diff --git a/include/clang/Driver/CC1Options.td b/include/clang/Driver/CC1Options.td
index 6a0d816..f3c6d11 100644
--- a/include/clang/Driver/CC1Options.td
+++ b/include/clang/Driver/CC1Options.td
@@ -323,6 +323,8 @@
   HelpText<"Generate output compatible with the standard GNU Objective-C runtime">;
 def std_EQ : Joined<"-std=">,
   HelpText<"Language standard to compile for">;
+def fmath_errno : Flag<"-fmath-errno">,
+  HelpText<"Require math functions to indicate errors by setting errno">;
 def fms_extensions : Flag<"-fms-extensions">,
   HelpText<"Accept some non-standard constructs used in Microsoft header files ">;
 def main_file_name : Separate<"-main-file-name">,
@@ -331,8 +333,6 @@
   HelpText<"Disable C++ copy constructor elision">;
 def fno_lax_vector_conversions : Flag<"-fno-lax-vector-conversions">,
   HelpText<"Disallow implicit conversions between vectors with a different number of elements or different element types">;
-def fno_math_errno : Flag<"-fno-math-errno">,
-  HelpText<"Don't require math functions to respect errno">;
 def fno_signed_char : Flag<"-fno-signed-char">,
   HelpText<"Char is unsigned">;
 def fno_operator_names : Flag<"-fno-operator-names">,
diff --git a/include/clang/Driver/ToolChain.h b/include/clang/Driver/ToolChain.h
index b3c2d05..cc8d438 100644
--- a/include/clang/Driver/ToolChain.h
+++ b/include/clang/Driver/ToolChain.h
@@ -87,10 +87,6 @@
 
   // Platform defaults information
 
-  /// IsMathErrnoDefault - Does this tool chain set -fmath-errno by
-  /// default.
-  virtual bool IsMathErrnoDefault() const = 0;
-
   /// IsBlocksDefault - Does this tool chain enable -fblocks by default.
   virtual bool IsBlocksDefault() const { return false; }
 
diff --git a/lib/Driver/ToolChains.cpp b/lib/Driver/ToolChains.cpp
index cc3febf..419787d 100644
--- a/lib/Driver/ToolChains.cpp
+++ b/lib/Driver/ToolChains.cpp
@@ -516,10 +516,6 @@
   return DAL;
 }
 
-bool Darwin::IsMathErrnoDefault() const {
-  return false;
-}
-
 bool Darwin::IsUnwindTablesDefault() const {
   // FIXME: Gross; we should probably have some separate target
   // definition, possibly even reusing the one in clang.
@@ -599,10 +595,6 @@
   return *T;
 }
 
-bool Generic_GCC::IsMathErrnoDefault() const {
-  return true;
-}
-
 bool Generic_GCC::IsUnwindTablesDefault() const {
   // FIXME: Gross; we should probably have some separate target
   // definition, possibly even reusing the one in clang.
diff --git a/lib/Driver/ToolChains.h b/lib/Driver/ToolChains.h
index be36344..374ad8c 100644
--- a/lib/Driver/ToolChains.h
+++ b/lib/Driver/ToolChains.h
@@ -38,7 +38,6 @@
 
   virtual Tool &SelectTool(const Compilation &C, const JobAction &JA) const;
 
-  virtual bool IsMathErrnoDefault() const;
   virtual bool IsUnwindTablesDefault() const;
   virtual const char *GetDefaultRelocationModel() const;
   virtual const char *GetForcedPicModel() const;
@@ -136,7 +135,6 @@
 
   virtual Tool &SelectTool(const Compilation &C, const JobAction &JA) const;
 
-  virtual bool IsMathErrnoDefault() const;
   virtual bool IsBlocksDefault() const {
     // Blocks default to on for 10.6 (darwin10) and beyond.
     return (DarwinVersion[0] > 9);
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 8f0af21..bf59abc 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -837,11 +837,11 @@
     break;
   }
 
-  // -fmath-errno is default.
-  if (!Args.hasFlag(options::OPT_fmath_errno,
+  // -fno-math-errno is default.
+  if (Args.hasFlag(options::OPT_fmath_errno,
                    options::OPT_fno_math_errno,
-                   getToolChain().IsMathErrnoDefault()))
-    CmdArgs.push_back("-fno-math-errno");
+                   false))
+    CmdArgs.push_back("-fmath-errno");
 
   Arg *Unsupported;
   if ((Unsupported = Args.getLastArg(options::OPT_MG)) ||
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 63f66fa..8817469 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -479,8 +479,8 @@
     Res.push_back("-fblocks");
   if (Opts.EmitAllDecls)
     Res.push_back("-femit-all-decls");
-  if (!Opts.MathErrno)
-    Res.push_back("-fno-math-errno");
+  if (Opts.MathErrno)
+    Res.push_back("-fmath-errno");
   if (Opts.OverflowChecking)
     Res.push_back("-ftrapv");
   if (Opts.HeinousExtensions)
@@ -1147,7 +1147,7 @@
   Opts.HeinousExtensions = Args.hasArg(OPT_fheinous_gnu_extensions);
   Opts.AccessControl = Args.hasArg(OPT_faccess_control);
   Opts.ElideConstructors = !Args.hasArg(OPT_fno_elide_constructors);
-  Opts.MathErrno = !Args.hasArg(OPT_fno_math_errno);
+  Opts.MathErrno = Args.hasArg(OPT_fmath_errno);
   Opts.InstantiationDepth = getLastArgIntValue(Args, OPT_ftemplate_depth, 99,
                                                Diags);
   Opts.NeXTRuntime = !Args.hasArg(OPT_fgnu_runtime);
diff --git a/test/CodeGen/libcalls.c b/test/CodeGen/libcalls.c
index fe12f4a..a96176a 100644
--- a/test/CodeGen/libcalls.c
+++ b/test/CodeGen/libcalls.c
@@ -1,7 +1,7 @@
-// RUN: %clang_cc1 -emit-llvm -o %t %s -triple i386-unknown-unknown
+// RUN: %clang_cc1 -fmath-errno -emit-llvm -o %t %s -triple i386-unknown-unknown
 // RUN: grep "declare " %t | count 6
 // RUN: grep "declare " %t | grep "@llvm." | count 1
-// RUN: %clang_cc1 -fno-math-errno -emit-llvm -o %t %s -triple i386-unknown-unknown
+// RUN: %clang_cc1 -emit-llvm -o %t %s -triple i386-unknown-unknown
 // RUN: grep "declare " %t | count 6
 // RUN: grep "declare " %t | grep -v "@llvm." | count 0
 
diff --git a/test/Driver/analyze.c b/test/Driver/analyze.c
index 2f850bc..359b0e0 100644
--- a/test/Driver/analyze.c
+++ b/test/Driver/analyze.c
@@ -6,4 +6,3 @@
 
 // CHECK: "-analyze"
 // CHECK: "-target-feature" "+sse"
-// CHECK: "-fno-math-errno"
diff --git a/test/Driver/clang_f_opts.c b/test/Driver/clang_f_opts.c
index c4ea430..f1d6759 100644
--- a/test/Driver/clang_f_opts.c
+++ b/test/Driver/clang_f_opts.c
@@ -1,11 +1,11 @@
-// RUN: %clang -### -S -fblocks -fbuiltin -fmath-errno -fcommon -fpascal-strings -fno-blocks -fno-builtin -fno-math-errno -fno-common -fno-pascal-strings -fblocks -fbuiltin -fmath-errno -fcommon -fpascal-strings %s 2>&1 | FileCheck -check-prefix=CHECK-OPTIONS1 %s
-// RUN: %clang -### -S -fblocks -fbuiltin -fmath-errno -fcommon -fpascal-strings -fno-blocks -fno-builtin -fno-math-errno -fno-common -fno-pascal-strings -fno-show-source-location -fshort-wchar %s 2>&1 | FileCheck -check-prefix=CHECK-OPTIONS2 %s
+// RUN: %clang -### -S -fblocks -fbuiltin -fno-math-errno -fcommon -fpascal-strings -fno-blocks -fno-builtin -fmath-errno -fno-common -fno-pascal-strings -fblocks -fbuiltin -fmath-errno -fcommon -fpascal-strings %s 2>&1 | FileCheck -check-prefix=CHECK-OPTIONS1 %s
+// RUN: %clang -### -S -fblocks -fbuiltin -fno-math-errno -fcommon -fpascal-strings -fno-blocks -fno-builtin -fmath-errno -fno-common -fno-pascal-strings -fno-show-source-location -fshort-wchar %s 2>&1 | FileCheck -check-prefix=CHECK-OPTIONS2 %s
 // RUN: %clang -### -fshort-enums %s 2>&1 | FileCheck -check-prefix=CHECK-SHORT-ENUMS %s
 
 // CHECK-OPTIONS1: -fblocks
 // CHECK-OPTIONS1: -fpascal-strings
 
-// CHECK-OPTIONS2: -fno-math-errno
+// CHECK-OPTIONS2: -fmath-errno
 // CHECK-OPTIONS2: -fno-builtin
 // CHECK-OPTIONS2: -fshort-wchar
 // CHECK-OPTIONS2: -fno-common
diff --git a/test/Sema/unused-expr.c b/test/Sema/unused-expr.c
index f5c64e6..68503bd 100644
--- a/test/Sema/unused-expr.c
+++ b/test/Sema/unused-expr.c
@@ -1,8 +1,8 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -fno-math-errno %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s
 
 int foo(int X, int Y);
 
-double sqrt(double X);  // implicitly const because of -fno-math-errno!
+double sqrt(double X);  // implicitly const because of no -fmath-errno!
 
 void bar(volatile int *VP, int *P, int A,
          _Complex double C, volatile _Complex double VC) {
@@ -24,7 +24,7 @@
   __real__ C;          // expected-warning {{expression result unused}}
   __real__ VC;
   
-  // We know this can't change errno because of -fno-math-errno.
+  // We know this can't change errno because of no -fmath-errno.
   sqrt(A);  // expected-warning {{ignoring return value of function declared with const attribute}}
 }