Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -85,16 +85,19 @@
   HelpText<"Assume thread-local variables are defined in the executable">;
 def _SLASH_GR : CLFlag<"GR">, HelpText<"Enable emission of RTTI data">;
 def _SLASH_GR_ : CLFlag<"GR-">, HelpText<"Disable emission of RTTI data">;
+def _SLASH_GF : CLIgnoredFlag<"GF">, HelpText<"Enable string pooling (default)">;
 def _SLASH_GF_ : CLFlag<"GF-">, HelpText<"Disable string pooling">,
   Alias<fwritable_strings>;
-def _SLASH_GS : CLFlag<"GS">, HelpText<"Enable buffer security check">;
+def _SLASH_GS : CLFlag<"GS">, HelpText<"Enable buffer security check (default)">;
 def _SLASH_GS_ : CLFlag<"GS-">, HelpText<"Disable buffer security check">;
-def _SLASH_Gs : CLJoined<"Gs">, HelpText<"Set stack probe size">,
-  Alias<mstack_probe_size>;
+// FIXME: Not sure /Gs really means /Gs0 (see PR39074).
+def : CLFlag<"Gs">, HelpText<"Same as /Gs0">, Alias<mstack_probe_size>, AliasArgs<["0"]>;
+def _SLASH_Gs : CLJoined<"Gs">,
+  HelpText<"Set stack probe size (default 4096)">, Alias<mstack_probe_size>;
 def _SLASH_Gy : CLFlag<"Gy">, HelpText<"Put each function in its own section">,
   Alias<ffunction_sections>;
 def _SLASH_Gy_ : CLFlag<"Gy-">,
-  HelpText<"Don't put each function in its own section">,
+  HelpText<"Don't put each function in its own section (default)">,
   Alias<fno_function_sections>;
 def _SLASH_Gw : CLFlag<"Gw">, HelpText<"Put each data item in its own section">,
   Alias<fdata_sections>;
@@ -109,18 +112,43 @@
   Alias<I>;
 def _SLASH_J : CLFlag<"J">, HelpText<"Make char type unsigned">,
   Alias<funsigned_char>;
-def _SLASH_O0 : CLFlag<"O0">, Alias<O0>;
-// /Oy- is handled by the /O option because /Oy- only has an effect on 32-bit.
-def _SLASH_O : CLJoined<"O">, HelpText<"Optimization level">;
-def _SLASH_Od : CLFlag<"Od">, HelpText<"Disable optimization">, Alias<O0>;
-def _SLASH_Oi : CLFlag<"Oi">, HelpText<"Enable use of builtin functions">,
-  Alias<fbuiltin>;
-def _SLASH_Oi_ : CLFlag<"Oi-">, HelpText<"Disable use of builtin functions">,
-  Alias<fno_builtin>;
-def _SLASH_Os : CLFlag<"Os">, HelpText<"Optimize for size">, Alias<O>,
-  AliasArgs<["s"]>;
-def _SLASH_Ot : CLFlag<"Ot">, HelpText<"Optimize for speed">, Alias<O>,
-  AliasArgs<["2"]>;
+
+// The _SLASH_O option handles all the /O flags, but we also provide separate
+// aliased options to provide separate help messages.
+def _SLASH_O : CLJoined<"O">,
+  HelpText<"Set multiple /O flags at once; e.g. '/O2y-' is the same as '/O2 /y-'">,
+  MetaVarName<"<flags>">;
+// FIXME: Not sure why we have -O0 here; MSVC doesn't support that.
+def : CLFlag<"O0">, Alias<O0>, HelpText<"Disable optimization">;
+def : CLFlag<"O1">, Alias<_SLASH_O>, AliasArgs<["1"]>,
+  HelpText<"Optimize for size (equivalent to /Og /Os /Oy /Ob2 /Gs /GF /Gy)">;
+def : CLFlag<"O2">, Alias<_SLASH_O>, AliasArgs<["2"]>,
+  HelpText<"Optimize for speed (equivalent to /Og /Oi /Ot /Oy /Ob2 /Gs /GF /Gy)">;
+def : CLFlag<"Ob0">, Alias<_SLASH_O>, AliasArgs<["b0"]>,
+  HelpText<"Disable function inlining">;
+def : CLFlag<"Ob1">, Alias<_SLASH_O>, AliasArgs<["b1"]>,
+  HelpText<"Only inline functions which are (explicitly or implicitly) marked inline">;
+def : CLFlag<"Ob2">, Alias<_SLASH_O>, AliasArgs<["b2"]>,
+  HelpText<"Inline functions as deemed beneficial by the compiler">;
+def : CLFlag<"Od">, Alias<_SLASH_O>, AliasArgs<["d"]>,
+  HelpText<"Disable optimization">;
+def : CLFlag<"Og">, Alias<_SLASH_O>, AliasArgs<["g"]>,
+  HelpText<"No effect">;
+def : CLFlag<"Oi">, Alias<_SLASH_O>, AliasArgs<["i"]>,
+  HelpText<"Enable use of builtin functions">;
+def : CLFlag<"Oi-">, Alias<_SLASH_O>, AliasArgs<["i-"]>,
+  HelpText<"Disable use of builtin functions">;
+def : CLFlag<"Os">, Alias<_SLASH_O>, AliasArgs<["s"]>,
+  HelpText<"Optimize for size">;
+def : CLFlag<"Ot">, Alias<_SLASH_O>, AliasArgs<["t"]>,
+  HelpText<"Optimize for speed">;
+def : CLFlag<"Ox">, Alias<_SLASH_O>, AliasArgs<["x"]>,
+  HelpText<"Deprecated (equivalent to /Og /Oi /Ot /Oy /Ob2); use /O2 instead">;
+def : CLFlag<"Oy">, Alias<_SLASH_O>, AliasArgs<["y"]>,
+  HelpText<"Enable frame pointer omission (x86 only)">;
+def : CLFlag<"Oy-">, Alias<_SLASH_O>, AliasArgs<["y-"]>,
+  HelpText<"Disable frame pointer omission (x86 only, default)">;
+
 def _SLASH_QUESTION : CLFlag<"?">, Alias<help>,
   HelpText<"Display available options">;
 def _SLASH_Qvec : CLFlag<"Qvec">,
@@ -326,10 +354,8 @@
 def _SLASH_FC : CLIgnoredFlag<"FC">;
 def _SLASH_Fd : CLIgnoredJoined<"Fd">;
 def _SLASH_FS : CLIgnoredFlag<"FS">;
-def _SLASH_GF : CLIgnoredFlag<"GF">;
 def _SLASH_kernel_ : CLIgnoredFlag<"kernel-">;
 def _SLASH_nologo : CLIgnoredFlag<"nologo">;
-def _SLASH_Og : CLIgnoredFlag<"Og">;
 def _SLASH_openmp_ : CLIgnoredFlag<"openmp-">;
 def _SLASH_permissive_ : CLIgnoredFlag<"permissive-">;
 def _SLASH_RTC : CLIgnoredJoined<"RTC">;
Index: lib/Driver/ToolChains/MSVC.cpp
===================================================================
--- lib/Driver/ToolChains/MSVC.cpp
+++ lib/Driver/ToolChains/MSVC.cpp
@@ -1378,6 +1378,7 @@
       }
       break;
     case 'g':
+      A->claim();
       break;
     case 'i':
       if (I + 1 != E && OptStr[I + 1] == '-') {
Index: test/Driver/Xarch.c
===================================================================
--- test/Driver/Xarch.c
+++ test/Driver/Xarch.c
@@ -1,10 +1,10 @@
-// RUN: %clang -target i386-apple-darwin9 -m32 -Xarch_i386 -O2 %s -S -### 2>&1 | FileCheck -check-prefix=O2ONCE %s
-// O2ONCE: "-O2"
-// O2ONCE-NOT: "-O2"
+// RUN: %clang -target i386-apple-darwin9 -m32 -Xarch_i386 -O3 %s -S -### 2>&1 | FileCheck -check-prefix=O3ONCE %s
+// O3ONCE: "-O3"
+// O3ONCE-NOT: "-O3"
 
-// RUN: %clang -target i386-apple-darwin9 -m64 -Xarch_i386 -O2 %s -S -### 2>&1 | FileCheck -check-prefix=O2NONE %s
-// O2NONE-NOT: "-O2"
-// O2NONE: argument unused during compilation: '-Xarch_i386 -O2'
+// RUN: %clang -target i386-apple-darwin9 -m64 -Xarch_i386 -O3 %s -S -### 2>&1 | FileCheck -check-prefix=O3NONE %s
+// O3NONE-NOT: "-O3"
+// O3NONE: argument unused during compilation: '-Xarch_i386 -O3'
 
 // RUN: not %clang -target i386-apple-darwin9 -m32 -Xarch_i386 -o -Xarch_i386 -S %s -S -Xarch_i386 -o 2>&1 | FileCheck -check-prefix=INVALID %s
 // INVALID: error: invalid Xarch argument: '-Xarch_i386 -o'