Note seeing LLVM output from rust

Seeing LLVM output from Rust

Short answer

In your build directory, invoke the Rust compiler with the --save-temps flag. It will look something like:

stage2/bin/rustc ../rust/src/test/run-pass/hello.rs -o test/run-pass/hello.stage2 -O --save-temps

An LLVM bitcode file will now be sitting in test/run-pass/hello.bc. Run it through llvm-dis (which may be found in, e.g., rust/llvm/x86_64-unknown-linux-gnu/Release+Asserts/bin/):

llvm-dis test/run-pass/hello.bc

This will produce a .ll file that you can look at:

less test/run-pass/hello.ll

Longer answer

Recall that if the test you want to run is in test/run-pass/hello.rs, the invocation (as described in The Rust test suite page) is

make check TESTNAME=test/run-pass/hello.rs

If you stick a VERBOSE=1 in front of that, you’ll see what’s happening under the hood. So, for instance:

VERBOSE=1 make check TESTNAME=test/run-pass/hello.rs

will spit out a bunch of verbose output, including (if you scroll up a ways) various calls to compiletest, the test runner, one of which will look something like:

stage2/bin/compiletest --compile-lib-path stage2/lib --run-lib-path stage2/lib/rustc/i686-unknown-linux-gnu/lib --rustc-path stage2/bin/rustc --stage-id stage2 --rustcflags "-O" test/run-pass/hello.rs --verbose  --src-base ../rust/src/test/run-pass/ --build-base test/run-pass/ --mode run-pass --runtool "/usr/bin/valgrind --leak-check=full --error-exitcode=100 --quiet --suppressions=../rust/src/etc/x86.supp"

You can manually add the --save-temps flag to the --rustcflags list, and the test runner will pass it along to the compiler. So the entire invocation, for this particular test, becomes:

stage2/bin/compiletest --compile-lib-path stage2/lib --run-lib-path stage2/lib/rustc/i686-unknown-linux-gnu/lib --rustc-path stage2/bin/rustc --stage-id stage2 --rustcflags "-O --save-temps" test/run-pass/hello.rs --verbose  --src-base ../rust/src/test/run-pass/ --build-base test/run-pass/ --mode run-pass --runtool "/usr/bin/valgrind --leak-check=full --error-exitcode=100 --quiet --suppressions=../rust/src/etc/x86.supp"

After doing that, your LLVM bitcode file will be sitting in test/run-pass/hello.bc and you can disassemble it with llvm-dis as described above.