[Clang][CodeGen] Add comments. NFC.

Author: dtcxzyw

clang/lib/CodeGen/CGExprScalar.cpp

@@ -4238,6 +4238,13 @@ static Value *emitPointerArithmetic(CodeGenFunction &CGF,
   else
     elemTy = CGF.ConvertTypeForMem(elementType);
 
+  // Some versions of glibc __PTR_ALIGN macro use idioms that add an index to a
+  // null pointer in order to cast the aligned integer value back to a pointer.
+  // This is undefined behavior, but we have to add a workaround for it.
+  //
+  // Unlike the above check `BinaryOperator::isNullPointerArithmeticExtension`,
+  // we still generate a GEP with a null-pointer base instead of inttoptr.
+  // This means that it's also UB to dereference a pointer created that way.
   if (CGF.getLangOpts().PointerOverflowDefined ||
       (!CGF.SanOpts.has(SanitizerKind::PointerOverflow) &&
        CGF.isUnderlyingBasePointerConstantNull(pointerOperand)))