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

For both MSVC and MinGW targets, the compiler generates calls to
functions for probing the stack, in functions that allocate a larger
amount of stack space.

The exact behaviour of these functions differ per architecture (some
decrement the stack, some actually decrement the stack pointer,
some only probe the stack). In MSVC mode, the compiler always
generates calls to a symbol named "__chkstk". In MinGW mode, the
symbol is named "__alloca" on i386 and "___chkstk_ms" on x86_64,
but the functions behave exactly the same as their MSVC counterparts
despite the differing names.

(On i386, these names are the raw symbol names - if considering
a C level function name with the extra implicit leading underscore,
they would be called "_chkstk" and "_alloca".)

Remove the misleading duplicate and unused functions. These were
added in fbfed86 /
c27de5b (adding "___chkstk_ms"
for both architectures, even if that symbol name only was used
on x86_64) and 40eb83b
(adding "__alloca" and "___chkstk", even if the former only was
used on i386, and the latter seeming like a misspelled form of
the MSVC function, with three underscores instead of two).

The x86_64 "___chkstk" was doubly surprising as that function had
the same behaviour as the function used on i386, while the
"__chkstk" that MSVC emitted calls to should behave exactly like
the preexisting "___chkstk_ms".

Remove the unused functions, and rename the misspelled MSVC-like
symbols to the correct name that MSVC mode actually uses.

Note that these files aren't assembled at all when building
compiler-rt builtins in MSVC mode, as they are expected to be
provided by MSVC libraries when building code in MSVC mode.

Differential Revision: https://reviews.llvm.org/D159139

compiler-rt/lib/builtins/CMakeLists.txt

@@ -330,7 +330,6 @@ if (NOT MSVC)
     set(x86_64_SOURCES
       ${x86_64_SOURCES}
       x86_64/chkstk.S
-      x86_64/chkstk2.S
     )
   endif()
 
@@ -363,7 +362,6 @@ if (NOT MSVC)
   if (WIN32)
     set(i386_SOURCES
       ${i386_SOURCES}
-      i386/chkstk.S
       i386/chkstk2.S
     )
   endif()

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

@@ -14,7 +14,7 @@
 .text
 .balign 4
 DEFINE_COMPILERRT_FUNCTION(_alloca) // _chkstk and _alloca are the same function
-DEFINE_COMPILERRT_FUNCTION(__chkstk)
+DEFINE_COMPILERRT_FUNCTION(_chkstk)
         push   %ecx
         cmp    $0x1000,%eax
         lea    8(%esp),%ecx     // esp before calling this routine -> ecx
@@ -35,7 +35,7 @@ DEFINE_COMPILERRT_FUNCTION(__chkstk)
         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(_chkstk)
 END_COMPILERRT_FUNCTION(_alloca)
 
 #endif // __i386__

compiler-rt/lib/builtins/x86_64/chkstk.S

@@ -18,6 +18,7 @@
 .text
 .balign 4
 DEFINE_COMPILERRT_FUNCTION(___chkstk_ms)
+DEFINE_COMPILERRT_FUNCTION(__chkstk)
         push   %rcx
         push   %rax
         cmp    $0x1000,%rax
@@ -35,6 +36,7 @@ DEFINE_COMPILERRT_FUNCTION(___chkstk_ms)
         pop    %rax
         pop    %rcx
         ret
+END_COMPILERRT_FUNCTION(__chkstk)
 END_COMPILERRT_FUNCTION(___chkstk_ms)
 
 #endif // __x86_64__