[Support] Don't modify the current EH context during stack unwinding

Copy it instead. Otherwise, key registers (such as RBP) may get zeroed
out by the stack unwinder.

Fixes CrashRecoveryTest.DumpStackCleanup with MSVC in release builds.

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D73809

(cherry picked from commit b074acb82f7e75a189fa7933b09627241b166121)

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Support/Windows/Signals.inc b/llvm/lib/Support/Windows/Signals.inc
index 8b525f1bd4a..09e19ae41f1 100644
--- a/llvm/lib/Support/Windows/Signals.inc
+++ b/llvm/lib/Support/Windows/Signals.inc
@@ -820,7 +820,13 @@ static LONG WINAPI LLVMUnhandledExceptionFilter(LPEXCEPTION_POINTERS ep) {
                    << "\n";
   }
 
-  LocalPrintStackTrace(llvm::errs(), ep ? ep->ContextRecord : nullptr);
+  // Stack unwinding appears to modify the context. Copy it to preserve the
+  // caller's context.
+  CONTEXT ContextCopy;
+  if (ep)
+    memcpy(&ContextCopy, ep->ContextRecord, sizeof(ContextCopy));
+
+  LocalPrintStackTrace(llvm::errs(), ep ? &ContextCopy : nullptr);
 
   return EXCEPTION_EXECUTE_HANDLER;
 }