OpenTitan RISC-V toolchain

This directory contains the Bazel configuration for OpenTitan’s RISC-V toolchain.

This LLVM toolchain comes from the lowrisc-toolchains repository. See third_party/lowrisc/BUILD.lowrisc_toolchain.bazel for changing the toolchain version.

Configuration

There are four rules used to configure the toolchain:

1. cc_toolchain: groups flags, features, and tools into a toolchain.
2. cc_tool_map: assigns tools to actions.
3. cc_args: defines flags to add to tools based on actions.
4. cc_feature: allows cc_args flags to be conditionally enabled.

To add new flags to a tool in the toolchain, define a new cc_args target and assign it to some actions (e.g. compiling C code, linking, etc.). Add the new flags to cc_toolchain.args.

To make flags optional, define a new cc_feature for those cc_args. Features can be enabled at the command line using bazel --features=$feature_name. Add the flags to cc_toolchain.known_features and optionally to cc_toolchain.enabled_features.

Bazel has three built-in features called dbg, fastbuild, and opt that can be used to enable and disable flags at different optimization levels.