compiler/rustc_mir_transform/src/normalize_array_len.rs

//! 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

use crate::MirPass;
use rustc_data_structures::fx::FxIndexMap;
use rustc_data_structures::intern::Interned;
use rustc_index::bit_set::BitSet;
use rustc_index::vec::IndexVec;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, ReErased, Region, TyCtxt};

const MAX_NUM_BLOCKS: usize = 800;
const MAX_NUM_LOCALS: usize = 3000;

pub struct NormalizeArrayLen;

impl<'tcx> MirPass<'tcx> for NormalizeArrayLen {
    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
        // See #105929
        sess.mir_opt_level() >= 4 && sess.opts.unstable_opts.unsound_mir_opts
    }

    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        // early returns for edge cases of highly unrolled functions
        if body.basic_blocks.len() > MAX_NUM_BLOCKS {
            return;
        }
        if body.local_decls.len() > MAX_NUM_LOCALS {
            return;
        }
        normalize_array_len_calls(tcx, body)
    }
}

pub fn normalize_array_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
    // We don't ever touch terminators, so no need to invalidate the CFG cache
    let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
    let local_decls = &mut body.local_decls;

    // do a preliminary analysis to see if we ever have locals of type `[T;N]` or `&[T;N]`
    let mut interesting_locals = BitSet::new_empty(local_decls.len());
    for (local, decl) in local_decls.iter_enumerated() {
        match decl.ty.kind() {
            ty::Array(..) => {
                interesting_locals.insert(local);
            }
            ty::Ref(.., ty, Mutability::Not) => match ty.kind() {
                ty::Array(..) => {
                    interesting_locals.insert(local);
                }
                _ => {}
            },
            _ => {}
        }
    }
    if interesting_locals.is_empty() {
        // we have found nothing to analyze
        return;
    }
    let num_intesting_locals = interesting_locals.count();
    let mut state = FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
    let mut patches_scratchpad =
        FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
    let mut replacements_scratchpad =
        FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
    for block in basic_blocks {
        // make length calls for arrays [T; N] not to decay into length calls for &[T]
        // that forbids constant propagation
        normalize_array_len_call(
            tcx,
            block,
            local_decls,
            &interesting_locals,
            &mut state,
            &mut patches_scratchpad,
            &mut replacements_scratchpad,
        );
        state.clear();
        patches_scratchpad.clear();
        replacements_scratchpad.clear();
    }
}

struct Patcher<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    patches_scratchpad: &'a FxIndexMap<usize, usize>,
    replacements_scratchpad: &'a mut FxIndexMap<usize, Local>,
    local_decls: &'a mut IndexVec<Local, LocalDecl<'tcx>>,
    statement_idx: usize,
}

impl<'tcx> Patcher<'_, 'tcx> {
    fn patch_expand_statement(
        &mut self,
        statement: &mut Statement<'tcx>,
    ) -> Option<std::vec::IntoIter<Statement<'tcx>>> {
        let idx = self.statement_idx;
        if let Some(len_statemnt_idx) = self.patches_scratchpad.get(&idx).copied() {
            let mut statements = Vec::with_capacity(2);

            // we are at statement that performs a cast. The only sound way is
            // to create another local that performs a similar copy without a cast and then
            // use this copy in the Len operation

            match &statement.kind {
                StatementKind::Assign(box (
                    ..,
                    Rvalue::Cast(
                        CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
                        operand,
                        _,
                    ),
                )) => {
                    match operand {
                        Operand::Copy(place) | Operand::Move(place) => {
                            // create new local
                            let ty = operand.ty(self.local_decls, self.tcx);
                            let local_decl = LocalDecl::with_source_info(ty, statement.source_info);
                            let local = self.local_decls.push(local_decl);
                            // make it live
                            let mut make_live_statement = statement.clone();
                            make_live_statement.kind = StatementKind::StorageLive(local);
                            statements.push(make_live_statement);
                            // copy into it

                            let operand = Operand::Copy(*place);
                            let mut make_copy_statement = statement.clone();
                            let assign_to = Place::from(local);
                            let rvalue = Rvalue::Use(operand);
                            make_copy_statement.kind =
                                StatementKind::Assign(Box::new((assign_to, rvalue)));
                            statements.push(make_copy_statement);

                            // to reorder we have to copy and make NOP
                            statements.push(statement.clone());
                            statement.make_nop();

                            self.replacements_scratchpad.insert(len_statemnt_idx, local);
                        }
                        _ => {
                            unreachable!("it's a bug in the implementation")
                        }
                    }
                }
                _ => {
                    unreachable!("it's a bug in the implementation")
                }
            }

            self.statement_idx += 1;

            Some(statements.into_iter())
        } else if let Some(local) = self.replacements_scratchpad.get(&idx).copied() {
            let mut statements = Vec::with_capacity(2);

            match &statement.kind {
                StatementKind::Assign(box (into, Rvalue::Len(place))) => {
                    let add_deref = if let Some(..) = place.as_local() {
                        false
                    } else if let Some(..) = place.local_or_deref_local() {
                        true
                    } else {
                        unreachable!("it's a bug in the implementation")
                    };
                    // replace len statement
                    let mut len_statement = statement.clone();
                    let mut place = Place::from(local);
                    if add_deref {
                        place = self.tcx.mk_place_deref(place);
                    }
                    len_statement.kind =
                        StatementKind::Assign(Box::new((*into, Rvalue::Len(place))));
                    statements.push(len_statement);

                    // make temporary dead
                    let mut make_dead_statement = statement.clone();
                    make_dead_statement.kind = StatementKind::StorageDead(local);
                    statements.push(make_dead_statement);

                    // make original statement NOP
                    statement.make_nop();
                }
                _ => {
                    unreachable!("it's a bug in the implementation")
                }
            }

            self.statement_idx += 1;

            Some(statements.into_iter())
        } else {
            self.statement_idx += 1;
            None
        }
    }
}

fn normalize_array_len_call<'tcx>(
    tcx: TyCtxt<'tcx>,
    block: &mut BasicBlockData<'tcx>,
    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
    interesting_locals: &BitSet<Local>,
    state: &mut FxIndexMap<Local, usize>,
    patches_scratchpad: &mut FxIndexMap<usize, usize>,
    replacements_scratchpad: &mut FxIndexMap<usize, Local>,
) {
    for (statement_idx, statement) in block.statements.iter_mut().enumerate() {
        match &mut statement.kind {
            StatementKind::Assign(box (place, rvalue)) => {
                match rvalue {
                    Rvalue::Cast(
                        CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
                        operand,
                        cast_ty,
                    ) => {
                        let Some(local) = place.as_local() else { return };
                        match operand {
                            Operand::Copy(place) | Operand::Move(place) => {
                                let Some(operand_local) = place.local_or_deref_local() else { return; };
                                if !interesting_locals.contains(operand_local) {
                                    return;
                                }
                                let operand_ty = local_decls[operand_local].ty;
                                match (operand_ty.kind(), cast_ty.kind()) {
                                    (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
                                        if of_ty_src == of_ty_dst {
                                            // this is a cast from [T; N] into [T], so we are good
                                            state.insert(local, statement_idx);
                                        }
                                    }
                                    // current way of patching doesn't allow to work with `mut`
                                    (
                                        ty::Ref(
                                            Region(Interned(ReErased, _)),
                                            operand_ty,
                                            Mutability::Not,
                                        ),
                                        ty::Ref(
                                            Region(Interned(ReErased, _)),
                                            cast_ty,
                                            Mutability::Not,
                                        ),
                                    ) => {
                                        match (operand_ty.kind(), cast_ty.kind()) {
                                            // current way of patching doesn't allow to work with `mut`
                                            (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
                                                if of_ty_src == of_ty_dst {
                                                    // this is a cast from [T; N] into [T], so we are good
                                                    state.insert(local, statement_idx);
                                                }
                                            }
                                            _ => {}
                                        }
                                    }
                                    _ => {}
                                }
                            }
                            _ => {}
                        }
                    }
                    Rvalue::Len(place) => {
                        let Some(local) = place.local_or_deref_local() else {
                            return;
                        };
                        if let Some(cast_statement_idx) = state.get(&local).copied() {
                            patches_scratchpad.insert(cast_statement_idx, statement_idx);
                        }
                    }
                    _ => {
                        // invalidate
                        state.remove(&place.local);
                    }
                }
            }
            _ => {}
        }
    }

    let mut patcher = Patcher {
        tcx,
        patches_scratchpad: &*patches_scratchpad,
        replacements_scratchpad,
        local_decls,
        statement_idx: 0,
    };

    block.expand_statements(|st| patcher.patch_expand_statement(st));
}