README: Add compiler-rt cloning step

[jbruechert]

I hope this is correct, at least it makes it build again for me.

[bjorn3]

This shouldn't be needed. compiler-builtins which is built as part of the rust standard library should already provide all necessary intrinsics. What error do you get without?

[bjorn3]

Looks like compiler-builtins itself reads this env var if the "c" package feature is enabled, which it shouldn:t at least on x86. What arch do you use?

[jbruechert]

I'm on x86_64. The error I'm getting is

error: failed to run custom build command for `compiler_builtins v0.1.67`

Caused by:
  process didn't exit successfully: `/home/jonahb/dev/rust/rustc_codegen_gcc/build_sysroot/target/debug/build/compiler_builtins-8535d9b204370609/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=build.rs
  cargo:compiler-rt=/home/jonahb/.cargo/registry/src/i.8713187.xyz-1ecc6299db9ec823/compiler_builtins-0.1.67/compiler-rt
  cargo:rustc-cfg=feature="unstable"
  cargo:rustc-cfg=feature="mem-unaligned"

  --- stderr
  thread 'main' panicked at 'RUST_COMPILER_RT_ROOT is not set', /home/jonahb/.cargo/registry/src/i.8713187.xyz-1ecc6299db9ec823/compiler_builtins-0.1.67/build.rs:491:21
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[bjorn3]

I'm on x86_64 too. I have never seen this error before. I have no clue why this would happen on x86_64.

[antoyo]

That's related to my changes to support non-native integers: some builtins were missing in compiler-builtins, so I had to use compiler-rt to get them.

[antoyo]

Thanks!