Index: include/llvm/IR/Constants.h
===================================================================
--- include/llvm/IR/Constants.h
+++ include/llvm/IR/Constants.h
@@ -1076,7 +1076,8 @@
   /// \param OnlyIfReducedTy see \a getWithOperands() docs.
   static Constant *get(unsigned Opcode, Constant *C1, Constant *C2,
                        unsigned Flags = 0, Type *OnlyIfReducedTy = nullptr,
-                       FastMathFlags FMF = FastMathFlags());
+                       FastMathFlags FMF = FastMathFlags(),
+                       bool Strict = false);
 
   /// \brief Return an ICmp or FCmp comparison operator constant expression.
   ///
Index: lib/Analysis/ConstantFolding.cpp
===================================================================
--- lib/Analysis/ConstantFolding.cpp
+++ lib/Analysis/ConstantFolding.cpp
@@ -1028,7 +1028,8 @@
         return C;
     }
 
-    return ConstantExpr::get(Opcode, Ops[0], Ops[1], 0, nullptr, FMF);
+    return ConstantExpr::get(Opcode, Ops[0], Ops[1], 0, nullptr,
+                             FMF, /* Strict = */ true);
   }
 
   switch (Opcode) {
Index: lib/IR/Constants.cpp
===================================================================
--- lib/IR/Constants.cpp
+++ lib/IR/Constants.cpp
@@ -1871,7 +1871,8 @@
 
 Constant *ConstantExpr::get(unsigned Opcode, Constant *C1, Constant *C2,
                             unsigned Flags, Type *OnlyIfReducedTy,
-                            FastMathFlags FMF) {
+                            FastMathFlags FMF,
+                            bool Strict) {
   // Check the operands for consistency first.
   assert(Opcode >= Instruction::BinaryOpsBegin &&
          Opcode <  Instruction::BinaryOpsEnd   &&
@@ -1944,6 +1945,11 @@
   if (OnlyIfReducedTy == C1->getType())
     return nullptr;
 
+  // ConstantExpression can't store these flags, which can lead to going against
+  // them.
+  if (Strict && (FMF.keepExceptions() || FMF.keepRounding()))
+    return nullptr;
+
   Constant *ArgVec[] = { C1, C2 };
   ConstantExprKeyType Key(Opcode, ArgVec, 0, Flags);
 
Index: test/Transforms/EarlyCSE/fpenv.ll
===================================================================
--- /dev/null
+++ test/Transforms/EarlyCSE/fpenv.ll
@@ -0,0 +1,24 @@
+; %val.true and %val.false should not be moved inside phi-node as constant
+; expressions as such transformation looses fast-math flags.
+; RUN: opt < %s -S -early-cse | FileCheck %s
+
+define double @do-not-turn-into-constexpr(double %x) {
+; CHECK-LABEL: @do-not-turn-into-constexpr(
+; CHECK: %val.true = fmul
+; CHECK: %val.false = fmul
+entry:
+  %cmp = fcmp oeq double %x, 0.000000e+00
+  br i1 %cmp, label %cond.true, label %cond.false
+
+cond.true:                                        ; preds = %entry
+  %val.true = fmul kexc double 1.000000e+300, 1.000000e+300
+  br label %cond.end
+
+cond.false:                                       ; preds = %entry
+  %val.false = fmul kexc double 1.000000e-300, 1.000000e-300
+  br label %cond.end
+
+cond.end:                                         ; preds = %cond.false, %cond.true
+  %cond = phi double [ %val.true, %cond.true ], [ %val.false, %cond.false ]
+  ret double %cond
+}