GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows (#148036)

chris-eibl · web-flow · commit cbd81d59cfea · 2026-04-12T08:49:18.000+03:00
Rather than failing late when compiling e.g. a debug configuration ``` build.bat -c debug --tail-call-interp ``` with hundreds of ``` error C4737: Unable to perform required tail call. Performance may be degraded. ``` -- fail early with an explicit error message for configurations that are not supported by MSVC. This is a follow-up on #140513 / #140548
diff --git a/Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst b/Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst
@@ -0,0 +1,2 @@
+Fail fast with an explicit and clear error message if tail-calling is not
+possible for MSVC builds on Windows. Patch by Chris Eibl.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
@@ -749,4 +749,13 @@
   <Target Name="_DeletePyBuildDirTxt" BeforeTargets="PrepareForBuild">
     <Delete Files="$(OutDir)pybuilddir.txt" />
   </Target>
+
+  <Target Name="_CheckTailCalling" BeforeTargets="PrepareForBuild" Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">
+    <Error Text="MSVC supports tail-calling only for x64."
+           Condition="$(Platform) != 'x64'" />
+    <Error Text="Platform toolset >= v145 is required for tail-calling."
+           Condition="$(PlatformToolset.Replace('v', '0')) &lt; '145'" />
+    <Error Text="MSVC requires optimization to be enabled for tail-calling."
+           Condition="$(Configuration) == 'Debug'" />
+  </Target>
 </Project>

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Fail fast with an explicit and clear error message if tail-calling is not`
	`2`	`+possible for MSVC builds on Windows. Patch by Chris Eibl.`