[release/10.0] Apply changes for building with clang-21 #121124
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Pull Request Overview
This PR addresses build failures when using clang-21 on Fedora 43 by adapting to a breaking change introduced in clang-20. The clang-20 compiler split the -fwrapv flag into separate -fwrapv and -fwrapv-pointer flags, and this PR migrates to using -fno-strict-overflow which implies both behaviors.
Key Changes:
- Replaced
-fwrapvwith-fno-strict-overflowto handle signed arithmetic overflow behavior in a way compatible with clang-20+ - Fixed type casting issues in CoreCLR debug interface code to avoid enum-to-int implicit conversion errors
- Added
-fno-strict-aliasingflag to match MSVC behavior
Reviewed Changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
src/mono/CMakeLists.txt |
Replaced -fwrapv with -fno-strict-overflow and updated comments to explain the clang-20+ behavior |
src/coreclr/debug/di/rsthread.cpp |
Added explicit cast from m_userState to CorDebugUserState enum and changed local variable type to int |
src/coreclr/debug/di/rspriv.h |
Changed m_userState member variable type from CorDebugUserState to int and updated kInvalidUserState constant accordingly |
eng/native/configurecompiler.cmake |
Replaced -fwrapv with -fno-strict-overflow and added -fno-strict-aliasing flag with updated comments |
This was referenced Oct 28, 2025
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This was referenced Oct 28, 2025
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Customer Impact
These issues were reported in #119706 as problems with clang-21 on Fedora 43. The investigation uncovered that clang introduced a potentially breaking change in clang-20 that we do not currently consume. These build changes impact VMR related builds when linux distrobutions performing source build adopt clang-21.
clang-20 breaking change log - https://releases.llvm.org/20.1.0/tools/clang/docs/ReleaseNotes.html#potentially-breaking-changes.
This PR contains the minimal changes needed to fix issues from the following PRs #120644 and #120775.
Regression
Moving to the new clang-21 compiler will cause the runtime to crash. This is a regression from using .NET 10 and the default compiler of clang-19.
Testing
This has been validated using various legs and examples to demonstrate the usage of undefined behavior these flags convert into "defined" behavior in C/C++.
Risk
Low. This makes clang-21 more permissive and is limited to non official product builds.