Static global will not work for XIP Mike From: Andrew (EFI) Fish Sent: Thursday, May 18, 2023 9:49 AM To: edk2-devel-groups-io ; Kinney, Michael D Cc: Ni, Ray ; Rebecca Cran 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 > 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 > Sent: Thursday, May 18, 2023 3:00 AM To: devel@edk2.groups.io Cc: Kinney, Michael D >; Rebecca Cran >; Ni, Ray > 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