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

["Rebecca Cran" <rebecca@bsdio.com>]

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>
> *Sent:* Friday, May 19, 2023 8:42 AM
> *To:* devel@edk2.groups.io; Kinney, Michael D <michael.d.kinney@intel.com>
> *Cc:* Ni, Ray <ray.ni@intel.com>; Rebecca Cran <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> 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>
>     *Sent:*Thursday, May 18, 2023 1:45 PM
>     *To:*edk2-devel-groups-io <devel@edk2.groups.io>; Andrew Fish
>     <afish@apple.com>
>     *Cc:*Kinney, Michael D <michael.d.kinney@intel.com>; Ni, Ray
>     <ray.ni@intel.com>; Rebecca Cran <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
>     <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>
>
>     	
>
>     <image001.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>
>
>     Thanks,
>
>     Andrew Fish
>
>
>
>
>         On May 18, 2023, at 11:45 AM, Andrew Fish viagroups.io
>         <http://groups.io/><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
>         <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> 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> 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>
>                 *Sent:*Thursday, May 18, 2023 10:09 AM
>                 *To:*Kinney, Michael D <michael.d.kinney@intel.com>
>                 *Cc:*edk2-devel-groups-io <devel@edk2.groups.io>; Ni,
>                 Ray <ray.ni@intel.com>; Rebecca Cran <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> wrote:
>
>                     Static global will not work for XIP
>
>                     Mike
>
>                     *From:*Andrew (EFI) Fish <afish@apple.com>
>                     *Sent:*Thursday, May 18, 2023 9:49 AM
>                     *To:*edk2-devel-groups-io <devel@edk2.groups.io>;
>                     Kinney, Michael D <michael.d.kinney@intel.com>
>                     *Cc:*Ni, Ray <ray.ni@intel.com>; Rebecca Cran
>                     <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> 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>
>                         *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
>
>     
>