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

[via cfe-commits]

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@@ -125,6 +137,25 @@ class TypeDescriptor {
     return 1 << (TypeInfo >> 1);
   }
 
+  const char *getBitIntBitCountPointer() const {
+    DCHECK(isBitIntTy());
+    DCHECK(isSignedBitIntTy());
+    // Scan Name for zero and return the next address
+    const char *p = getTypeName();
+    while (*p != '\0')
+      ++p;
+    // Return the next address
+    return p + 1;
+  }
+
+  unsigned getIntegerBitCount() const {
+    DCHECK(isIntegerTy());
+    if (isSignedBitIntTy())
+      return *reinterpret_cast<const u32 *>(getBitIntBitCountPointer());
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earnol wrote:

Well. I do agree with this statement, however do we have a way to know whether target has a strict alignment or not? Preferably on the define level or you would suggest use always use memcpy? 
I considered this option, but was really reluctant to add memcpy dependency into libubsan.
So may be the better option is gather it byte by byte a strict alignment platform.

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