Re: [edk2-devel] CpuDeadLoop() is optimized by compiler

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

[-- Attachment #1.1: Type: text/plain, Size: 30205 bytes --]

Hi Ray,

I know this is an old topic, but I think I have a new idea that works for XIP code.  We can update loop to compare a volatile global to a volatile local.  This forces 2 reads and a comparison on every loop iteration.  The local variable can be set to 1 to exit the loop without modifying the global variable.  I tried this with VS2019 with max opt enabled and I was able to exit loop by setting Index to 1 in a debugger.

diff --git a/MdePkg/Library/BaseLib/CpuDeadLoop.c b/MdePkg/Library/BaseLib/CpuDeadLoop.c
index b3b7548fa5..393c4290ed 100644
--- a/MdePkg/Library/BaseLib/CpuDeadLoop.c
+++ b/MdePkg/Library/BaseLib/CpuDeadLoop.c
@@ -9,6 +9,8 @@
#include <Base.h>
#include <Library/BaseLib.h>

+static volatile UINTN  mDeadLoopComparator = 0;^M
+^M
/**
   Executes an infinite loop.

@@ -26,7 +28,7 @@ CpuDeadLoop (
{
   volatile UINTN  Index;

-  for (Index = 0; Index == 0;) {
+  for (Index = mDeadLoopComparator; Index == mDeadLoopComparator;) {^M
     CpuPause ();
   }
}

Mike

From: Kinney, Michael D <michael.d.kinney@intel.com>
Sent: Tuesday, October 31, 2023 7:19 AM
To: Ni, Ray <ray.ni@intel.com>; Andrew (EFI) Fish <afish@apple.com>; edk2-devel-groups-io <devel@edk2.groups.io>; Rebecca Cran <rebecca@bsdio.com>; Hernandez Miramontes, Jose Miguel <jose.miguel.hernandez.miramontes@intel.com>
Cc: Kinney, Michael D <michael.d.kinney@intel.com>
Subject: RE: [edk2-devel] CpuDeadLoop() is optimized by compiler

Right. But if you break in with debugger, you can still skip over the jmp instruction and continue.

I agree XIP does not allow variable value to be updated, but we would never want to do that or all future dead loops in non XIP code would not loop.

Mike

From: Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>
Sent: Tuesday, October 31, 2023 1:31 AM
To: Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>; Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>; edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>; Hernandez Miramontes, Jose Miguel <jose.miguel.hernandez.miramontes@intel.com<mailto:jose.miguel.hernandez.miramontes@intel.com>>
Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler

Mike,
This is not friendly for XIP code. With XIP code, the global variable is not able to be updated as it sits in read-only SPI flash.

Thanks,
Ray
________________________________
From: Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Sent: Tuesday, October 31, 2023 11:37 AM
To: Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>; edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>; Hernandez Miramontes, Jose Miguel <jose.miguel.hernandez.miramontes@intel.com<mailto:jose.miguel.hernandez.miramontes@intel.com>>
Cc: Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Subject: RE: [edk2-devel] CpuDeadLoop() is optimized by compiler


Does using a static volatile global instead of a volatile local work?



Mike



From: Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>
Sent: Monday, October 30, 2023 7:52 PM
To: Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>; edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>; Hernandez Miramontes, Jose Miguel <jose.miguel.hernandez.miramontes@intel.com<mailto:jose.miguel.hernandez.miramontes@intel.com>>
Cc: Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler



It's been a while.



Is there any better solution? Can we go with assembly solution?



Thanks,

Ray

________________________________

From: Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>
Sent: Saturday, May 20, 2023 12:31 AM
To: edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>
Cc: Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler



I don't think the atomic is going to help. The compiler honored the volatile by doing a read, but assumed it would never change due to scoping. As you can see in my example if the compiler thinks DeadLoopCount can be changed it will put the check back in and assume the function can return. So an assembly function that does nothing called IncreaseScope()

 would fix this issue too.





void IncreaseScope(int *ptr);





void CpuDeadLoopFix(void) {



volatile

int DeadLoopCount = 0;



while(DeadLoopCount ==

0) {



IncreaseScope(&DeadLoopCount);



}



}





void CpuDeadLoop(void) {



volatile

int DeadLoopCount = 0;



while(DeadLoopCount ==

0);



}





Gives us:





voltbl

SEGMENT



voltbl

ENDS



voltbl

SEGMENT



voltbl

ENDS





DeadLoopCount$ =

48



CpuDeadLoopFix PROC

; COMDAT



$LN12:



sub

rsp, 40

; 00000028H



mov

DWORD PTR

DeadLoopCount$[rsp],

0



jmp

SHORT $LN10@CpuDeadLoo



$LL2@CpuDeadLoo:



lea

rcx, QWORD

PTR DeadLoopCount$[rsp]



call

IncreaseScope



$LN10@CpuDeadLoo:



mov

eax, DWORD

PTR DeadLoopCount$[rsp]



test

eax, eax



je

SHORT $LL2@CpuDeadLoo



add

rsp, 40

; 00000028H



ret

0



CpuDeadLoopFix ENDP





DeadLoopCount$ =

8



CpuDeadLoop PROC

; COMDAT



mov

DWORD PTR

DeadLoopCount$[rsp],

0



$LL2@CpuDeadLoo:



mov

eax, DWORD

PTR DeadLoopCount$[rsp]



jmp

SHORT $LL2@CpuDeadLoo



CpuDeadLoop ENDP







Compiler Explorer<https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>

godbolt.org<https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>

[cid:image001.png@01DAB6AA.1D74CB60]<https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>



Thanks,



Andrew Fish



PS I'm still not 100% sure it is a compiler bug. Some times things like this are due to the order the compiler applies the optimizations, and changing the order can change the behavior.





On May 19, 2023, at 8:31 AM, Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>> wrote:



Just to add more data, I also tried with "volatile sig_atomic_t" as someone suggested and both "/volatile:iso" and "/volatile:ms" with no change in results.


--

Rebecca Cran


On 5/18/23 20:53, Ni, Ray wrote:

I think all the options we considered are workarounds. These might break again if compiler is "cleverer" in future. Unless some Cxx spec clearly guarantees that.

I like Mike's idea to use assembly implementation for CpuDeadLoop. The assembly can simply "jmp $" then "ret".

I didn't find a dead-loop intrinsic function in MSVC.

Any better idea?

Thanks,

Ray

*From:* Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>
*Sent:* Friday, May 19, 2023 8:42 AM
*To:* devel@edk2.groups.io<mailto:devel@edk2.groups.io>; Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
*Cc:* Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>
*Subject:* Re: [edk2-devel] CpuDeadLoop() is optimized by compiler

Mike,

Sorry static was just to scope the name to the file since it is a lib, not to make it work.

That is a cool site. I learned about it complaining about stuff to the compiler team on our internal clang Slack channel as they use it to answer my questions.

Thanks,

Andrew Fish



   On May 18, 2023, at 2:42 PM, Michael D Kinney
   <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>> wrote:

   Using that tool, the following fragment seems to generate the
   right code. Volatile is required.  Static is optional.

   staticvolatileint mDeadLoopCount =0;

   void

   CpuDeadLoop(

   void

     )

   {

   while(mDeadLoopCount ==0);

   }

   GCC

   ===

   CpuDeadLoop():

   .L2:

   moveax,DWORDPTRmDeadLoopCount[rip]

   testeax,eax

   je.L2

   ret

   CLANG

   =====

   CpuDeadLoop():# @CpuDeadLoop()

   .LBB0_1:                          # =>This Inner Loop Header:Depth=1

   cmpdwordptr[rip+_ZL14mDeadLoopCount],0

   je.LBB0_1

   ret

   Mike

   *From:*Andrew (EFI) Fish <afish@apple.com<mailto:afish@apple.com>>
   *Sent:*Thursday, May 18, 2023 1:45 PM
   *To:*edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Andrew Fish
   <afish@apple.com<mailto:afish@apple.com>>
   *Cc:*Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>; Ni, Ray
   <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>
   *Subject:*Re: [edk2-devel] CpuDeadLoop() is optimized by compiler

   Whoops wrong compiler. Here is an update. I added the flags so
   this one reproduces the issue.

   Compiler Explorer
   <https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>

   godbolt.org<http://godbolt.org/>
   <https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>



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   Thanks,

   Andrew Fish




       On May 18, 2023, at 11:45 AM, Andrew Fish viagroups.io<http://viagroups.io/>
       <http://groups.io/><afish=apple.com@groups.io<mailto:afish=apple.com@groups.io>> wrote:

       Mike,

       This is a good way to play around with fixes, and to report
       bugs. You can see the assembler for different compilers with
       different flag.

       Compiler Explorer
       <https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>

       godbolt.org<http://godbolt.org/>
       <https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>



       <favicon.png>
       <https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGIM6SuADJ4DJgAcj4ARpjEINIADqgKhE4MHt6%2B/qRJKY4CIWGRLDFx0naYDmlCBEzEBBk%2BfgEVVQI1dQSFEdGx8ba19Y1ZLYNdoT0lfZIAlLaoXsTI7BwA9KsA1AAqAJ4JmBs7C8QbaFgbCLGYpBskG7SoTOgbhhuYqqwJ9AB0JhoAgqECBsFABHLx1TAQIEbBg%2BGYbEwAdisAI26I2xEwBEWDFhPg2ACp8SwTABmVH/ZEAEQ4c1onAArLw/BwtKRUJw3NZrCDjstEWYyTxSARNHS5gBrECMjT6TiSFnijmcXgKEBysVsumkOCwJBoFgJOixciUQ3G%2BhxYBcMxcPh0AixdUQKLKqKhOo7Tgiw1sQQAeQYtG92tIWBYhmA4jD%2BCxVQAbph1WH3pUvE6fbwgZgGWHaHgosQvR4sMqCMQ8Cws3MqAZgAoAGp4TAAdwD%2B1ZIv4ghEYnYUhkgkUKnUYd09oMRhQPMs%2BkL6sgc1QCXyDBTAFoA2SNhvI0sEOTqQpJTsDJLMGrc5U1y4GO5PE09MEJsVSnpcqkBMM/PbP2vujfPp7VaNcOiGR8shA682gYcDxiKXo4hAsYfz0BQxkApCJDmBR%2BQHUUsRWHh6SZJUw05DhVAADgANg3WjJA2YBkGQDZbW%2BLgNggblLGsG5cEIO5zGFG4PCNE0ThErgZl4LUtDmCADVQCSrTNCALUklApxtSQNDlGhaCdYgXTdMMPWYYhQ19FT/QIIMQ2VCMoxjdk4xvPAkxTdk02QDNiOzQRc2VAsixLDAVnZCsqxrPh6ybFt207LMh2EURxEHHt5CUNRlV0AIdJnPi51CxcIGXVc0k3bdd33ZBDzJY9T3PS9bBg28IFcND7RfRCpmQnJki/dJIN/Qa8jSLD%2BvQ9rqlQ0aZvsMDMNfbCUM6bqBk6Kb3xk%2BZFmWPQK0wALSI4ZlSFZdlKJo%2BjGNOHT2Mkb4NBe7jeKsOcNkEogpKFe0NnEy1YkFMkzFk0VxUUpB8CoKh1KyvsMukLKR1y/MEHVSdMdhqgCD2dg5WITHsmJhRcfx/YNTOi6rt4SjqTwOGNmbNsQduhimJYtiOK4hMFA2Dn7uQR6pBel65KhqUZTlPNFUu5VKLVbJ5IleUODMcjrtVSHtRmOYkxMtJ4iAA>

       Sorry I'm traveling and in Cupertino with lots of meetings so
       I did not have time to adjust the compiler flags....

       Thanks,

       Andrew Fish




           On May 18, 2023, at 10:24 AM, Andrew (EFI) Fish
           <afish@apple.com<mailto:afish@apple.com>> wrote:

           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.kinney@intel.com<mailto:michael.d.kinney@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 <afish@apple.com<mailto:afish@apple.com>>
               *Sent:*Thursday, May 18, 2023 10:09 AM
               *To:*Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
               *Cc:*edk2-devel-groups-io <devel@edk2.groups.io<mailto:devel@edk2.groups.io>>; Ni,
               Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@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.kinney@intel.com<mailto:michael.d.kinney@intel.com>> wrote:

                   Static global will not work for XIP

                   Mike

                   *From:*Andrew (EFI) Fish <afish@apple.com<mailto:afish@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.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
                   *Cc:*Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>; Rebecca Cran
                   <rebecca@bsdio.com<mailto:rebecca@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.kinney@intel.com<mailto:michael.d.kinney@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 2^nd 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<mailto:ray.ni@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.kinney@intel.com<mailto:michael.d.kinney@intel.com>>; Rebecca Cran
                       <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>; Ni, Ray <ray.ni@intel.com<mailto: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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