Reimplement NormalizeArrayLen based on SsaLocals #107172
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,288 +1,104 @@ | ||
| //! This pass eliminates casting of arrays into slices when their length | ||
| //! is taken using `.len()` method. Handy to preserve information in MIR for const prop | ||
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| use crate::ssa::SsaLocals; | ||
| use crate::MirPass; | ||
| use rustc_index::vec::IndexVec; | ||
| use rustc_middle::mir::visit::*; | ||
| use rustc_middle::mir::*; | ||
| use rustc_middle::ty::{self, TyCtxt}; | ||
| use rustc_mir_dataflow::impls::borrowed_locals; | ||
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| pub struct NormalizeArrayLen; | ||
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| impl<'tcx> MirPass<'tcx> for NormalizeArrayLen { | ||
| fn is_enabled(&self, sess: &rustc_session::Session) -> bool { | ||
| sess.mir_opt_level() >= 3 | ||
| } | ||
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| #[instrument(level = "trace", skip(self, tcx, body))] | ||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| debug!(def_id = ?body.source.def_id()); | ||
| normalize_array_len_calls(tcx, body) | ||
| } | ||
| } | ||
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| fn normalize_array_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id()); | ||
| let borrowed_locals = borrowed_locals(body); | ||
| let ssa = SsaLocals::new(tcx, param_env, body, &borrowed_locals); | ||
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| let slice_lengths = compute_slice_length(tcx, &ssa, body); | ||
| debug!(?slice_lengths); | ||
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| Replacer { tcx, slice_lengths }.visit_body_preserves_cfg(body); | ||
| } | ||
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| fn compute_slice_length<'tcx>( | ||
| tcx: TyCtxt<'tcx>, | ||
| ssa: &SsaLocals, | ||
| body: &Body<'tcx>, | ||
| ) -> IndexVec<Local, Option<ty::Const<'tcx>>> { | ||
| let mut slice_lengths = IndexVec::from_elem(None, &body.local_decls); | ||
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| for (local, rvalue) in ssa.assignments(body) { | ||
| match rvalue { | ||
| Rvalue::Cast( | ||
| CastKind::Pointer(ty::adjustment::PointerCast::Unsize), | ||
| operand, | ||
| cast_ty, | ||
| ) => { | ||
| let operand_ty = operand.ty(body, tcx); | ||
| debug!(?operand_ty); | ||
| if let Some(operand_ty) = operand_ty.builtin_deref(true) | ||
| && let ty::Array(_, len) = operand_ty.ty.kind() | ||
| && let Some(cast_ty) = cast_ty.builtin_deref(true) | ||
| && let ty::Slice(..) = cast_ty.ty.kind() | ||
|
Comment on lines
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There was a problem hiding this comment. (This is just me trying to understand code rather than suggesting any particular change to be made – you can largely ignore this.) Is this check necessary at all? I guess it is a nice defense-in-depth measure, but what other things an unsizing cast could be to? A dynamic object? There was a problem hiding this comment. Yes, |
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| { | ||
| slice_lengths[local] = Some(*len); | ||
| } | ||
| } | ||
| // The length information is stored in the fat pointer, so we treat `operand` as a value. | ||
| Rvalue::Use(operand) => { | ||
| if let Some(rhs) = operand.place() && let Some(rhs) = rhs.as_local() { | ||
| slice_lengths[local] = slice_lengths[rhs]; | ||
| } | ||
| } | ||
| // The length information is stored in the fat pointer. | ||
| // Reborrowing copies length information from one pointer to the other. | ||
| Rvalue::Ref(_, _, rhs) | Rvalue::AddressOf(_, rhs) => { | ||
| if let [PlaceElem::Deref] = rhs.projection[..] { | ||
| slice_lengths[local] = slice_lengths[rhs.local]; | ||
| } | ||
| } | ||
| _ => {} | ||
| } | ||
| } | ||
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| slice_lengths | ||
| } | ||
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| struct Replacer<'tcx> { | ||
| tcx: TyCtxt<'tcx>, | ||
| slice_lengths: IndexVec<Local, Option<ty::Const<'tcx>>>, | ||
| } | ||
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| impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> { | ||
| fn tcx(&self) -> TyCtxt<'tcx> { | ||
| self.tcx | ||
| } | ||
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| fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, loc: Location) { | ||
| if let Rvalue::Len(place) = rvalue | ||
| && let [PlaceElem::Deref] = &place.projection[..] | ||
| && let Some(len) = self.slice_lengths[place.local] | ||
| { | ||
| *rvalue = Rvalue::Use(Operand::Constant(Box::new(Constant { | ||
| span: rustc_span::DUMMY_SP, | ||
| user_ty: None, | ||
| literal: ConstantKind::from_const(len, self.tcx), | ||
| }))); | ||
| } | ||
| self.super_rvalue(rvalue, loc); | ||
| } | ||
| } | ||
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Shouldn’t the comment above be removed at this point?