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| 1 | +//! This pass lowers calls to core::slice::len to just Len op. |
| 2 | +//! It should run before inlining! |
| 3 | +
|
| 4 | +use crate::transform::MirPass; |
| 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}; |
| 9 | + |
| 10 | +pub struct LowerSliceLenCalls; |
| 11 | + |
| 12 | +impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls { |
| 13 | + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| 14 | + lower_slice_len_calls(tcx, body) |
| 15 | + } |
| 16 | +} |
| 17 | + |
| 18 | +pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| 19 | + let language_items = tcx.lang_items(); |
| 20 | + let slice_len_fn_item_def_id = if let Some(slice_len_fn_item) = language_items.slice_len_fn() { |
| 21 | + slice_len_fn_item |
| 22 | + } else { |
| 23 | + // there is no language item to compare to :) |
| 24 | + return; |
| 25 | + }; |
| 26 | + |
| 27 | + let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut(); |
| 28 | + |
| 29 | + for block in basic_blocks { |
| 30 | + // lower `<[_]>::len` calls |
| 31 | + lower_slice_len_call(tcx, block, &*local_decls, slice_len_fn_item_def_id); |
| 32 | + } |
| 33 | +} |
| 34 | + |
| 35 | +struct SliceLenPatchInformation<'tcx> { |
| 36 | + add_statement: Statement<'tcx>, |
| 37 | + new_terminator_kind: TerminatorKind<'tcx>, |
| 38 | +} |
| 39 | + |
| 40 | +fn lower_slice_len_call<'tcx>( |
| 41 | + tcx: TyCtxt<'tcx>, |
| 42 | + block: &mut BasicBlockData<'tcx>, |
| 43 | + local_decls: &IndexVec<Local, LocalDecl<'tcx>>, |
| 44 | + slice_len_fn_item_def_id: DefId, |
| 45 | +) { |
| 46 | + let mut patch_found: Option<SliceLenPatchInformation<'_>> = None; |
| 47 | + |
| 48 | + let terminator = block.terminator(); |
| 49 | + match &terminator.kind { |
| 50 | + TerminatorKind::Call { |
| 51 | + func, |
| 52 | + args, |
| 53 | + destination: Some((dest, bb)), |
| 54 | + cleanup: None, |
| 55 | + from_hir_call: true, |
| 56 | + .. |
| 57 | + } => { |
| 58 | + // some heuristics for fast rejection |
| 59 | + if args.len() != 1 { |
| 60 | + return; |
| 61 | + } |
| 62 | + let arg = match args[0].place() { |
| 63 | + Some(arg) => arg, |
| 64 | + None => return, |
| 65 | + }; |
| 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) |
| 72 | + // goto bb1 |
| 73 | + |
| 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 = StatementKind::Assign(Box::new((*dest, r_value))); |
| 78 | + let add_statement = Statement { |
| 79 | + kind: len_statement_kind, |
| 80 | + source_info: terminator.source_info.clone(), |
| 81 | + }; |
| 82 | + |
| 83 | + // modify terminator into simple Goto |
| 84 | + let new_terminator_kind = TerminatorKind::Goto { target: bb.clone() }; |
| 85 | + |
| 86 | + let patch = SliceLenPatchInformation { add_statement, new_terminator_kind }; |
| 87 | + |
| 88 | + patch_found = Some(patch); |
| 89 | + } |
| 90 | + _ => {} |
| 91 | + } |
| 92 | + } |
| 93 | + _ => {} |
| 94 | + } |
| 95 | + |
| 96 | + if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found { |
| 97 | + block.statements.push(add_statement); |
| 98 | + block.terminator_mut().kind = new_terminator_kind; |
| 99 | + } |
| 100 | +} |
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