Struct rustc_mir::transform::nrvo::RenameReturnPlace

pub struct RenameReturnPlace;

This pass looks for MIR that always copies the same local into the return place and eliminates the copy by renaming all uses of that local to _0.

This allows LLVM to perform an optimization similar to the named return value optimization (NRVO) that is guaranteed in C++. This avoids a stack allocation and memcpy for the relatively common pattern of allocating a buffer on the stack, mutating it, and returning it by value like so:

fn foo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
    let mut buf = [0; 1024];
    init(&mut buf);
    buf
}

For now, this pass is very simple and only capable of eliminating a single copy. A more general version of copy propagation, such as the one based on non-overlapping live ranges in #47954 and #71003, could yield even more benefits.

Trait Implementations

impl<'tcx> MirPass<'tcx> for RenameReturnPlace

fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>)

fn name(&self) -> Cow<'_, str>