================
@@ -2489,6 +2489,81 @@ are listed below.
$ clang -fuse-ld=lld -Oz -Wl,--icf=safe -fcodegen-data-use code.cc
+Strict Aliasing
+---------------
+
+The C and C++ standards require accesses to objects in memory to use l-values
of
+an appropriate type for the object. This is called *strict aliasing* or
+*type-based alias analysis*. Strict aliasing enhances a variety of powerful
+memory optimizations, including reordering, combining, and eliminating memory
+accesses. These optimizations can lead to unexpected behavior in code that
+violates the strict aliasing rules. For example:
+
+.. code-block:: c++
+
+ void advance(size_t *index, double *data) {
+ double value = data[*index];
+ /* Clang may assume that this store does not change the contents of
`data`. */
+ *index += 1;
+ /* Clang may assume that this store does not change the contents of
`index`. */
+ data[*index] = value;
+ /* Either of these facts may create significant optimization
opportunities
+ if Clang is able to inline this function. */
+ }
+
+Strict aliasing can be explicitly enabled with ``-fstrict-aliasing`` and
+disabled with ``-fno-strict-aliasing``. ``clang-cl`` defaults to
+``-fno-strict-aliasing``; see :ref:`Strict aliasing in clang-cl.
+<clang_cl_strict_aliasing>`. Otherwise, Clang defaults to
``-fstrict-aliasing``.
+
+C and C++ specify slightly different rules for strict aliasing. To improve
+language interoperability, Clang allows two types to alias if either language
+would permit it. This includes applying the C++ similar types rule to C,
+allowing ``int **`` to alias ``int const * const *``. Clang also relaxes the
+standard aliasing rules in the following ways:
+
+* All integer types of the same size are permitted to alias each other,
+ including signed and unsigned types.
+* ``void*`` is permitted to alias any pointer type, ``void**`` is permitted to
+ alias any pointer to pointer type, and so on.
+
+Code which violates strict aliasing has undefined behavior. A program that
+works in one version of Clang may not work in another because of changes to the
+optimizer. Clang provides a `:ref:TypeSanitizer <TypeSanitizer>` to help detect
----------------
rjmccall wrote:
```suggestion
optimizer. Clang provides a :ref:`TypeSanitizer <TypeSanitizer>` to help detect
```
https://github.com/llvm/llvm-project/pull/122116
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