Update Windows builds to use ClangCL #549

zanieb · 2025-03-04T22:55:38Z

Adding the flag as described in python/cpython#129907

ClangCL looks to provide a 20% runtime performance and build times were purportedly better as well.

This also would let us turn on the tail-calling interpreter for another 10% performance boost.

Fidget-Spinner · 2025-03-05T17:35:15Z

cpython-windows/build.py

@@ -653,6 +653,9 @@ def run_msbuild(
        # This can also work around known incompatibilities with the Windows 11
        # SDK as of at least CPython 3.9.7.
        f"/property:DefaultWindowsSDKVersion={windows_sdk_version}",
+        # Use ClangCL for better build and runtime performance
+        # https://github.com/python/cpython/issues/130090
+        "/p:PlatformToolset=ClangCL",


Probably should guard only on 3.14. (It's the first version with Clang PGO, that will guarantee better performance).

Also, requires python/cpython#130040 to pass on CI. For some reason there's a one line patch required.

If you are using clang versions below 19, this will furthermore need a fix for python/cpython#130213, due to the problems with intrinsics.

~~We're using LLVM 19 here (and presumably 20 soon — #553)~~ actually, realized we're only setting up a LLVM toolchain on Unix right now. I will look into the version we're using on Windows.

We should teach the build system to pull a pre-built LLVM toolchain from somewhere. The official releases from e.g. https://github.com/llvm/llvm-project/releases/tag/llvmorg-20.1.0 should be fine.

If we use the toolchain built into the GitHub Actions Runner, it will randomly be updated out from under us. Given some sensitivity we've seen around LLVM versions lately, this spooky-action-at-a-distance will likely randomly cause CI to fail.

Best to make CI as deterministic and reproducible as possible.

zanieb · 2025-03-05T20:00:49Z

The latest benchmark is at python/cpython#129907 (comment) which shows some regressions, but an average 19% improvement.

zanieb · 2025-03-18T21:56:35Z

Interesting, it failed with

cpython>        (ClCompile target) ->
cpython>          C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.43.34808\include\vcruntime.h(188,30): error : typedef redefinition with different types ('unsigned long long' vs 'unsigned int') [C:\Users\RUNNER~1\AppData\Local\Temp\python-build-73gmp1o2\Python-3.14.0a6\PCbuild\pythoncore.vcxproj]
cpython>          C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\lib\clang\19\include\__stddef_ptrdiff_t.h(18,26): error : typedef redefinition with different types ('int' vs 'long long') [C:\Users\RUNNER~1\AppData\Local\Temp\python-build-73gmp1o2\Python-3.14.0a6\PCbuild\pythoncore.vcxproj]
cpython>          C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.43.34808\include\vcruntime.h(188,30): error : typedef redefinition with different types ('unsigned long long' vs 'unsigned int') [C:\Users\RUNNER~1\AppData\Local\Temp\python-build-73gmp1o2\Python-3.14.0a6\PCbuild\pythoncore.vcxproj]
cpython>          C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\lib\clang\19\include\__stddef_ptrdiff_t.h(18,26): error : typedef redefinition with different types ('int' vs 'long long') [C:\Users\RUNNER~1\AppData\Local\Temp\python-build-73gmp1o2\Python-3.14.0a6\PCbuild\pythoncore.vcxproj]

chris-eibl · 2025-03-18T22:12:46Z

AFAIR this means it is compiling in 323bit mode. Try adding -m64.

See python/cpython@263870d.

Interesting, according to python/cpython@d8a1cf4 this should be part of v3.14.0a6

chris-eibl · 2025-03-18T22:41:25Z

But I see it is passed e.g. here:

-flto=thin -m64 -Wno-deprecated-non-prototype -Wno-unused-label -Wno-pointer-sign -Wno-incompatible-pointer-types-discards-qualifiers -Wno-unused-function  "..\Modules\_heapqmodule.c"

but not here

 /Z7 /nologo /W3 /WX- /diagnostics:column /O2 /Oi /D HACL_CAN_COMPILE_VEC256 /D _Py_HAVE_ZLIB /D _USRDLL /D Py_BUILD_CORE /D Py_BUILD_CORE_BUILTIN /D Py_ENABLE_SHARED /D "MS_DLL_ID=\"3.14\"" /D _Py_USING_PGO=1 /D WIN32 /D "PY3_DLLNAME=L\"python3\"" /D _WIN64 /D NDEBUG /D PyStats /D _WINDLL /GF /MD /GS /Gy /fp:precise /Fo"C:\Users\RUNNER~1\AppData\Local\Temp\python-build-2aa_wvqi\Python-3.14.0a6\PCbuild\obj\314amd64_PGInstrument\pythoncore\\" /Gd /TC /arch:AVX2 ..\Modules\_hacl\Hacl_Hash_Blake2b_Simd256.c

But in the line above for \Hacl_Hash_Blake2s.c all the clang stuff is present like for _heapqmodule.c above :-o

chris-eibl · 2025-03-18T22:51:34Z

Try adding "/p:PreferredToolArchitecture=x64". Before python/cpython@263870d that was the trick that did it for me. Maybe it is still needed, although I back then did not need it anymore. That was a back and forth.

I will re-investigate ...

zanieb · 2025-03-18T23:04:57Z

We also do some non-standard things (like, we hack the project files), so it's possible it's a problem there.

chris-eibl · 2025-03-18T23:10:34Z

Interestingly, I can reproduce on cpython main and alpha6. I cannot build without "/p:PreferredToolArchitecture=x64" anymore, too.
Weird, but I have an idea.

Need some sleep though now :)

zanieb · 2025-03-18T23:47:39Z

Cool that gets us to

cpython> copying C:\Users\RUNNER~1\AppData\Local\Temp\python-build-hctl79hu\Python-3.14.0a6\PCbuild\amd64\python314.exp
Traceback (most recent call last):
  File "C:\a\python-build-standalone\python-build-standalone\cpython-windows\build.py", line 1816, in <module>
    sys.exit(main())
             ^^^^^^
  File "C:\a\python-build-standalone\python-build-standalone\cpython-windows\build.py", line 1790, in main
    tar_path = build_cpython(
               ^^^^^^^^^^^^^^
  File "C:\a\python-build-standalone\python-build-standalone\cpython-windows\build.py", line 1524, in build_cpython
    build_info = collect_python_build_artifacts(
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\a\python-build-standalone\python-build-standalone\cpython-windows\build.py", line 1079, in collect_python_build_artifacts
    shutil.copyfile(source, dest)
  File "C:\hostedtoolcache\windows\Python\3.12.9\x64\Lib\shutil.py", line 260, in copyfile
    with open(src, 'rb') as fsrc:
         ^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\python-build-hctl79hu\\Python-3.14.0a6\\PCbuild\\amd64\\python314.exp'

which is from the copy at

python-build-standalone/cpython-windows/build.py

Lines 1072 to 1079 in 0935643

    
           exts = ("lib", "exp") 
        
           for ext in exts: 
        
               source = outputs_path / ("python%s%s.%s" % (python_majmin, lib_suffix, ext)) 
        
               dest = core_dir / ("python%s%s.%s" % (python_majmin, lib_suffix, ext)) 
        
               log("copying %s" % source) 
        
               shutil.copyfile(source, dest)

I presume this file is not produced upstream as well? I don't know if it's of consequence — I'm woefully lacking in knowledge of .exp files / Windows dynamic linking details.

chris-eibl · 2025-03-19T17:10:34Z

Ok, I've dug deeper this time - please see python/cpython#131473 for details.

Regarding the *.exp file: most probably the gold linker does not write that, I did not find anything in the net, yet. Don't know whether this is a clang-cl compatibility issue or a wrong switch, etc, in the cPython build.

zanieb · 2025-03-19T17:17:53Z

A nice improvement in build times

i686-pc-windows-msvc / 3.14 / pgo 29m -> 22m
x86_64-pc-windows-msvc / 3.14 / pgo 26m -> 18m

Fidget-Spinner · 2025-03-28T11:51:56Z

Tail calling for Windows clangcl has been merged https://github.com/python/cpython/pull/130040/files#diff-4295039788386c2986b811f2a6b59cf088be59822b24b4f1243c9e9b184f65fbR109

zanieb added the ci:skip label Mar 4, 2025

Fidget-Spinner reviewed Mar 5, 2025

View reviewed changes

chris-eibl mentioned this pull request Mar 7, 2025

gh-130090: Support PGO for clang-cl python/cpython#129907

Merged

zanieb added platform:windows Specific to the Windows platform python:3.14 and removed ci:skip labels Mar 17, 2025

Update Windows builds to use ClangCL

16c9a50

zanieb force-pushed the zb/clangcl branch from 32db823 to 16c9a50 Compare March 17, 2025 20:37

debug: Try setting some clang compatibility flags?

2636c55

debug: explicitly set -m64?

4033e0d

debug: try "/p:PreferredToolArchitecture=x64"

0935643

Drop "exp" file

10a69b0

chris-eibl mentioned this pull request Mar 19, 2025

clang-cl on Windows still needs PreferredToolArchitecture python/cpython#131473

Closed

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Update Windows builds to use ClangCL #549

Update Windows builds to use ClangCL #549

zanieb commented Mar 4, 2025 •

edited

Loading

Fidget-Spinner Mar 5, 2025 •

edited

Loading

Fidget-Spinner Mar 5, 2025

chris-eibl Mar 9, 2025

zanieb Mar 9, 2025 •

edited

Loading

indygreg Mar 9, 2025

zanieb commented Mar 5, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 19, 2025

zanieb commented Mar 19, 2025

Fidget-Spinner commented Mar 28, 2025

Update Windows builds to use ClangCL #549

Are you sure you want to change the base?

Update Windows builds to use ClangCL #549

Conversation

zanieb commented Mar 4, 2025 • edited Loading

Fidget-Spinner Mar 5, 2025 • edited Loading

Choose a reason for hiding this comment

Fidget-Spinner Mar 5, 2025

Choose a reason for hiding this comment

chris-eibl Mar 9, 2025

Choose a reason for hiding this comment

zanieb Mar 9, 2025 • edited Loading

Choose a reason for hiding this comment

indygreg Mar 9, 2025

Choose a reason for hiding this comment

zanieb commented Mar 5, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 18, 2025

zanieb commented Mar 18, 2025

chris-eibl commented Mar 19, 2025

zanieb commented Mar 19, 2025

Fidget-Spinner commented Mar 28, 2025

zanieb commented Mar 4, 2025 •

edited

Loading

Fidget-Spinner Mar 5, 2025 •

edited

Loading

zanieb Mar 9, 2025 •

edited

Loading