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.opts.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, local_decls, _) =
31 body.basic_blocks_local_decls_mut_and_var_debug_info_no_invalidate();
33 for block in basic_blocks {
34 // lower `<[_]>::len` calls
35 lower_slice_len_call(tcx, block, &*local_decls, slice_len_fn_item_def_id);
39 struct SliceLenPatchInformation<'tcx> {
40 add_statement: Statement<'tcx>,
41 new_terminator_kind: TerminatorKind<'tcx>,
44 fn lower_slice_len_call<'tcx>(
46 block: &mut BasicBlockData<'tcx>,
47 local_decls: &IndexVec<Local, LocalDecl<'tcx>>,
48 slice_len_fn_item_def_id: DefId,
50 let mut patch_found: Option<SliceLenPatchInformation<'_>> = None;
52 let terminator = block.terminator();
53 match &terminator.kind {
54 TerminatorKind::Call {
63 // some heuristics for fast rejection
67 let Some(arg) = args[0].place() else { return };
68 let func_ty = func.ty(local_decls, tcx);
69 match func_ty.kind() {
70 ty::FnDef(fn_def_id, _) if fn_def_id == &slice_len_fn_item_def_id => {
71 // perform modifications
72 // from something like `_5 = core::slice::<impl [u8]>::len(move _6) -> bb1`
73 // into `_5 = Len(*_6)
76 // make new RValue for Len
77 let deref_arg = tcx.mk_place_deref(arg);
78 let r_value = Rvalue::Len(deref_arg);
79 let len_statement_kind =
80 StatementKind::Assign(Box::new((*destination, r_value)));
82 Statement { kind: len_statement_kind, source_info: terminator.source_info };
84 // modify terminator into simple Goto
85 let new_terminator_kind = TerminatorKind::Goto { target: *bb };
87 let patch = SliceLenPatchInformation { add_statement, new_terminator_kind };
89 patch_found = Some(patch);
97 if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found {
98 block.statements.push(add_statement);
99 block.terminator_mut().kind = new_terminator_kind;