Revert "[compiler-rt] [builtins] Remove unused/misnamed x86 chkstk functions"

This reverts parts of commit 885d7b7,
and adds verbose comments explaining all the variants of this
function, for clarity for future readers.

It turns out that those functions actually weren't misnamed or
unused after all: Apparently Clang doesn't match GCC when it comes
to what stack probe function is referenced on i386 mingw. GCC < 4.6
references a symbol named "___chkstk", with three leading underscores,
and GCC >= 4.6 references "___chkstk_ms".

Restore these functions, to allow linking object files built with
GCC with compiler-rt.

Author: mstorsjo

compiler-rt/lib/builtins/CMakeLists.txt

@@ -388,6 +388,7 @@ if (NOT MSVC)
   if (WIN32)
     set(i386_SOURCES
       ${i386_SOURCES}
+      i386/chkstk.S
       i386/chkstk2.S
     )
   endif()

compiler-rt/lib/builtins/i386/chkstk.S

@@ -0,0 +1,40 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "../assembly.h"
+
+// _chkstk routine
+// This routine is windows specific
+// http://msdn.microsoft.com/en-us/library/ms648426.aspx
+//
+// This function does not decrement %esp at the end.
+
+// GCC after 4.6 generates calls to "___chkstk_ms". For other variants of
+// this function, which do decrement %esp, see chkstk2.S.
+
+#ifdef __i386__
+
+.text
+.balign 4
+DEFINE_COMPILERRT_FUNCTION(__chkstk_ms)
+        push   %ecx
+        push   %eax
+        cmp    $0x1000,%eax
+        lea    12(%esp),%ecx
+        jb     1f
+2:
+        sub    $0x1000,%ecx
+        test   %ecx,(%ecx)
+        sub    $0x1000,%eax
+        cmp    $0x1000,%eax
+        ja     2b
+1:
+        sub    %eax,%ecx
+        test   %ecx,(%ecx)
+        pop    %eax
+        pop    %ecx
+        ret
+END_COMPILERRT_FUNCTION(__chkstk_ms)
+
+#endif // __i386__

compiler-rt/lib/builtins/i386/chkstk2.S

@@ -11,9 +11,26 @@
 // This routine is windows specific
 // http://msdn.microsoft.com/en-us/library/ms648426.aspx
 
+// Clang on i386 mingw generates calls to "_alloca" (which gets decorated to
+// "__alloca").
+//
+// GCC before 4.6 generated calls a symbol which after decoration is named
+// "___chkstk", with three leading underscores. We provide that here as well.
+//
+// MSVC produces calls to the symbol "__chkstk", with two leading underscores.
+// That one has the same signature as this one - but we don't provide that
+// symbol here. (If we'd do that, we should do it in a separate object file
+// to avoid potential symbol collisions - see
+// commit 248aeac1ad2cf4f583490dd1312a5b448d2bb8cc for details.)
+//
+// GCC after 4.6 generates calls to "___chkstk_ms", which does not decrement
+// %esp - that function is defined in chkstk.S.
+
 .text
 .balign 4
 DEFINE_COMPILERRT_FUNCTION(_alloca) // _chkstk and _alloca are the same function
+// This gets decorated into "___chkstk"; GCC < 4.6 references this symbol.
+DEFINE_COMPILERRT_FUNCTION(__chkstk)
         push   %ecx
         cmp    $0x1000,%eax
         lea    8(%esp),%ecx     // esp before calling this routine -> ecx
@@ -34,6 +51,7 @@ DEFINE_COMPILERRT_FUNCTION(_alloca) // _chkstk and _alloca are the same function
         push   (%eax)           // push return address onto the stack
         sub    %esp,%eax        // restore the original value in eax
         ret
+END_COMPILERRT_FUNCTION(__chkstk)
 END_COMPILERRT_FUNCTION(_alloca)
 
 #endif // __i386__