On Thu, May 18, 2023 at 10:59 AM Ni, Ray <ray...@intel.com> wrote:
>
> Hi,
>
> Starting from certain version of Visual Studio C compiler (I don’t have the
> exact version. I am using VS2019), CpuDeadLoop is now optimized quite well by
> compiler.
>
>
>
> The optimization is so “good” that it becomes harder for developers to break
> out of the deadloop.
>
>
>
> I copied the assembly instructions as below for your reference.
>
> The compiler does not generate instructions that jump out of the loop when
> the Index is not zero.
>
> So in order to break out of the loop, developers need to:
>
> Manually adjust rsp by increasing 40
> Manually “ret”
>
>
>
> I am not sure if anyone has interest to re-write this function so that
> compiler can be “fooled” again.
>
> Thanks,
> Ray
>
>
>
> =======================
>
> ; Function compile flags: /Ogspy
>
> ; File e:\work\edk2\MdePkg\Library\BaseLib\CpuDeadLoop.c
>
> ; COMDAT CpuDeadLoop
>
> _TEXT SEGMENT
>
> Index$ = 48
>
> CpuDeadLoop PROC
> ; COMDAT
>
>
>
> ; 26 : {
>
>
>
> $LN12:
>
> 00000 48 83 ec 28 sub rsp, 40
> ; 00000028H
>
>
>
> ; 27 : volatile UINTN Index;
>
> ; 28 :
>
> ; 29 : for (Index = 0; Index == 0;) {
>
>
>
> 00004 48 c7 44 24 30
>
> 00 00 00 00 mov QWORD PTR Index$[rsp], 0
>
> $LN10@CpuDeadLoo:
>
>
>
> ; 30 : CpuPause ();
>
>
>
> 0000d 48 8b 44 24 30 mov rax, QWORD PTR Index$[rsp]
>
> 00012 e8 00 00 00 00 call CpuPause
>
> 00017 eb f4 jmp SHORT $LN10@CpuDeadLoo
>
> CpuDeadLoop ENDP
>
> _TEXT ENDS
>
> END
Hi Ray,
Can you try something like this? https://godbolt.org/z/x7P1PqY59
Seems to work, but godbolt does not support MSVC LTO :/
--
Pedro
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