NOnohuck13·Apr 7, 2021
Bazel differentiates between the "host" environment (your dev box) the "execution" environment (where the compiler runs) and the "target" environment (e.g. RISCv5)
Edit: there's a confusing number of ways of specifying these things in your build, e.g. old crosstool files, platforms/constraints, toolchains. A stylized 20k foot view is:
Each build target specifies two different kinds of inputs: sources (code, libraries) and "tools" (compilers). A reproducible build requires fully-specifying not just the sources but all the tools you use to build them.
Obviously cross-compiling for RISCv5 requires different compiler flags than x86_64. So instead of depending on "gcc" you'd depend on an abstract "toolchain" target which defines ways to invoke different version(s) of gcc based on your host, execution, and target platforms.
In practice, you
Read the rest
wouldn't write
toolchains yourself, you'd depend on existing implementations provided by library code, e.g. many many third party language rules here: https://github.com/jin/awesome-bazel#rules
And you _probably_ wouldn't depend on a specific toolchain in every single rule, you'd define a global one for your project.
"platforms" and "constraints" together let you define more fine-grained ways different environments differ (os, cpu, etc) to avoid enumerating the combinatoric explosion of build flavors across different dimensions.
HTH, caveat, I have not done cross-compilation in anger. Someone hopefully will correct me if my understanding is flawed.