[clang] [compiler-rt] [ubsan] Display correct runtime messages for negative _BitInt (PR #96240)

[via cfe-commits]

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@@ -3324,6 +3359,20 @@ llvm::Constant *CodeGenFunction::EmitCheckTypeDescriptor(QualType T) {
       DiagnosticsEngine::ak_qualtype, (intptr_t)T.getAsOpaquePtr(), StringRef(),
       StringRef(), std::nullopt, Buffer, std::nullopt);
 
+  if (IsBitInt) {
+    // The Structure is: 0 to end the string, 32 bit unsigned integer in target
+    // endianness, zero.
+    char S[6] = {'\0', '\0', '\0', '\0', '\0', '\0'};
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earnol wrote:

It is not used explicitly and serves as a guard byte. The idea is to have 2 consecutive zero bytes at the end of the line. 

https://github.com/llvm/llvm-project/pull/96240