Re: [edk2-devel] CpuDeadLoop() is optimized by compiler

["Rebecca Cran" <rebecca@bsdio.com>]

When I use CpuDeadLoop for debugging on Aarch64 I have symbols loaded so I can just do ‘set Index=1’ and resume, but it sounds like the issue is that people want to sometimes debug without symbols/source, and the generated assembly is making that difficult.

Rebecca

On Thu, May 18, 2023, at 9:36 AM, Michael D Kinney wrote:
> Hi Ray,
> 
> So the code generated does deadloop, but is just not easy to resume 
> from as we have been able to do in the past.
> 
> We use CpuDeadloop() for 2 purposes.  One is a terminal condition with 
> no reason to ever continue.
> 
> The 2nd is a debug aide for developers to halt the system at a specific 
> location and then continue from that point, usually with a debugger, to 
> step through code to an area to evaluate unexpected behavior.
> 
> We may have to do a NASM implementation of CpuDeadloop() to make sure 
> it meets both use cases.
> 
> Mike
> 
> *From:* Ni, Ray <ray.ni@intel.com> 
> *Sent:* Thursday, May 18, 2023 3:00 AM
> *To:* devel@edk2.groups.io
> *Cc:* Kinney, Michael D <michael.d.kinney@intel.com>; Rebecca Cran 
> <rebecca@bsdio.com>; Ni, Ray <ray.ni@intel.com>
> *Subject:* CpuDeadLoop() is optimized by compiler
> 
> 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:
>  1. Manually adjust rsp by increasing 40
>  2. 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
> 
> 
>