Provide a build of lli that can run current rustc's bytecode output

[mzabaluev]

I want to have a tool in the source tree of a project, to use it during the project's build.
It would be great to write the tool in Rust, but if I compile it into a native executable, there will be problems with cross-compiling builds. A better option would be to generate LLVM bytecode and run it with lli, but the version of LLVM provided by my Linux distribution (3.4) fails to run bytecode produced by Rust.

It seems that any user of the bytecode would need to build and install the LLVM used by Rust. Another issue is that it's currently a fork of upstream LLVM, so anything that requires the upstream LLVM on the same system may suffer compatibility problems in turn. It would be a better solution to build and install a rust-lli along with the compiler, at least until Rust can depend on a reasonably old stable version of the upstream LLVM.

[kmcallister]

I'm not sure I understand the use case for lli here. You can't take bytecode from rustc that thinks it's targetting one platform and then compile/run it on another platform. size_of::<int>() and processing of #[cfg] directives will be wrong, for example. It may work in some limited cases but it's not really a safe or reasonable thing to do.

Since you need to compile things differently for host vs. target, you might as well use the normal compilation path for both.

[thestinger]

It would be a better solution to build and install a rust-lli along with the compiler, at least until Rust can depend on a reasonably old stable version of the upstream LLVM.

This is already supported via the --llvm-root parameter for configure. The only things in Rust's fork of LLVM are a compile-time optimization and a NullCheckElimination pass. Both could probably be pushed upstream anyway.

[thestinger]

Since LLVM bitcode isn't portable across platforms / architectures, I don't think the use case here is valid.