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<p class="MsoNormal">Hi Ray,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So the code generated does deadloop, but is just not easy to resume from as we have been able to do in the past.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We use CpuDeadloop() for 2 purposes. One is a terminal condition with no reason to ever continue.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The 2<sup>nd</sup> 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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We may have to do a NASM implementation of CpuDeadloop() to make sure it meets both use cases.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Mike<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="mso-ligatures:none">From:</span></b><span style="mso-ligatures:none"> Ni, Ray <ray.ni@intel.com>
<br>
<b>Sent:</b> Thursday, May 18, 2023 3:00 AM<br>
<b>To:</b> devel@edk2.groups.io<br>
<b>Cc:</b> Kinney, Michael D <michael.d.kinney@intel.com>; Rebecca Cran <rebecca@bsdio.com>; Ni, Ray <ray.ni@intel.com><br>
<b>Subject:</b> CpuDeadLoop() is optimized by compiler<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The optimization is so “good” that it becomes harder for developers to break out of the deadloop.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I copied the assembly instructions as below for your reference.<o:p></o:p></p>
<p class="MsoNormal">The compiler does not generate instructions that jump out of the loop when the Index is not zero.<o:p></o:p></p>
<p class="MsoNormal">So in order to break out of the loop, developers need to:<o:p></o:p></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3">Manually adjust rsp by increasing 40<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3">Manually “ret”<o:p></o:p></li></ol>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am not sure if anyone has interest to re-write this function so that compiler can be “fooled” again.<o:p></o:p></p>
<p class="MsoNormal">Thanks,<br>
Ray<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">=======================<o:p></o:p></p>
<p class="MsoNormal">; Function compile flags: /Ogspy<o:p></o:p></p>
<p class="MsoNormal">; File e:\work\edk2\MdePkg\Library\BaseLib\CpuDeadLoop.c<o:p></o:p></p>
<p class="MsoNormal">; COMDAT CpuDeadLoop<o:p></o:p></p>
<p class="MsoNormal">_TEXT SEGMENT<o:p></o:p></p>
<p class="MsoNormal">Index$ = 48<o:p></o:p></p>
<p class="MsoNormal">CpuDeadLoop PROC ; COMDAT<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">; 26 : {<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">$LN12:<o:p></o:p></p>
<p class="MsoNormal"> 00000 48 83 ec 28 sub rsp, 40 ; 00000028H<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">; 27 : volatile UINTN Index;<o:p></o:p></p>
<p class="MsoNormal">; 28 : <o:p></o:p></p>
<p class="MsoNormal">; 29 : for (Index = 0; Index == 0;) {<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> 00004 48 c7 44 24 30<o:p></o:p></p>
<p class="MsoNormal"> 00 00 00 00 mov QWORD PTR Index$[rsp], 0<o:p></o:p></p>
<p class="MsoNormal">$LN10@CpuDeadLoo:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">; 30 : CpuPause ();<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> 0000d 48 8b 44 24 30 mov rax, QWORD PTR Index$[rsp]<o:p></o:p></p>
<p class="MsoNormal"> 00012 e8 00 00 00 00 call CpuPause<o:p></o:p></p>
<p class="MsoNormal"> 00017 eb f4 jmp SHORT $LN10@CpuDeadLoo<o:p></o:p></p>
<p class="MsoNormal">CpuDeadLoop ENDP<o:p></o:p></p>
<p class="MsoNormal">_TEXT ENDS<o:p></o:p></p>
<p class="MsoNormal">END<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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