======================================
Adoption Guide for ``-fbounds-safety``
======================================

.. contents::
   :local:

Where to get ``-fbounds-safety``
================================

The open sourcing to llvm.org's ``llvm-project`` is still on going and the
feature is not available yet. In the mean time, the preview implementation is
available
`here <https://github.com/swiftlang/llvm-project/tree/stable/20240723>`_ in a
fork of ``llvm-project``. Please follow
`Building LLVM with CMake <https://llvm.org/docs/CMake.html>`_ to build the
compiler.

Feature flag
============

Pass ``-fbounds-safety`` as a Clang compilation flag for the C file that you
want to adopt. We recommend adopting the model file by file, because adoption
requires some effort to add bounds annotations and fix compiler diagnostics.

Include ``ptrcheck.h``
======================

``ptrcheck.h`` is a Clang toolchain header to provide definition of the bounds
annotations such as ``__counted_by``, ``__counted_by_or_null``, ``__sized_by``,
etc. In the LLVM source tree, the header is located in
``llvm-project/clang/lib/Headers/ptrcheck.h``.


Add bounds annotations on pointers as necessary
===============================================

Annotate pointers on struct fields and function parameters if they are pointing
to an array of object, with appropriate bounds annotations. Please see
:doc:`BoundsSafety` to learn what kind of bounds annotations are available and
their semantics. Note that local pointer variables typically don't need bounds
annotations because they are implicitly a wide pointer (``__bidi_indexable``)
that automatically carries the bounds information.

Address compiler diagnostics
============================

Once you pass ``-fbounds-safety`` to compile a C file, you will see some new
compiler warnings and errors, which guide adoption of ``-fbounds-safety``.
Consider the following example:

.. code-block:: c

   #include <ptrcheck.h>

   void init_buf(int *p, int n) {
      for (int i = 0; i < n; ++i)
         p[i] = 0; // error: array subscript on single pointer 'p' must use a constant index of 0 to be in bounds
   }

The parameter ``int *p`` doesn't have a bounds annotation, so the compiler will
complain about the code indexing into it (``p[i]``) as it assumes that ``p`` is
pointing to a single ``int`` object or null. To address the diagnostics, you
should add a bounds annotation on ``int *p`` so that the compiler can reason
about the safety of the array subscript. In the following example, ``p`` is now
``int *__counted_by(n)``, so the compiler will allow the array subscript with
additional run-time checks as necessary.

.. code-block:: c

   #include <ptrcheck.h>

   void init_buf(int *__counted_by(n) p, int n) {
      for (int i = 0; i < n; ++i)
         p[i] = 0; // ok; `p` is now has a type with bounds annotation.
   }

Run test suites to fix new run-time traps
=========================================

Adopting ``-fbounds-safety`` may cause your program to trap if it violates
bounds safety or it has incorrect adoption. Thus, it is necessary to perform
run-time testing of your program to gain confidence that it won't trap at
run time.

Repeat the process for each remaining file
==========================================

Once you've done with adopting a single C file, please repeat the same process
for each remaining C file that you want to adopt.