Hi Michael,

I'm on gcc 13.1 compiling for Windows x64, with the MinGW UCRT64 runtime
library

best regards,
Julian

On Thu, Jun 29, 2023 at 9:27 PM Michael Matz <matz@suse.de> wrote:

> Hello,
>
> On Thu, 29 Jun 2023, Julian Waters via Gcc wrote:
>
> > int main() {
> >     asm ("nop" "\n"
> >          "\t" "nop" "\n"
> >          "\t" "nop");
> >
> >     asm volatile ("nop" "\n"
> >                   "\t" "nop" "\n"
> >                   "\t" "nop");
> > }
> >
> > objdump --disassemble-all -M intel -M intel-mnemonic a.exe >
> disassembly.txt
> >
> > 00000001400028c0 <main>:
> >    1400028c0: 48 83 ec 28             sub    rsp,0x28
> >    1400028c4: e8 37 ec ff ff            call   140001500 <__main>
> >    1400028c9: 90                            nop
> >    1400028ca: 90                            nop
> >    1400028cb: 90                            nop
> >    1400028cc: 31 c0                       xor    eax,eax
> >    1400028cd: 48 83 c4 28             add    rsp,0x28
> >    1400028ce: c3                            ret
> >
> > Note how there are only 3 nops above when there should be 6, as the
> first 3
> > have been deleted by the compiler. With the patch, the correct 6 nops
> > instead of 3 are compiled into the final code.
> >
> > Of course, the above was compiled with the most extreme optimizations
> > available to stress test the possible bug, -O3, -ffunction-sections
> > -fdata-sections -Wl,--gc-sections -flto=auto. Compiled as C++17 and intel
> > assembly syntax
>
> Works just fine here:
>
> % cat xx.c
> int main() {
>     asm ("nop" "\n"
>          "\t" "nop" "\n"
>          "\t" "nop");
>
>     asm volatile ("nop" "\n"
>                   "\t" "nop" "\n"
>                   "\t" "nop");
> }
>
> % g++ -v
> ...
> gcc version 12.2.1 20230124 [revision
> 193f7e62815b4089dfaed4c2bd34fd4f10209e27] (SUSE Linux)
>
> % g++ -std=c++17 -flto=auto -O3 -ffunction-sections -fdata-sections xx.c
> % objdump --disassemble-all -M intel -M intel-mnemonic a.out
> ...
> 0000000000401020 <main>:
>   401020:       90                      nop
>   401021:       90                      nop
>   401022:       90                      nop
>   401023:       90                      nop
>   401024:       90                      nop
>   401025:       90                      nop
>   401026:       31 c0                   xor    eax,eax
>   401028:       c3                      ret
>   401029:       0f 1f 80 00 00 00 00    nop    DWORD PTR [rax+0x0]
> ...
>
> Testing with recent trunk works as well with no differences in output.
> This is for x86_64-linux.
>
> So, as suspected, something else is broken for you.  Which compiler
> version are you using?  (And we need to check if it's something in the
> mingw target)
>
>
> Ciao,
> Michael.
>