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...@intel.com>
> *Sent:* Thursday, May 18, 2023 3:00 AM
> *To:* devel@edk2.groups.io
> *Cc:* Kinney, Michael D <michael.d.kin...@intel.com>; Rebecca Cran
> <rebe...@bsdio.com>; Ni, Ray <ray...@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
>
>
>
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