diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index ed207e6..3281e36 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -68,6 +68,9 @@
     if (A->getOption().matches(options::OPT_O0))
       return 0;
 
+    if (A->getOption().matches(options::OPT_Ofast))
+      return 3;
+
     assert (A->getOption().matches(options::OPT_O));
 
     StringRef S(A->getValue());
@@ -97,6 +100,13 @@
   return 0;
 }
 
+static bool isOptimizationLevelFast(ArgList &Args) {
+  if (Arg *A = Args.getLastArg(options::OPT_O_Group))
+    if (A->getOption().matches(options::OPT_Ofast))
+      return true;
+  return false;
+}
+
 static void addWarningArgs(ArgList &Args, std::vector<std::string> &Warnings) {
   for (arg_iterator I = Args.filtered_begin(OPT_W_Group),
          E = Args.filtered_end(); I != E; ++I) {
@@ -324,7 +334,8 @@
   Opts.ForbidGuardVariables = Args.hasArg(OPT_fforbid_guard_variables);
   Opts.UseRegisterSizedBitfieldAccess = Args.hasArg(
     OPT_fuse_register_sized_bitfield_access);
-  Opts.RelaxedAliasing = Args.hasArg(OPT_relaxed_aliasing);
+  Opts.RelaxedAliasing = Args.hasArg(OPT_relaxed_aliasing) ||
+    isOptimizationLevelFast(Args);
   Opts.StructPathTBAA = Args.hasArg(OPT_struct_path_tbaa);
   Opts.DwarfDebugFlags = Args.getLastArgValue(OPT_dwarf_debug_flags);
   Opts.MergeAllConstants = !Args.hasArg(OPT_fno_merge_all_constants);
@@ -1302,7 +1313,8 @@
   // inlining enabled.
   Opts.NoInlineDefine = !Opt || Args.hasArg(OPT_fno_inline);
 
-  Opts.FastMath = Args.hasArg(OPT_ffast_math);
+  Opts.FastMath = Args.hasArg(OPT_ffast_math) ||
+    isOptimizationLevelFast(Args);
   Opts.FiniteMathOnly = Args.hasArg(OPT_ffinite_math_only);
 
   Opts.RetainCommentsFromSystemHeaders =