perf: cranelift-codegen-meta: reduce llvm-lines in calls to Path::join

[dishmaker]

-0.1% less llvm lines

You can just close this PR if it's wasting your time 😄

[cfallin]

@dishmaker can I ask what the overall goal with these PRs is? You're optimizing the size of build-time meta-code, and in tiny (trivial 0.1% shifts) ways and in some places in ways that makes the code less idiomatic or harder to trace through (e.g. prior vec-push vs. array-literal PR), which doesn't really affect Cranelift's critical path in any real way as far as I'm aware. Is there a broader goal of making binaries smaller or somesuch?

[dishmaker]

I'm trying to reduce compile times in debug and binary size in release.

cranelift-codegen depends on:

• cranelift-codegen (build-script), which depends on:
  • cranelift-codegen-meta

[cfallin]

Sure, faster builds of the build script itself are a priority; we've indeed been careful to keep deps minimal there.

That said I'm not sure "0.1% fewer LLVM IR lines" justifies a lot of PRs, review traffic, and sometimes nontrivial changes to control flow if the build-time effect is also in that neighborhood (and it could be less if trivial differences are optimized away early in the LLVM pipeline). Have you measured a speedup in build time? I'm happy to see these through if you've got that data. Thanks!