[clang] [Clang] add wraps and no_wraps attributes (PR #115094)

[Justin Stitt via cfe-commits]

================
@@ -433,6 +433,26 @@ Attribute Changes in Clang
 - Fix a bug where clang doesn't automatically apply the ``[[gsl::Owner]]`` or
   ``[[gsl::Pointer]]`` to STL explicit template specialization decls. (#GH109442)
 
+- Introduced ``__attribute__((wraps))`` which can be added to type or variable
+  declarations. Using an attributed type or variable in an arithmetic
+  expression will define the overflow behavior for that expression as having
+  two's complement wrap-around. These expressions will not be instrumented by
+  overflow sanitizers nor will they cause integer overflow warnings. They also
+  cannot be optimized away by some eager UB optimizations as the behavior of
+  the arithmetic is no longer "undefined".
+
+  There is also ``__attribute__((no_wraps))`` which can be added to types or
+  variable declarations. Types or variables with this attribute may be
+  instrumented by overflow sanitizers, if enabled. Note that this matches the
+  default behavior of integer types. So, in most cases, ``no_wraps`` serves
----------------
JustinStitt wrote:

When used alongside very strict SSCLs there is great utility to be gained from `no_wraps`.

In the kernel we want to use an SSCL ignorelist like this:
```
[{signed-integer-overflow,unsigned-integer-overflow}]
type:*
```

then annotate specific types like `size_t` in source:

``` cpp
typedef __kernel_size_t    size_t __attribute__((no_wraps));
```


https://github.com/llvm/llvm-project/pull/115094