Index: lib/builtins/i386/chkstk.S
===================================================================
--- lib/builtins/i386/chkstk.S
+++ lib/builtins/i386/chkstk.S
@@ -3,7 +3,8 @@
 
 #include "../assembly.h"
 
-// _chkstk routine
+// _chkstk (_alloca) routine - probe stack between %esp and (%esp-%eax) in 4k increments,
+// then decrement %esp by %eax.  Preserves all registers except %esp and flags.
 // This routine is windows specific
 // http://msdn.microsoft.com/en-us/library/ms648426.aspx
 
@@ -11,7 +12,8 @@
 
 .text
 .balign 4
-DEFINE_COMPILERRT_FUNCTION(__chkstk_ms)
+DEFINE_COMPILERRT_FUNCTION(_alloca) // _chkstk and _alloca are the same function
+DEFINE_COMPILERRT_FUNCTION(_chkstk)
         push   %ecx
         push   %eax
         cmp    $0x1000,%eax
@@ -19,16 +21,18 @@
         jb     1f
 2:
         sub    $0x1000,%ecx
-        orl    $0,(%ecx)
+        test   %ecx,(%ecx)
         sub    $0x1000,%eax
         cmp    $0x1000,%eax
         ja     2b
 1:
         sub    %eax,%ecx
-        orl    $0,(%ecx)
         pop    %eax
-        pop    %ecx
+        mov    %ecx,%esp          // install the new top of stack pointer into esp
+        mov    -8(%esp,%eax),%ecx // restore ecx (esp+eax point 4 bytes above the return address)
+        push   -4(%esp,%eax)      // push return address onto the stack
         ret
-END_COMPILERRT_FUNCTION(__chkstk_ms)
+END_COMPILERRT_FUNCTION(_chkstk)
+END_COMPILERRT_FUNCTION(_alloca)
 
 #endif // __i386__
Index: lib/builtins/x86_64/chkstk.S
===================================================================
--- lib/builtins/x86_64/chkstk.S
+++ lib/builtins/x86_64/chkstk.S
@@ -24,13 +24,13 @@
         jb     1f
 2:
         sub    $0x1000,%rcx
-        orl    $0,(%rcx)
+        test   %rcx,(%rcx)
         sub    $0x1000,%rax
         cmp    $0x1000,%rax
         ja     2b
 1:
         sub    %rax,%rcx
-        orl    $0,(%rcx)
+        test   %rcx,(%rcx)
         pop    %rax
         pop    %rcx
         ret