Re: CpuDeadLoop() is optimized by compiler

["Michael D Kinney" <michael.d.kinney@intel.com>]

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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



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