ASAN/MSAN: document use of sanitizers.

(cherry picked from commit 030ff40, ref #18067)

doc/devdocs/C.rst

@@ -12,3 +12,4 @@
    backtraces
    debuggingtips
    valgrind
+   sanitizers

doc/devdocs/sanitizers.rst

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+*****************
+Sanitizer support
+*****************
+
+General considerations
+----------------------
+
+Using Clang's sanitizers obviously require you to use Clang (``USECLANG=1``), but there's
+another catch: most sanitizers require a run-time library, provided by the host compiler,
+while the instrumented code generated by Julia's JIT relies on functionality from that
+library. This implies that the LLVM version of your host compiler matches that of the LLVM
+library used within Julia.
+
+An easy solution is to have an dedicated build folder for providing a matching toolchain, by
+building with ``BUILD_LLVM_CLANG=1`` and overriding ``LLVM_USE_CMAKE=1`` (Autotool-based
+builds are incompatible with ASAN). You can then refer to this toolchain from another build
+folder by specifying ``USECLANG=1`` while overriding the ``CC`` and ``CXX`` variables.
+
+
+Address Sanitizer (ASAN)
+------------------------
+
+For detecting or debugging memory bugs, you can use Clang's `address sanitizer (ASAN)
+<http://clang.llvm.org/docs/AddressSanitizer.html>`_. By compiling with
+``SANITIZE=1`` you enable ASAN for the Julia compiler and its generated code. In addition,
+you can specify ``LLVM_SANITIZE=1`` to sanitize the LLVM library as well. Note that these
+options incur a high performance and memory cost. For example, using ASAN for Julia and LLVM
+makes ``testall1`` takes 8-10 times as long while using 20 times as much memory (this can
+be reduced to respectively a factor of 3 and 4 by using the options described below).
+
+By default, Julia sets the ``allow_user_segv_handler=1`` ASAN flag, which is required for
+signal delivery to work properly. You can define other options using the ``ASAN_OPTIONS``
+environment flag, in which case you'll need to repeat the default option mentioned before.
+For example, memory usage can be reduced by specifying ``fast_unwind_on_malloc=0`` and
+``malloc_context_size=2``, at the cost of backtrace accuracy. For now, Julia also sets
+``detect_leaks=0``, but this should be removed in the future.
+
+
+Memory Sanitizer (MSAN)
+-----------------------
+
+For detecting use of uninitialized memory, you can use Clang's `memory sanitizer (MSAN)
+<http://clang.llvm.org/docs/MemorySanitizer.html>`_ by compiling with
+``SANITIZE_MEMORY=1``.