {
+; CHECK-LABEL: @pow_sitofp_float_base_no_fast(
+; CHECK-NEXT:    [[SUBFP:%.*]] = sitofp i32 [[X:%.*]] to float
+; CHECK-NEXT:    [[POWI:%.*]] = tail call float @llvm.pow.f32(float [[BASE:%.*]], float [[SUBFP]])
+; CHECK-NEXT:    [[RES:%.*]] = fpext float [[POWI]] to double
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %subfp = sitofp i32 %x to float
+  %powi = tail call float @llvm.pow.f32(float %base, float %subfp)
+  %res = fpext float %powi to double
+  ret double %res
+}
+
+define double @pow_sitofp_double_base_no_fast(double %base, i32 %x) {
+; CHECK-LABEL: @pow_sitofp_double_base_no_fast(
+; CHECK-NEXT:    [[SUBFP:%.*]] = sitofp i32 [[X:%.*]] to double
+; CHECK-NEXT:    [[POWI:%.*]] = tail call double @llvm.pow.f64(double [[BASE:%.*]], double [[SUBFP]])
+; CHECK-NEXT:    ret double [[POWI]]
+;
+  %subfp = sitofp i32 %x to double
+  %powi = tail call double @llvm.pow.f64(double %base, double %subfp)
+  ret double %powi
+}
+
+define float @pow_sitofp_const_base_exp_i64_no_fast(i64 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_exp_i64_no_fast(
+; CHECK-NEXT:    [[SUBFP:%.*]] = uitofp i64 [[X:%.*]] to float
+; CHECK-NEXT:    [[RES:%.*]] = tail call float @llvm.pow.f32(float 7.000000e+00, float [[SUBFP]])
+; CHECK-NEXT:    ret float [[RES]]
+;
+  %subfp = uitofp i64 %x to float
+  %res = tail call float @llvm.pow.f32(float 7.000000e+00, float %subfp)
+  ret float %res
+}
+
+declare float @llvm.pow.f32(float, float)
+declare double @llvm.pow.f64(double, double)