@adaamss3 There aren't instructions but the basic approach I would take is:
- Create a custom target-spec that turns on/off the cpu features you need.
- On a i686-class machine, configure rustc with
./configure --host=i686-unknown-linux-gnu,i686-unknown-linux-gnu-custom, where the latter triple is your custom target. - Build that to create a rustc that will run your i386, and use that compiler with your custom target spec to build additional Rust software.
If you want an i386-compatible rustc that doesn't need a custom target to produce i386-compatible code I think you would need to edit the built-in target spec when building your compiler.