Index: llvm/include/llvm/Transforms/Utils/SimplifyLibCalls.h
===================================================================
--- llvm/include/llvm/Transforms/Utils/SimplifyLibCalls.h
+++ llvm/include/llvm/Transforms/Utils/SimplifyLibCalls.h
@@ -133,6 +133,7 @@
   Value *optimizeCAbs(CallInst *CI, IRBuilder<> &B);
   Value *optimizeCos(CallInst *CI, IRBuilder<> &B);
   Value *optimizePow(CallInst *CI, IRBuilder<> &B);
+  Value *replacePowWithCbrt(CallInst *Pow, IRBuilder<> &B);
   Value *replacePowWithSqrt(CallInst *Pow, IRBuilder<> &B);
   Value *optimizeExp2(CallInst *CI, IRBuilder<> &B);
   Value *optimizeFMinFMax(CallInst *CI, IRBuilder<> &B);
Index: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1119,6 +1119,48 @@
   return InnerChain[Exp];
 }
 
+/// Use cube root in place of pow(x, +/-0.333...).
+Value *LibCallSimplifier::replacePowWithCbrt(CallInst *Pow, IRBuilder<> &B) {
+  // Only in finite and normal math.
+  if (!Pow->isFast())
+    return nullptr;
+
+  Type *Ty = Pow->getType();
+  Type::TypeID TyID = Ty->getTypeID();
+  // TODO: Handle SP FP as well, though the argument seems to never match below.
+  if (TyID != Type::FloatTyID && TyID != Type::DoubleTyID)
+    return nullptr;
+
+  ConstantFP *Arg2C = dyn_cast<ConstantFP>(Pow->getArgOperand(1));
+  if (!Arg2C)
+    return nullptr;
+
+  const APFloat SPPlusThird(1.0f / 3.0f), SPMinusThird(-1.0f / 3.0f),
+                DPPlusThird(1.0  / 3.0),  DPMinusThird(-1.0  / 3.0);
+  bool isPlusThird  = Arg2C->isExactlyValue((TyID == Type::FloatTyID)
+                                            ? SPPlusThird : DPPlusThird),
+       isMinusThird = Arg2C->isExactlyValue((TyID == Type::FloatTyID)
+                                            ? SPMinusThird : DPMinusThird);
+  if (!isPlusThird && !isMinusThird)
+    return nullptr;
+
+  if (!hasUnaryFloatFn(TLI, Ty, LibFunc_cbrt, LibFunc_cbrtf, LibFunc_cbrtl))
+    return nullptr;
+
+  // Fast-math flags from the pow() are propagated to all replacement ops.
+  IRBuilder<>::FastMathFlagGuard Guard(B);
+  B.setFastMathFlags(Pow->getFastMathFlags());
+  Value *Cbrt = emitUnaryFloatFnCall(Pow->getArgOperand(0),
+                                     TLI->getName(LibFunc_cbrt), B,
+                                     Pow->getCalledFunction()->getAttributes());
+
+  // If this is pow(x, -0.333...), get the reciprocal.
+  if (isMinusThird)
+    Cbrt = B.CreateFDiv(ConstantFP::get(Ty, 1.0), Cbrt);
+
+  return Cbrt;
+}
+
 /// Use square root in place of pow(x, +/-0.5).
 Value *LibCallSimplifier::replacePowWithSqrt(CallInst *Pow, IRBuilder<> &B) {
   // TODO: There is some subset of 'fast' under which these transforms should
@@ -1217,6 +1259,9 @@
   if (Value *Sqrt = replacePowWithSqrt(CI, B))
     return Sqrt;
 
+  if (Value *Cbrt = replacePowWithCbrt(CI, B))
+    return Cbrt;
+
   ConstantFP *Op2C = dyn_cast<ConstantFP>(Op2);
   if (!Op2C)
     return Ret;
Index: llvm/test/Transforms/InstCombine/pow-cbrt.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/pow-cbrt.ll
@@ -0,0 +1,120 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+define double @pow_intrinsic_third_fast(double %x) {
+; CHECK-LABEL: @pow_intrinsic_third_fast
+; CHECK-NEXT:    [[POW:%.*]] = call fast double @cbrt(double %x)
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call fast double @llvm.pow.f64(double %x, double 0x3fd5555555555555)
+  ret double %pow
+}
+
+define float @powf_intrinsic_third_fast(float %x) {
+; CHECK-LABEL: @powf_intrinsic_third_fast
+; CHECK-NEXT:    [[POW:%.*]] = call fast float @cbrtf(float %x)
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call fast float @llvm.pow.f32(float %x, float 0x3fd5555560000000)
+  ret float %pow
+}
+
+define double @pow_intrinsic_third_approx(double %x) {
+; CHECK-LABEL: @pow_intrinsic_third_approx
+; CHECK-NEXT:    [[POW:%.*]] = call afn double @llvm.pow.f64(double %x, double 0x3FD5555555555555)
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call afn double @llvm.pow.f64(double %x, double 0x3fd5555555555555)
+  ret double %pow
+}
+
+define float @powf_intrinsic_third_approx(float %x) {
+; CHECK-LABEL: @powf_intrinsic_third_approx(
+; CHECK-NEXT:    [[POW:%.*]] = call afn float @llvm.pow.f32(float %x, float 0x3FD5555560000000)
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call afn float @llvm.pow.f32(float %x, float 0x3fd5555560000000)
+  ret float %pow
+}
+
+define double @pow_libcall_third_fast(double %x) {
+; CHECK-LABEL: @pow_libcall_third_fast
+; CHECK-NEXT:    [[POW:%.*]] = call fast double @cbrt(double %x)
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call fast double @pow(double %x, double 0x3fd5555555555555)
+  ret double %pow
+}
+
+define float @powf_libcall_third_fast(float %x) {
+; CHECK-LABEL: @powf_libcall_third_fast
+; CHECK-NEXT:    [[POW:%.*]] = call fast float @cbrtf(float %x)
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call fast float @powf(float %x, float 0x3fd5555560000000)
+  ret float %pow
+}
+
+define double @pow_intrinsic_negthird_fast(double %x) {
+; CHECK-LABEL: @pow_intrinsic_negthird_fast
+; CHECK-NEXT:    [[CBR:%.*]] = call fast double @cbrt(double %x)
+; CHECK-NEXT:    [[POW:%.*]] = fdiv fast double 1.000000e+00, [[CBR]]
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call fast double @llvm.pow.f64(double %x, double 0xbfd5555555555555)
+  ret double %pow
+}
+
+define float @powf_intrinsic_negthird_fast(float %x) {
+; CHECK-LABEL: @powf_intrinsic_negthird_fast
+; CHECK-NEXT:    [[CBR:%.*]] = call fast float @cbrtf(float %x)
+; CHECK-NEXT:    [[POW:%.*]] = fdiv fast float 1.000000e+00, [[CBR]]
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call fast float @llvm.pow.f32(float %x, float 0xbfd5555560000000)
+  ret float %pow
+}
+
+define double @pow_intrinsic_negthird_approx(double %x) {
+; CHECK-LABEL: @pow_intrinsic_negthird_approx
+; CHECK-NEXT:    [[POW:%.*]] = call afn double @llvm.pow.f64(double %x, double 0xBFD5555555555555)
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call afn double @llvm.pow.f64(double %x, double 0xbfd5555555555555)
+  ret double %pow
+}
+
+define float @powf_intrinsic_negthird_approx(float %x) {
+; CHECK-LABEL: @powf_intrinsic_negthird_approx(
+; CHECK-NEXT:    [[POW:%.*]] = call afn float @llvm.pow.f32(float %x, float 0xBFD5555560000000)
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call afn float @llvm.pow.f32(float %x, float 0xbfd5555560000000)
+  ret float %pow
+}
+
+define double @pow_libcall_negthird_fast(double %x) {
+; CHECK-LABEL: @pow_libcall_negthird_fast
+; CHECK-NEXT:    [[CBR:%.*]] = call fast double @cbrt(double %x)
+; CHECK-NEXT:    [[POW:%.*]] = fdiv fast double 1.000000e+00, [[CBR]]
+; CHECK-NEXT:    ret double [[POW]]
+;
+  %pow = call fast double @pow(double %x, double 0xbfd5555555555555)
+  ret double %pow
+}
+
+define float @powf_libcall_negthird_fast(float %x) {
+; CHECK-LABEL: @powf_libcall_negthird_fast
+; CHECK-NEXT:    [[CBR:%.*]] = call fast float @cbrtf(float %x)
+; CHECK-NEXT:    [[POW:%.*]] = fdiv fast float 1.000000e+00, [[CBR]]
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %pow = call fast float @powf(float %x, float 0xbfd5555560000000)
+  ret float %pow
+}
+
+declare double @llvm.pow.f64(double, double) #0
+declare float @llvm.pow.f32(float, float) #0
+declare double @pow(double, double)
+declare float @powf(float, float)
+
+attributes #0 = { nounwind readnone speculatable }