1 //! This pass lowers calls to core::slice::len to just Len op.
2 //! It should run before inlining!
5 use rustc_hir::def_id::DefId;
6 use rustc_index::vec::IndexVec;
7 use rustc_middle::mir::*;
8 use rustc_middle::ty::{self, TyCtxt};
10 pub struct LowerSliceLenCalls;
12 impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls {
13 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
14 sess.mir_opt_level() > 0
17 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
18 lower_slice_len_calls(tcx, body)
22 pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
23 let language_items = tcx.lang_items();
24 let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
25 // there is no language item to compare to :)
29 // The one successor remains unchanged, so no need to invalidate
30 let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
31 for block in basic_blocks {
32 // lower `<[_]>::len` calls
33 lower_slice_len_call(tcx, block, &body.local_decls, slice_len_fn_item_def_id);
37 struct SliceLenPatchInformation<'tcx> {
38 add_statement: Statement<'tcx>,
39 new_terminator_kind: TerminatorKind<'tcx>,
42 fn lower_slice_len_call<'tcx>(
44 block: &mut BasicBlockData<'tcx>,
45 local_decls: &IndexVec<Local, LocalDecl<'tcx>>,
46 slice_len_fn_item_def_id: DefId,
48 let mut patch_found: Option<SliceLenPatchInformation<'_>> = None;
50 let terminator = block.terminator();
51 match &terminator.kind {
52 TerminatorKind::Call {
61 // some heuristics for fast rejection
65 let Some(arg) = args[0].place() else { return };
66 let func_ty = func.ty(local_decls, tcx);
67 match func_ty.kind() {
68 ty::FnDef(fn_def_id, _) if fn_def_id == &slice_len_fn_item_def_id => {
69 // perform modifications
70 // from something like `_5 = core::slice::<impl [u8]>::len(move _6) -> bb1`
71 // into `_5 = Len(*_6)
74 // make new RValue for Len
75 let deref_arg = tcx.mk_place_deref(arg);
76 let r_value = Rvalue::Len(deref_arg);
77 let len_statement_kind =
78 StatementKind::Assign(Box::new((*destination, r_value)));
80 Statement { kind: len_statement_kind, source_info: terminator.source_info };
82 // modify terminator into simple Goto
83 let new_terminator_kind = TerminatorKind::Goto { target: *bb };
85 let patch = SliceLenPatchInformation { add_statement, new_terminator_kind };
87 patch_found = Some(patch);
95 if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found {
96 block.statements.push(add_statement);
97 block.terminator_mut().kind = new_terminator_kind;