Sep 26, 2019 Tags: devblog, programming, rust
It’s a Rust tool announcement this time: llvm-passgen!1
llvm-passgen is a very small tool: all it does is generate valid build and source skeletons for LLVM passes.
This is a trivial task (it’s only two or three files, depending on how you count), but one that I frequently forget how to do correctly:
I consistently forget which LLVM CMake files I want to
include within my
llvm-passgen provides a sensible set of defaults. In particular,
LLVMConfig is always
included, preventing many of the annoying compiler/linker flag issues that show up with custom
Similarly, llvm-passgen uses LLVM’s
add_llvm_library helper to specify your pass.
No more fiddling with
add_library to approximate what LLVM expects from your build.
Setting up an LLVM pass is an exercise in tedium — there are a few basic lines and variables that every pass’s source code needs, and debugging their absence can be annoying. llvm-passgen’s pass templates ensure that your boilerplate is never forgotten.
Where appropriate, llvm-passgen’s templates provide
stubs. These are easy to forget and accidentally replace with constructor usage, which
LLVM explicitly disallows for basic block and function passes!
Using llvm-passgen is really easy. You can install it from
1 $ cargo install llvm-passgen
By default, llvm-passgen will generate
1 2 3 4 llvm-passgen Foo cd Foo/build cmake .. make
You can make it generate other kinds of passes with the
block are currently understood:
1 2 3 4 llvm-passgen Bar --kind module cd Bar/build cmake .. make
You can also ask llvm-passgen to construct the pass in a different parent directory with
1 2 3 4 llvm-passgen Baz --dest /tmp --kind block cd /tmp/Baz/build cmake .. make
Depending on your setup,
cmake .. may not discover LLVM’s CMake configuration by default.
If you see errors stemming from the
include directives, you can specify
1 2 3 cd build LLVM_DIR=/path/to/llvm/cmake cmake .. make
LLVM has several other useful passes beyond the ones currently supported by llvm-passgen —
adding support for e.g.
CallGraphSCCPass (among others) is an obvious next step.
Parameterizing the CMake template might also make sense — the generated
currently makes assumptions about LLVM’s build that appear to hold true for LLVM 6 and newer,
but may not be the case for older versions.