[llvm-dev] (Question regarding the) incomplete "builtins library" of "Compiler-RT"

[Craig Topper via llvm-dev]

__chkstk_ms is used in x86_64-mingw https://godbolt.org/z/dajIju


~Craig


On Fri, Nov 30, 2018 at 1:17 PM Stefan Kanthak via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> "Friedman, Eli" <efriedma at codeaurora.org> wrote:
>
> > On 11/30/2018 8:31 AM, Stefan Kanthak via llvm-dev wrote:
> >> Hi @ll,
> >>
> >> compiler-rt implements (for example) the MSVC (really Windows)
> >> specific routines compiler-rt/lib/builtins/i386/chkstk.S and
> >> compiler-rt/lib/builtins/x86_64/chkstk.S as __chkstk_ms()
> >> See <http://msdn.microsoft.com/en-us/library/ms648426.aspx>
> >>
> >> Is there any special reason why compiler-rt doesn't implement
> >> other MSVC specific functions (alias builtins or "compiler
> >> intrinsics") for which clang/LLVM but generates calls?
> >
> > There are two "Windows" targets supported by LLVM: one using the MinGW
> > runtime, and the other using the MSVC runtime. If you're targeting the
> > MSVC runtime, it provides _aulldiv and friends; if you're targeting
> > MinGW, LLVM won't refer to those routines. So I can't see a scenario
> > where we would need those routines in compiler-rt.
>
> What's the purpose of __chkstk_ms() then, implemented in compiler-rt?
>
> 1. LLVM does NOT create calls to __chkstk_ms();
>
> 2. both the MSVC and the MinGW runtime provide their own __chkstk()
>    as well as their own __alloca() routines.
>
> --- llvm.c ---
> int main(int argc) {
>     int stack[1111];
>     return _alloca(argc);
> }
> --- EOF ---
>
> Compiled with "-target i386-mingw" this generates the following code
> (see <https://godbolt.org/z/yPk4Jo>):
>
> _main: # @main
>     push  ebp
>     mov   ebp, esp
>     mov   eax, 4456
>     call  __alloca
>     mov   eax, dword ptr [ebp + 8]
>     mov   dword ptr [ebp - 4452], eax # 4-byte Spill
>     call  ___main
>     mov   dword ptr [ebp - 4], 0
>     mov   eax, dword ptr [ebp + 8]
>     mov   dword ptr [esp], eax
>     call  __alloca
>     add   esp, 4456
>     pop   ebp
>     ret
>
>
> Compiled with "-target i386-win32" this generates the following code
> (see <https://godbolt.org/z/Hc8jaL>):
>
> _main: # @main
>     push  ebp
>     mov   ebp, esp
>     push  esi
>     and   esp, -16
>     mov   eax, 4480
>     call  __chkstk
>     mov   esi, esp
>     mov   eax, dword ptr [ebp + 8]
>     mov   dword ptr [esi + 4464], 0
>     mov   ecx, dword ptr [ebp + 8]
>     add   ecx, 3
>     and   ecx, -4
>     mov   dword ptr [esi + 12], eax # 4-byte Spill
>     mov   eax, ecx
>     call  __chkstk
>     mov   ecx, esp
>     and   ecx, -16
>     mov   esp, ecx
>     mov   eax, ecx
>     lea   esp, [ebp - 4]
>     pop   esi
>     pop   ebp
>     ret
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