Change cg_ssa's get_param to borrow the builder mutably

[RDambrosio016]

This is a small change to make get_param more flexible for codegens that may need to modify things when retrieving function parameters.

This will currently only be used by rustc_codegen_nvvm (my own project), but may be useful to more codegens in the future.

This is needed because cg_nvvm needs to remap certain types to libnvvm-friendly types, such as i128 -> <2 x i64>. Because cg_ssa does not give mutable access to the builder, i resorted to using a mutex:

    fn get_param(&self, index: usize) -> Self::Value {
        let val = llvm::get_param(self.llfn(), index as c_uint);
        trace!("Get param `{:?}`", val);
        unsafe {
            let llfnty = LLVMRustGetFunctionType(self.llfn());
            let map = self.remapped_integer_args.borrow();
            if let Some((_, key)) = map.get(llfnty) {
                if let Some((_, new_ty)) = key.iter().find(|t| t.0 == index) {
                    trace!("Casting irregular param {:?} to {:?}", val, new_ty);
                    return transmute_llval(
                        *self.llbuilder.lock().unwrap(),
                        &self.cx,
                        val,
                        *new_ty,
                    );
                }
            }
            val
        }
    }

However, i predict this is pretty bad for performance, considering how much builders are called during codegen, so i would greatly appreciate having a more flexible API for this.

[rust-highfive]

Some changes occured to rustc_codegen_gcc

cc @antoyo

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @michaelwoerister (or someone else) soon.

Please see the contribution instructions for more information.

[RDambrosio016]

@rustbot label: +A-codegen +T-compiler

[bjorn3]

Makes sense to me. There are a lot more methods in cg_ssa that should probably take &mut self instead of using interior mutability.

r? @bjorn3

@bors r+

[bors]

📌 Commit 870b831 has been approved by bjorn3

[bjorn3]

@bors rollup=always