Add debug support for MSVC builds - patch 1 - #1263
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cpython-unix/build.py offers `debug` alongside noopt and pgo, and every Unix target declares it in ci-targets.yaml. Windows accepts only noopt/pgo and hardcodes configuration="Release", so there is no way to produce a debug interpreter. That matters for embedding. MSVC's pyconfig.h selects the import library from _DEBUG, which the debug CRT defines, so a consumer building with /MDd asks for python3XX_d.lib and cannot link against a release distribution. Follows the Unix spelling: `debug` is a peer of noopt and pgo in the option set, so debug+pgo is simply not a valid combination. The set is renamed to `options` to match, since debug is not an optimization. The rest is naming. A Debug configuration suffixes its artifacts with $(PyDebugExt), so PC/layout is passed --debug, and the executables, extension libraries and dependency libraries pick up the _d suffix. The tail-calling interpreter is disabled for Debug. PBS turns it on for 3.15 on x64, but [[msvc::musttail]] requires /O2 and under /Od MSVC reports C4737 for each dispatch site. CPython's early check for this was reverted and the build still fails as of September 2026. https://learn.microsoft.com/en-us/cpp/cpp/attributes#msvcmusttail python/cpython#148047 building.rst documents the option and the debug CRT requirement: the binaries import ucrtbased.dll and vcruntime*d.dll, which ship with Visual Studio and are not redistributable.
vadikmironov
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September 12, 2026 09:38
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This change is a part of set of changes addressing #26. This is exactly the same change as the first commit in #1262. It is raised separately for the ease of reviewing by maintainers while #1262 is to give an overview of the full set of patches (can be deleted once individual commits are merged).
What changes in
cpython-windows/build.py:debugis added to the option set besidenooptandpgo, with the same ordering as Unix build already uses. Same as with Unix, this removes illegal combinations likedebug+pgo. The set is renamed fromoptimizationstooptionsalso inline with the Unix build.configuration="Debug"is passed to msbuild andartifact_configaccordingly.PC/layoutgets--debugand that drives the_dartifact suffixes._dsuffix$(PyDebugExt)producesdocs/building.rstupdated with the debug option and the debug CRT requirementEvidence: every debug configuration was built with this commit as the base, validated and tested. 26 jobs across 3.10 to 3.15 on x64, x86 and arm64, VS 2022 and VS 2026: https://github.com/vadikmironov/omniglot-bazel-starter/actions/runs/34095975591. The details, the archive layout and the other findings are in #1262.