Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -90,8 +90,8 @@
   Alias<fwritable_strings>;
 def _SLASH_GS : CLFlag<"GS">, HelpText<"Enable buffer security check (default)">;
 def _SLASH_GS_ : CLFlag<"GS-">, HelpText<"Disable buffer security check">;
-// FIXME: Not sure /Gs really means /Gs0 (see PR39074).
-def : CLFlag<"Gs">, HelpText<"Same as /Gs0">, Alias<mstack_probe_size>, AliasArgs<["0"]>;
+def : CLFlag<"Gs">, HelpText<"Use stack probes (default)">,
+  Alias<mstack_probe_size>, AliasArgs<["4096"]>;
 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">,
@@ -121,9 +121,9 @@
 // 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)">;
+  HelpText<"Optimize for size (equivalent to /Og /Os /Oy /Ob2 /GF /Gy)">;
 def : CLFlag<"O2">, Alias<_SLASH_O>, AliasArgs<["2"]>,
-  HelpText<"Optimize for speed (equivalent to /Og /Oi /Ot /Oy /Ob2 /Gs /GF /Gy)">;
+  HelpText<"Optimize for speed (equivalent to /Og /Oi /Ot /Oy /Ob2 /GF /Gy)">;
 def : CLFlag<"Ob0">, Alias<_SLASH_O>, AliasArgs<["b0"]>,
   HelpText<"Disable function inlining">;
 def : CLFlag<"Ob1">, Alias<_SLASH_O>, AliasArgs<["b1"]>,
Index: test/Driver/cl-options.c
===================================================================
--- test/Driver/cl-options.c
+++ test/Driver/cl-options.c
@@ -101,7 +101,7 @@
 // Gy_-NOT: -ffunction-sections
 
 // RUN: %clang_cl /Gs -### -- %s 2>&1 | FileCheck -check-prefix=Gs %s
-// Gs: "-mstack-probe-size=0"
+// Gs: "-mstack-probe-size=4096"
 // RUN: %clang_cl /Gs0 -### -- %s 2>&1 | FileCheck -check-prefix=Gs0 %s
 // Gs0: "-mstack-probe-size=0"
 // RUN: %clang_cl /Gs4096 -### -- %s 2>&1 | FileCheck -check-prefix=Gs4096 %s