Index: include/llvm/IR/Module.h
===================================================================
--- include/llvm/IR/Module.h
+++ include/llvm/IR/Module.h
@@ -186,6 +186,7 @@
                                   ///< Format: (arch)(sub)-(vendor)-(sys0-(abi)
   void *NamedMDSymTab;            ///< NamedMDNode names.
   DataLayout DL;                  ///< DataLayout associated with the module
+  unsigned VersionNum;            ///< Module version number determined from parsing
 
   friend class Constant;
 
@@ -240,6 +241,9 @@
   /// @returns a string containing the module-scope inline assembly blocks.
   const std::string &getModuleInlineAsm() const { return GlobalScopeAsm; }
 
+  /// Get module version number.
+  const unsigned &getModuleVersionNum() const { return  VersionNum; }
+
   /// Get a RandomNumberGenerator salted for use with this module. The
   /// RNG can be seeded via -rng-seed=<uint64> and is salted with the
   /// ModuleID and the provided pass salt. The returned RNG should not
@@ -276,6 +280,9 @@
       GlobalScopeAsm += '\n';
   }
 
+  /// Set the module version number.
+  void setModuleVersionNum(unsigned V) { VersionNum = V; }
+
   /// Append to the module-scope inline assembly blocks.
   /// A trailing newline is added if the input doesn't have one.
   void appendModuleInlineAsm(StringRef Asm) {
Index: lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- lib/Bitcode/Reader/BitcodeReader.cpp
+++ lib/Bitcode/Reader/BitcodeReader.cpp
@@ -419,7 +419,7 @@
   if (Record.empty())
     return error("Invalid record");
   unsigned ModuleVersion = Record[0];
-  if (ModuleVersion > 2)
+  if (ModuleVersion > 3)
     return error("Invalid value");
   UseStrtab = ModuleVersion >= 2;
   return ModuleVersion;
@@ -1045,7 +1045,7 @@
   }
 }
 
-static FastMathFlags getDecodedFastMathFlags(unsigned Val) {
+static FastMathFlags getDecodedFastMathFlags(unsigned Val, bool UseOldFast) {
   FastMathFlags FMF;
   if (0 != (Val & FastMathFlags::AllowReassoc))
     FMF.setAllowReassoc();
@@ -1061,6 +1061,10 @@
     FMF.setAllowContract(true);
   if (0 != (Val & FastMathFlags::ApproxFunc))
     FMF.setApproxFunc();
+  // Support the old fast math, by bitcode version, as a mapping to reassoc 
+  // which equates to UnsafeAlgebra, use this to upgrade to current fast.
+  if (FMF.allowReassoc() && UseOldFast)
+    FMF.setFast();
   return FMF;
 }
 
@@ -3235,6 +3239,7 @@
       if (!VersionOrErr)
         return VersionOrErr.takeError();
       UseRelativeIDs = *VersionOrErr >= 1;
+      TheModule->setModuleVersionNum(*VersionOrErr);
       break;
     }
     case bitc::MODULE_CODE_TRIPLE: {  // TRIPLE: [strchr x N]
@@ -3352,6 +3357,7 @@
   InstructionList.clear();
   unsigned ModuleValueListSize = ValueList.size();
   unsigned ModuleMDLoaderSize = MDLoader->size();
+  bool UseOldFast = (TheModule->getModuleVersionNum() < 3);
 
   // Add all the function arguments to the value table.
   for (Argument &I : F->args())
@@ -3529,7 +3535,7 @@
           if (Record[OpNum] & (1 << bitc::PEO_EXACT))
             cast<BinaryOperator>(I)->setIsExact(true);
         } else if (isa<FPMathOperator>(I)) {
-          FastMathFlags FMF = getDecodedFastMathFlags(Record[OpNum]);
+          FastMathFlags FMF = getDecodedFastMathFlags(Record[OpNum], UseOldFast);
           if (FMF.any())
             I->setFastMathFlags(FMF);
         }
@@ -3796,7 +3802,7 @@
       bool IsFP = LHS->getType()->isFPOrFPVectorTy();
       FastMathFlags FMF;
       if (IsFP && Record.size() > OpNum+1)
-        FMF = getDecodedFastMathFlags(Record[++OpNum]);
+        FMF = getDecodedFastMathFlags(Record[++OpNum], UseOldFast);
 
       if (OpNum+1 != Record.size())
         return error("Invalid record");
@@ -4467,7 +4473,7 @@
 
       FastMathFlags FMF;
       if ((CCInfo >> bitc::CALL_FMF) & 1) {
-        FMF = getDecodedFastMathFlags(Record[OpNum++]);
+        FMF = getDecodedFastMathFlags(Record[OpNum++], UseOldFast);
         if (!FMF.any())
           return error("Fast math flags indicator set for call with no FMF");
       }
Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -139,7 +139,7 @@
 
 void BitcodeWriterBase::writeModuleVersion() {
   // VERSION: [version#]
-  Stream.EmitRecord(bitc::MODULE_CODE_VERSION, ArrayRef<uint64_t>{2});
+  Stream.EmitRecord(bitc::MODULE_CODE_VERSION, ArrayRef<uint64_t>{3});
 }
 
 /// Base class to manage the module bitcode writing, currently subclassed for