# 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. You can then refer to this toolchain from another build folder by specifying USECLANG=1 while overriding the CC and CXX variables.

To use one of of the sanitizers set SANITIZE=1 and then the appropriate flag for the sanitizer you want to use.

On macOS, this might need some extra flags also to work. Altogether, it might look like this, plus one or more of the SANITIZE_* flags listed below:

make -C deps USE_BINARYBUILDER_LLVM=0 LLVM_VER=svn stage-llvm

make -C src SANITIZE=1 USECLANG=1 \
CC=~+/deps/scratch/llvm-svn/build_Release/bin/clang \
CXX=~+/deps/scratch/llvm-svn/build_Release/bin/clang++ \
CPPFLAGS="-isysroot $(xcode-select -p)/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk" \ CXXFLAGS="-isystem$(xcode-select -p)/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1"

(or put these into your Make.user, so you don't need to remember them every time).

For detecting or debugging memory bugs, you can use Clang's address sanitizer (ASAN). By compiling with SANITIZE_ADDRESS=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.
For detecting use of uninitialized memory, you can use Clang's memory sanitizer (MSAN) by compiling with SANITIZE_MEMORY=1.
For debugging data-races and other threading related issues you can use Clang's thread sanitizer (TSAN) by compiling with SANITIZE_THREAD=1.