[edk2-devel] CpuDeadLoop() is optimized by compiler
Rebecca Cran
rebecca at bsdio.com
Fri May 19 15:31:18 UTC 2023
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 at apple.com>
> *Sent:* Friday, May 19, 2023 8:42 AM
> *To:* devel at edk2.groups.io; Kinney, Michael D <michael.d.kinney at intel.com>
> *Cc:* Ni, Ray <ray.ni at intel.com>; Rebecca Cran <rebecca at 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 at 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 at apple.com>
> *Sent:*Thursday, May 18, 2023 1:45 PM
> *To:*edk2-devel-groups-io <devel at edk2.groups.io>; Andrew Fish
> <afish at apple.com>
> *Cc:*Kinney, Michael D <michael.d.kinney at intel.com>; Ni, Ray
> <ray.ni at intel.com>; Rebecca Cran <rebecca at 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 at 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 at 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 at 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 at apple.com>
> *Sent:*Thursday, May 18, 2023 10:09 AM
> *To:*Kinney, Michael D <michael.d.kinney at intel.com>
> *Cc:*edk2-devel-groups-io <devel at edk2.groups.io>; Ni,
> Ray <ray.ni at intel.com>; Rebecca Cran <rebecca at 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 at intel.com> wrote:
>
> Static global will not work for XIP
>
> Mike
>
> *From:*Andrew (EFI) Fish <afish at apple.com>
> *Sent:*Thursday, May 18, 2023 9:49 AM
> *To:*edk2-devel-groups-io <devel at edk2.groups.io>;
> Kinney, Michael D <michael.d.kinney at intel.com>
> *Cc:*Ni, Ray <ray.ni at intel.com>; Rebecca Cran
> <rebecca at 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 at 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 at intel.com>
> *Sent:*Thursday, May 18, 2023 3:00 AM
> *To:*devel at edk2.groups.io
> *Cc:*Kinney, Michael D
> <michael.d.kinney at intel.com>; Rebecca Cran
> <rebecca at bsdio.com>; Ni, Ray <ray.ni at 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
>
> $LN10 at CpuDeadLoo:
>
> ; 30 : CpuPause ();
>
> 0000d 48 8b 44 24 30 mov rax, QWORD PTR
> Index$[rsp]
>
> 00012 e8 00 00 00 00 call CpuPause
>
> 00017 eb f4 jmp SHORT $LN10 at CpuDeadLoo
>
> CpuDeadLoop ENDP
>
> _TEXT ENDS
>
> END
>
>
>
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