Crate llvm_plugin

Expand description

This crate gives the ability to safely implement passes for the new LLVM pass manager, by leveraging the strongly typed interface provided by Inkwell.

If you have never developed LLVM passes before, you can take a look at the available examples. They will (hopefully) give you a better idea of how to use this crate.

If you want a deeper understanding of the many concepts surrounding the new LLVM pass manager, you should read the official LLVM documentation.

Getting started

An LLVM plugin is merely a dylib that is given a PassBuilder by the LLVM tool (e.g. opt, lld) loading it. A PassBuilder allows registering callbacks on specific actions being performed by the LLVM tool.

For instance, the --passes parameter of opt allows specifying a custom pass pipeline to be run on a given IR module. A plugin could therefore register a callback for parsing an element of the given pipeline (e.g. a pass name), in order to insert a custom pass to run by opt.

The following code illustrates the idea:

// A name and version is required.
#[llvm_plugin::plugin(name = "plugin_name", version = "0.1")]
fn plugin_registrar(builder: &mut PassBuilder) {
    // Add a callback to parse a name from the textual representation of
    // the pipeline to be run.
    builder.add_module_pipeline_parsing_callback(|name, manager| {
        if name == "custom-pass" {
            // the input pipeline contains the name "custom-pass",
            // so we add our custom pass to the pass manager
            manager.add_pass(CustomPass);

            // we notify the caller that we were able to parse
            // the given name
            PipelineParsing::Parsed
        } else {
            // in any other cases, we notify the caller that our
            // callback wasn't able to parse the given name
            PipelineParsing::NotParsed
        }
    });
}

struct CustomPass;
impl LlvmModulePass for CustomPass {
    fn run_pass(
        &self,
        module: &mut Module,
        manager: &ModuleAnalysisManager
    ) -> PreservedAnalyses {
        // transform the IR
    }
}

Now, executing this command would run our custom pass on the input module:

opt --load-pass-plugin=libplugin.so --passes=custom-pass module.bc -disable-output

However, executing this command would not (custom-pass2 cannot be parsed by our plugin):

opt --load-pass-plugin=libplugin.so --passes=custom-pass2 module.bc -disable-output

More callbacks are available, read the PassBuilder documentation for more details.

A note on Windows

On this platform, LLVM plugins need the LLVM symbols directly from the executable loading them (most of the time opt.exe or lld.exe). Therefore, you need to specify the additional feature win-link-opt or win-link-lld while building a plugin. The former will link the plugin to opt.lib, the latter being for lld.lib.