Mike,
I guess my other question… If this turns out to be a compiler bug should we
scope the change to the broken toolchain. I’m not sure what the right answer is
for that, but I want to ask the question?
Thanks,
Andrew Fish
> On May 18, 2023, at 10:19 AM, Michael D Kinney <michael.d.kin...@intel.com>
> wrote:
>
> Andrew,
>
> This might work for XIP. Set non const global to initial value that is
> expected value to stay in dead loop.
>
> UINTN mDeadLoopCount = 0;
>
> VOID
> CpuDeadLoop(
> VOID
> )
> {
> while (mDeadLoopCount == 0) {
> CpuPause();
> }
> }
>
> When deadloop is entered, developer can not change value of mDeadLoopCount,
> but they can use debugger to force exit loop and return from function.
>
> Mike
>
>
> From: Andrew (EFI) Fish <af...@apple.com <mailto:af...@apple.com>>
> Sent: Thursday, May 18, 2023 10:09 AM
> To: Kinney, Michael D <michael.d.kin...@intel.com
> <mailto:michael.d.kin...@intel.com>>
> Cc: edk2-devel-groups-io <devel@edk2.groups.io
> <mailto:devel@edk2.groups.io>>; Ni, Ray <ray...@intel.com
> <mailto:ray...@intel.com>>; Rebecca Cran <rebe...@bsdio.com
> <mailto:rebe...@bsdio.com>>
> Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler
>
> Mike,
>
> Good point, that is why we are using the stack ….
>
> The only other thing I can think of is to pass the address of Index to some
> inline assembler, or an asm no op function, to give it a side effect the
> compiler can’t resolve.
>
> Thanks,
>
> Andrew Fish
>
>
> On May 18, 2023, at 10:05 AM, Kinney, Michael D <michael.d.kin...@intel.com
> <mailto:michael.d.kin...@intel.com>> wrote:
>
> Static global will not work for XIP
>
> Mike
>
> From: Andrew (EFI) Fish <af...@apple.com <mailto:af...@apple.com>>
> Sent: Thursday, May 18, 2023 9:49 AM
> To: edk2-devel-groups-io <devel@edk2.groups.io
> <mailto:devel@edk2.groups.io>>; Kinney, Michael D <michael.d.kin...@intel.com
> <mailto:michael.d.kin...@intel.com>>
> Cc: Ni, Ray <ray...@intel.com <mailto:ray...@intel.com>>; Rebecca Cran
> <rebe...@bsdio.com <mailto:rebe...@bsdio.com>>
> Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler
>
> Mike,
>
> I pinged some compiler experts to see if our code is correct, or if the
> compiler has an issue. Seems to be trending compiler issue right now, but
> I’ve NOT gotten feedback from anyone on the spec committee yet.
>
> If we move Index to a static global that would likely work around the
> compiler issue.
>
> Thanks,
>
> Andrew Fish
>
>
>
> On May 18, 2023, at 8:36 AM, Michael D Kinney <michael.d.kin...@intel.com
> <mailto:michael.d.kin...@intel.com>> 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 <mailto:ray...@intel.com>>
> Sent: Thursday, May 18, 2023 3:00 AM
> To: devel@edk2.groups.io <mailto:devel@edk2.groups.io>
> Cc: Kinney, Michael D <michael.d.kin...@intel.com
> <mailto:michael.d.kin...@intel.com>>; Rebecca Cran <rebe...@bsdio.com
> <mailto:rebe...@bsdio.com>>; Ni, Ray <ray...@intel.com
> <mailto: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:
> 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
>
>
>
>
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