dotnet · ellahathaway · Dec 1, 2025
@@ -0,0 +1,145 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aaron R Robinson <arobins@microsoft.com>
+Date: Fri, 14 Nov 2025 11:01:28 -0800
+Subject: [PATCH] [release/9.0-staging] Add flags when the clang's major
+ version is > 20.0 (#121151)
+Backport: https://github.com/dotnet/runtime/pull/121151
+
+## Customer Impact
+
+- [x] Customer reported
+- [ ] Found internally
+
+These issues were reported in
+https://github.com/dotnet/runtime/issues/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 PR https://github.com/dotnet/runtime/pull/120775.
+
+.NET 10: https://github.com/dotnet/runtime/pull/121124
+.NET 8: https://github.com/dotnet/runtime/pull/121150
+
+## Regression
+
+- [ ] Yes
+- [x] No
+
+Build with the new clang-21 compiler will cause the runtime to crash.
+
+## 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 has zero impact on our production build since we specifically
+target clang-18. This is only valid for those partners that are using
+clang-20+.
+---
+ eng/native/configurecompiler.cmake | 18 +++++++++++++++---
+ src/coreclr/debug/di/rspriv.h      |  4 ++--
+ src/coreclr/debug/di/rsthread.cpp  | 12 ++++++------
+ 3 files changed, 23 insertions(+), 11 deletions(-)
+
+diff --git a/eng/native/configurecompiler.cmake b/eng/native/configurecompiler.cmake
+index 109b947e4eb..c114c03a9a1 100644
+--- a/eng/native/configurecompiler.cmake
++++ b/eng/native/configurecompiler.cmake
+@@ -526,9 +526,21 @@ if (CLR_CMAKE_HOST_UNIX)
+   # Disable frame pointer optimizations so profilers can get better call stacks
+   add_compile_options(-fno-omit-frame-pointer)
+
+-  # Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
+-  # using twos-complement representation (this is normally undefined according to the C++ spec).
+-  add_compile_options(-fwrapv)
++  if((CMAKE_C_COMPILER_ID STREQUAL "Clang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 20.0) OR
++      (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 20.0))
++    # Make signed overflow well-defined. Implies the following flags in clang-20 and above.
++    # -fwrapv - Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
++    # using twos-complement representation (this is normally undefined according to the C++ spec).
++    # -fwrapv-pointer - The same as -fwrapv but for pointers.
++    add_compile_options(-fno-strict-overflow)
++
++    # Suppress C++ strict aliasing rules. This matches our use of MSVC.
++    add_compile_options(-fno-strict-aliasing)
++  else()
++    # Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
++    # using twos-complement representation (this is normally undefined according to the C++ spec).
++    add_compile_options(-fwrapv)
++  endif()
+
+   if(CLR_CMAKE_HOST_APPLE)
+     # Clang will by default emit objc_msgSend stubs in Xcode 14, which ld from earlier Xcodes doesn't understand.
+diff --git a/src/coreclr/debug/di/rspriv.h b/src/coreclr/debug/di/rspriv.h
+index 7e2b49b3170..119ca6f7c08 100644
+--- a/src/coreclr/debug/di/rspriv.h
++++ b/src/coreclr/debug/di/rspriv.h
+@@ -6404,8 +6404,8 @@ private:
+     // Lazily initialized.
+     EXCEPTION_RECORD *  m_pExceptionRecord;
+
+-    static const CorDebugUserState kInvalidUserState = CorDebugUserState(-1);
+-    CorDebugUserState     m_userState;  // This is the current state of the
++    static const int kInvalidUserState = -1;
++    int                   m_userState;  // This is the current state of the
+                                         // thread, at the time that the
+                                         // left side synchronized
+
+diff --git a/src/coreclr/debug/di/rsthread.cpp b/src/coreclr/debug/di/rsthread.cpp
+index cd7f79867a5..8c4f3317eff 100644
+--- a/src/coreclr/debug/di/rsthread.cpp
++++ b/src/coreclr/debug/di/rsthread.cpp
+@@ -783,7 +783,7 @@ CorDebugUserState CordbThread::GetUserState()
+         m_userState = pDAC->GetUserState(m_vmThreadToken);
+     }
+
+-    return m_userState;
++    return (CorDebugUserState)m_userState;
+ }
+
+
+@@ -887,7 +887,7 @@ HRESULT CordbThread::CreateStepper(ICorDebugStepper ** ppStepper)
+ //Returns true if current user state of a thread is USER_WAIT_SLEEP_JOIN
+ bool CordbThread::IsThreadWaitingOrSleeping()
+ {
+-    CorDebugUserState userState = m_userState;
++    int userState = m_userState;
+     if (userState == kInvalidUserState)
+     {
+         //If m_userState is not ready, we'll read from DAC only part of it which
+@@ -3721,14 +3721,14 @@ HRESULT CordbUnmanagedThread::SetupFirstChanceHijackForSync()
+     LOG((LF_CORDB, LL_INFO10000, "CUT::SFCHFS: hijackCtx started as:\n"));
+     LogContext(GetHijackCtx());
+
+-    // Save the thread's full context for all platforms except for x86 because we need the 
++    // Save the thread's full context for all platforms except for x86 because we need the
+     // DT_CONTEXT_EXTENDED_REGISTERS to avoid getting incomplete information and corrupt the thread context
+     DT_CONTEXT context;
+-#ifdef TARGET_X86    
++#ifdef TARGET_X86
+     context.ContextFlags = DT_CONTEXT_FULL | DT_CONTEXT_EXTENDED_REGISTERS;
+ #else
+     context.ContextFlags = DT_CONTEXT_FULL;
+-#endif 
++#endif
+
+     BOOL succ = DbiGetThreadContext(m_handle, &context);
+     _ASSERTE(succ);
+@@ -3739,7 +3739,7 @@ HRESULT CordbUnmanagedThread::SetupFirstChanceHijackForSync()
+         LOG((LF_CORDB, LL_ERROR, "CUT::SFCHFS: DbiGetThreadContext error=0x%x\n", error));
+     }
+
+-#ifdef TARGET_X86    
++#ifdef TARGET_X86
+     GetHijackCtx()->ContextFlags = DT_CONTEXT_FULL | DT_CONTEXT_EXTENDED_REGISTERS;
+ #else
+     GetHijackCtx()->ContextFlags = DT_CONTEXT_FULL;