[clang] [TySan] Add initial documentation for Type Sanitizer (PR #123595)

[Aaron Ballman via cfe-commits]

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+================
+TypeSanitizer
+================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+The TypeSanitizer is a detector for strict type aliasing violations. It consists of a compiler
+instrumentation module and a run-time library. C/C++ has type-based aliasing rules, and LLVM 
+can exploit these for optimizations given the TBAA metadata Clang emits. In general, a pointer 
+of a given type cannot access an object of a different type, with only a few exceptions. 
+
+These rules aren't always apparent to users, which leads to code that violates these rules
+(e.g. for type punning). This can lead to optimization passes introducing bugs unless the 
+code is build with ``-fno-strict-aliasing``, sacrificing performance.
+
+TypeSanitizer is built to catch when these strict aliasing rules have been violated, helping 
+users find where such bugs originate in their code despite the code looking valid at first glance.
+
+As TypeSanitizer is still experimental, it can currently have a large impact on runtime speed, 
+memory use, and code size.
+
+How to build
+============
+
+Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_ and enable
+the ``compiler-rt`` runtime. An example CMake configuration that will allow
+for the use/testing of TypeSanitizer:
+
+.. code-block:: console
+
+   $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" <path to source>/llvm
+
+Usage
+=====
+
+Compile and link your program with ``-fsanitize=type`` flag.  The
+TypeSanitizer run-time library should be linked to the final executable, so
+make sure to use ``clang`` (not ``ld``) for the final link step. To
+get a reasonable performance add ``-O1`` or higher.
+TypeSanitizer by default doesn't print the full stack trace in error messages. Use ``TYSAN_OPTIONS=print_stacktrace=1`` 
+to print the full trace. To get nicer stack traces in error messages add ``-fno-omit-frame-pointer`` and 
+``-g``.  To get perfect stack traces you may need to disable inlining (just use ``-O1``) and tail call elimination 
+(``-fno-optimize-sibling-calls``).
+
+.. code-block:: console
+
+    % cat example_AliasViolation.c
+    int main(int argc, char **argv) {
+      int x = 100;
+      float *y = (float*)&x;
+      *y += 2.0f;          // Strict aliasing violation
+      return 0;
+    }
+
+    # Compile and link
+    % clang++ -g -fsanitize=type example_AliasViolation.cc
+
+The program will print an error message to stderr each time a strict aliasing violation is detected. 
----------------
AaronBallman wrote:

```suggestion
The program will print an error message to ``stderr`` each time a strict aliasing violation is detected. 
```

https://github.com/llvm/llvm-project/pull/123595