Index: llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h
+++ llvm/trunk/include/llvm/Analysis/TargetTransformInfo.h
@@ -586,6 +586,12 @@
   /// \brief Return true if the hardware has a fast square-root instruction.
   bool haveFastSqrt(Type *Ty) const;
 
+  /// Return true if it is faster to check if a floating-point value is NaN
+  /// (or not-NaN) versus a comparison against a constant FP zero value.
+  /// Targets should override this if materializing a 0.0 for comparison is
+  /// generally as cheap as checking for ordered/unordered.
+  bool isFCmpOrdCheaperThanFCmpZero(Type *Ty) const;
+
   /// \brief Return the expected cost of supporting the floating point operation
   /// of the specified type.
   int getFPOpCost(Type *Ty) const;
@@ -1009,6 +1015,7 @@
                                               bool *Fast) = 0;
   virtual PopcntSupportKind getPopcntSupport(unsigned IntTyWidthInBit) = 0;
   virtual bool haveFastSqrt(Type *Ty) = 0;
+  virtual bool isFCmpOrdCheaperThanFCmpZero(Type *Ty) = 0;
   virtual int getFPOpCost(Type *Ty) = 0;
   virtual int getIntImmCodeSizeCost(unsigned Opc, unsigned Idx, const APInt &Imm,
                                     Type *Ty) = 0;
@@ -1273,6 +1280,10 @@
   }
   bool haveFastSqrt(Type *Ty) override { return Impl.haveFastSqrt(Ty); }
 
+  bool isFCmpOrdCheaperThanFCmpZero(Type *Ty) override {
+    return Impl.isFCmpOrdCheaperThanFCmpZero(Ty);
+  }
+
   int getFPOpCost(Type *Ty) override { return Impl.getFPOpCost(Ty); }
 
   int getIntImmCodeSizeCost(unsigned Opc, unsigned Idx, const APInt &Imm,
Index: llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h
+++ llvm/trunk/include/llvm/Analysis/TargetTransformInfoImpl.h
@@ -316,6 +316,8 @@
 
   bool haveFastSqrt(Type *Ty) { return false; }
 
+  bool isFCmpOrdCheaperThanFCmpZero(Type *Ty) { return true; }
+  
   unsigned getFPOpCost(Type *Ty) { return TargetTransformInfo::TCC_Basic; }
 
   int getIntImmCodeSizeCost(unsigned Opcode, unsigned Idx, const APInt &Imm,
Index: llvm/trunk/include/llvm/CodeGen/BasicTTIImpl.h
===================================================================
--- llvm/trunk/include/llvm/CodeGen/BasicTTIImpl.h
+++ llvm/trunk/include/llvm/CodeGen/BasicTTIImpl.h
@@ -297,6 +297,10 @@
            TLI->isOperationLegalOrCustom(ISD::FSQRT, VT);
   }
 
+  bool isFCmpOrdCheaperThanFCmpZero(Type *Ty) {
+    return true;
+  }
+
   unsigned getFPOpCost(Type *Ty) {
     // By default, FP instructions are no more expensive since they are
     // implemented in HW.  Target specific TTI can override this.
Index: llvm/trunk/lib/Analysis/TargetTransformInfo.cpp
===================================================================
--- llvm/trunk/lib/Analysis/TargetTransformInfo.cpp
+++ llvm/trunk/lib/Analysis/TargetTransformInfo.cpp
@@ -281,6 +281,10 @@
   return TTIImpl->haveFastSqrt(Ty);
 }
 
+bool TargetTransformInfo::isFCmpOrdCheaperThanFCmpZero(Type *Ty) const {
+  return TTIImpl->isFCmpOrdCheaperThanFCmpZero(Ty);
+}
+
 int TargetTransformInfo::getFPOpCost(Type *Ty) const {
   int Cost = TTIImpl->getFPOpCost(Ty);
   assert(Cost >= 0 && "TTI should not produce negative costs!");
Index: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.h
===================================================================
--- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.h
+++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.h
@@ -125,6 +125,7 @@
   bool isLegalMaskedGather(Type *DataType);
   bool isLegalMaskedScatter(Type *DataType);
   bool hasDivRemOp(Type *DataType, bool IsSigned);
+  bool isFCmpOrdCheaperThanFCmpZero(Type *Ty);
   bool areInlineCompatible(const Function *Caller,
                            const Function *Callee) const;
   const TTI::MemCmpExpansionOptions *enableMemCmpExpansion(
Index: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
+++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -2537,6 +2537,10 @@
   return TLI->isOperationLegal(IsSigned ? ISD::SDIVREM : ISD::UDIVREM, VT);
 }
 
+bool X86TTIImpl::isFCmpOrdCheaperThanFCmpZero(Type *Ty) {
+  return false;
+}
+
 bool X86TTIImpl::areInlineCompatible(const Function *Caller,
                                      const Function *Callee) const {
   const TargetMachine &TM = getTLI()->getTargetMachine();
Index: llvm/trunk/lib/Transforms/Scalar/PartiallyInlineLibCalls.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/PartiallyInlineLibCalls.cpp
+++ llvm/trunk/lib/Transforms/Scalar/PartiallyInlineLibCalls.cpp
@@ -26,7 +26,8 @@
 
 
 static bool optimizeSQRT(CallInst *Call, Function *CalledFunc,
-                         BasicBlock &CurrBB, Function::iterator &BB) {
+                         BasicBlock &CurrBB, Function::iterator &BB,
+                         const TargetTransformInfo *TTI) {
   // There is no need to change the IR, since backend will emit sqrt
   // instruction if the call has already been marked read-only.
   if (Call->onlyReadsMemory())
@@ -39,7 +40,7 @@
   //
   // (after)
   // v0 = sqrt_noreadmem(src) # native sqrt instruction.
-  // if (v0 is a NaN)
+  // [if (v0 is a NaN) || if (src < 0)]
   //   v1 = sqrt(src)         # library call.
   // dst = phi(v0, v1)
   //
@@ -48,7 +49,8 @@
   // Create phi and replace all uses.
   BasicBlock *JoinBB = llvm::SplitBlock(&CurrBB, Call->getNextNode());
   IRBuilder<> Builder(JoinBB, JoinBB->begin());
-  PHINode *Phi = Builder.CreatePHI(Call->getType(), 2);
+  Type *Ty = Call->getType();
+  PHINode *Phi = Builder.CreatePHI(Ty, 2);
   Call->replaceAllUsesWith(Phi);
 
   // Create basic block LibCallBB and insert a call to library function sqrt.
@@ -65,7 +67,10 @@
   Call->addAttribute(AttributeList::FunctionIndex, Attribute::ReadNone);
   CurrBB.getTerminator()->eraseFromParent();
   Builder.SetInsertPoint(&CurrBB);
-  Value *FCmp = Builder.CreateFCmpOEQ(Call, Call);
+  Value *FCmp = TTI->isFCmpOrdCheaperThanFCmpZero(Ty)
+                    ? Builder.CreateFCmpORD(Call, Call)
+                    : Builder.CreateFCmpOGE(Call->getOperand(0),
+                                            ConstantFP::get(Ty, 0.0));
   Builder.CreateCondBr(FCmp, JoinBB, LibCallBB);
 
   // Add phi operands.
@@ -106,7 +111,7 @@
       case LibFunc_sqrtf:
       case LibFunc_sqrt:
         if (TTI->haveFastSqrt(Call->getType()) &&
-            optimizeSQRT(Call, CalledFunc, *CurrBB, BB))
+            optimizeSQRT(Call, CalledFunc, *CurrBB, BB, TTI))
           break;
         continue;
       default:
Index: llvm/trunk/test/CodeGen/X86/sqrt-partial.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/sqrt-partial.ll
+++ llvm/trunk/test/CodeGen/X86/sqrt-partial.ll
@@ -3,7 +3,7 @@
 
 ; PR31455 - https://bugs.llvm.org/show_bug.cgi?id=31455
 ; We have to assume that errno can be set, so we have to make a libcall in that case.
-; But it's better for perf to check that the argument is valid rather than the result of 
+; But it's better for perf to check that the argument is valid rather than the result of
 ; sqrtss/sqrtsd.
 ; Note: This is really a test of the -partially-inline-libcalls IR pass (and we have an IR test
 ; for that), but we're checking the final asm to make sure that comes out as expected too.
@@ -11,11 +11,11 @@
 define float @f(float %val) nounwind {
 ; CHECK-LABEL: f:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    sqrtss %xmm0, %xmm1
-; CHECK-NEXT:    ucomiss %xmm1, %xmm1
-; CHECK-NEXT:    jp .LBB0_2
+; CHECK-NEXT:    xorps %xmm1, %xmm1
+; CHECK-NEXT:    ucomiss %xmm1, %xmm0
+; CHECK-NEXT:    jb .LBB0_2
 ; CHECK-NEXT:  # BB#1: # %.split
-; CHECK-NEXT:    movaps %xmm1, %xmm0
+; CHECK-NEXT:    sqrtss %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ; CHECK-NEXT:  .LBB0_2: # %call.sqrt
 ; CHECK-NEXT:    jmp sqrtf # TAILCALL
@@ -26,11 +26,11 @@
 define double @d(double %val) nounwind {
 ; CHECK-LABEL: d:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    sqrtsd %xmm0, %xmm1
-; CHECK-NEXT:    ucomisd %xmm1, %xmm1
-; CHECK-NEXT:    jp .LBB1_2
+; CHECK-NEXT:    xorps %xmm1, %xmm1
+; CHECK-NEXT:    ucomisd %xmm1, %xmm0
+; CHECK-NEXT:    jb .LBB1_2
 ; CHECK-NEXT:  # BB#1: # %.split
-; CHECK-NEXT:    movapd %xmm1, %xmm0
+; CHECK-NEXT:    sqrtsd %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ; CHECK-NEXT:  .LBB1_2: # %call.sqrt
 ; CHECK-NEXT:    jmp sqrt # TAILCALL
Index: llvm/trunk/test/Transforms/PartiallyInlineLibCalls/X86/good-prototype.ll
===================================================================
--- llvm/trunk/test/Transforms/PartiallyInlineLibCalls/X86/good-prototype.ll
+++ llvm/trunk/test/Transforms/PartiallyInlineLibCalls/X86/good-prototype.ll
@@ -1,18 +1,20 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -partially-inline-libcalls -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 ; RUN: opt -S -passes=partially-inline-libcalls -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 
 define float @f(float %val) {
-; CHECK: @f
-; CHECK: entry:
-; CHECK-NEXT: %[[RES:.+]] = tail call float @sqrtf(float %val) #0
-; CHECK-NEXT: %[[CMP:.+]] = fcmp oeq float %[[RES]], %[[RES]]
-; CHECK-NEXT: br i1 %[[CMP]], label %[[EXIT:.+]], label %[[CALL:.+]]
-; CHECK: [[CALL]]:
-; CHECK-NEXT: %[[RES2:.+]] = tail call float @sqrtf(float %val){{$}}
-; CHECK-NEXT: br label %[[EXIT]]
-; CHECK: [[EXIT]]:
-; CHECK-NEXT: %[[RET:.+]] = phi float [ %[[RES]], %entry ], [ %[[RES2]], %[[CALL]] ]
-; CHECK-NEXT: ret float %[[RET]]
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RES:%.*]] = tail call float @sqrtf(float [[VAL:%.*]]) #0
+; CHECK-NEXT:    [[TMP0:%.*]] = fcmp oge float [[VAL]], 0.000000e+00
+; CHECK-NEXT:    br i1 [[TMP0]], label [[ENTRY_SPLIT:%.*]], label [[CALL_SQRT:%.*]]
+; CHECK:       call.sqrt:
+; CHECK-NEXT:    [[TMP1:%.*]] = tail call float @sqrtf(float [[VAL]])
+; CHECK-NEXT:    br label [[ENTRY_SPLIT]]
+; CHECK:       entry.split:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi float [ [[RES]], [[ENTRY:%.*]] ], [ [[TMP1]], [[CALL_SQRT]] ]
+; CHECK-NEXT:    ret float [[TMP2]]
+;
 entry:
   %res = tail call float @sqrtf(float %val)
   ret float %res