Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1322,12 +1322,12 @@
     APFloat BaseR = APFloat(1.0);
     BaseR.convert(BaseF->getSemantics(), APFloat::rmTowardZero, &Ignored);
     BaseR = BaseR / *BaseF;
-    bool IsInteger    = BaseF->isInteger(),
-         IsReciprocal = BaseR.isInteger();
+    bool IsInteger = BaseF->isInteger(), IsReciprocal = BaseR.isInteger();
     const APFloat *NF = IsReciprocal ? &BaseR : BaseF;
     APSInt NI(64, false);
     if ((IsInteger || IsReciprocal) &&
-        !NF->convertToInteger(NI, APFloat::rmTowardZero, &Ignored) &&
+        NF->convertToInteger(NI, APFloat::rmTowardZero, &Ignored) ==
+            APFloat::opOK &&
         NI > 1 && NI.isPowerOf2()) {
       double N = NI.logBase2() * (IsReciprocal ? -1.0 : 1.0);
       Value *FMul = B.CreateFMul(Expo, ConstantFP::get(Ty, N), "mul");
@@ -1410,11 +1410,20 @@
   return Sqrt;
 }
 
+static Value *createPowWithIntegerExponent(Value *Base, Value *Expo, Module *M,
+                                      IRBuilder<> &B) {
+  Value *Args[] = {Base, Expo};
+  Function *F = Intrinsic::getDeclaration(M, Intrinsic::powi, Base->getType());
+  return B.CreateCall(F, Args);
+}
+
 Value *LibCallSimplifier::optimizePow(CallInst *Pow, IRBuilder<> &B) {
-  Value *Base = Pow->getArgOperand(0), *Expo = Pow->getArgOperand(1);
+  Value *Base = Pow->getArgOperand(0);
+  Value *Expo = Pow->getArgOperand(1);
   Function *Callee = Pow->getCalledFunction();
   StringRef Name = Callee->getName();
   Type *Ty = Pow->getType();
+  Module *M = Pow->getModule();
   Value *Shrunk = nullptr;
   bool Ignored;
 
@@ -1438,6 +1447,10 @@
   if (match(Base, m_FPOne()))
     return Base;
 
+  // powf(x, sitofp(n)) -> powi(x, n)
+  if (auto *IntToFP = dyn_cast<SIToFPInst>(Expo))
+    return createPowWithIntegerExponent(Base, IntToFP->getOperand(0), M, B);
+
   if (Value *Exp = replacePowWithExp(Pow, B))
     return Exp;
 
@@ -1462,9 +1475,12 @@
   if (Value *Sqrt = replacePowWithSqrt(Pow, B))
     return Sqrt;
 
+  if (!Pow->isFast())
+    return Shrunk;
+
   // 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.
@@ -1488,9 +1504,8 @@
         if (!Expo2.isInteger())
           return nullptr;
 
-        Sqrt =
-            getSqrtCall(Base, Pow->getCalledFunction()->getAttributes(),
-                        Pow->doesNotAccessMemory(), Pow->getModule(), B, TLI);
+        Sqrt = getSqrtCall(Base, Pow->getCalledFunction()->getAttributes(),
+                           Pow->doesNotAccessMemory(), M, B, TLI);
       }
 
       // We will memoize intermediate products of the Addition Chain.
@@ -1513,6 +1528,14 @@
 
       return FMul;
     }
+
+    APSInt IntExpo(32, false);
+    // powf(x, C) -> powi(x, C) iff C is a constant integer value
+    if (ExpoF->convertToInteger(IntExpo, APFloat::rmTowardZero, &Ignored) ==
+        APFloat::opOK) {
+      return createPowWithIntegerExponent(
+          Base, ConstantInt::get(B.getInt32Ty(), IntExpo), M, B);
+    }
   }
 
   return Shrunk;
Index: test/Transforms/InstCombine/pow_fp_int.ll
===================================================================
--- test/Transforms/InstCombine/pow_fp_int.ll
+++ test/Transforms/InstCombine/pow_fp_int.ll
@@ -0,0 +1,124 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; PR42190
+
+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_const_base_power_of_2_fast(i32 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_power_of_2_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call fast float @llvm.powi.f32(float 1.600000e+01, 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 16.000000e+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 double @powf_exp_const_int_fast(double %base) {
+; CHECK-LABEL: @powf_exp_const_int_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call fast double @llvm.powi.f64(double [[BASE:%.*]], i32 40)
+; CHECK-NEXT:    ret double [[TMP1]]
+;
+  %res = tail call fast double @llvm.pow.f64(double %base, double 4.000000e+01)
+  ret double %res
+}
+
+define double @pow_sitofp_const_base_no_fast(i32 %x) {
+; CHECK-LABEL: @pow_sitofp_const_base_no_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = call 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 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:    [[TMP1:%.*]] = call 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 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:    [[TMP1:%.*]] = call double @llvm.powi.f64(double [[BASE:%.*]], i32 [[X:%.*]])
+; CHECK-NEXT:    ret double [[TMP1]]
+;
+  %subfp = sitofp i32 %x to double
+  %powi = tail call double @llvm.pow.f64(double %base, double %subfp)
+  ret double %powi
+}
+
+; Negative tests
+define double @powf_exp_const_int_no_fast(double %base) {
+; CHECK-LABEL: @powf_exp_const_int_no_fast(
+; CHECK-NEXT:    [[RES:%.*]] = tail call double @llvm.pow.f64(double [[BASE:%.*]], double 4.000000e+01)
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %res = tail call double @llvm.pow.f64(double %base, double 4.000000e+01)
+  ret double %res
+}
+
+define double @powf_exp_const_not_int_fast(double %base) {
+; CHECK-LABEL: @powf_exp_const_not_int_fast(
+; CHECK-NEXT:    [[RES:%.*]] = tail call fast double @llvm.pow.f64(double [[BASE:%.*]], double 3.750000e+01)
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %res = tail call fast double @llvm.pow.f64(double %base, double 3.750000e+01)
+  ret double %res
+}
+
+define double @powf_exp_const_not_int_no_fast(double %base) {
+; CHECK-LABEL: @powf_exp_const_not_int_no_fast(
+; CHECK-NEXT:    [[RES:%.*]] = tail call double @llvm.pow.f64(double [[BASE:%.*]], double 3.750000e+01)
+; CHECK-NEXT:    ret double [[RES]]
+;
+  %res = tail call double @llvm.pow.f64(double %base, double 3.750000e+01)
+  ret double %res
+}
+
+declare float @llvm.pow.f32(float, float)
+declare double @llvm.pow.f64(double, double)