src/librustc_mir/transform/simplify_try.rs

//! The general point of the optimizations provided here is to simplify something like:
//!
//! ```rust
//! match x {
//!     Ok(x) => Ok(x),
//!     Err(x) => Err(x)
//! }
//! ```
//!
//! into just `x`.

use crate::transform::{MirPass, MirSource, simplify};
use rustc::ty::{TyCtxt, Ty};
use rustc::mir::*;
use rustc_target::abi::VariantIdx;
use itertools::Itertools as _;

/// Simplifies arms of form `Variant(x) => Variant(x)` to just a move.
///
/// This is done by transforming basic blocks where the statements match:
///
/// ```rust
/// _LOCAL_TMP = ((_LOCAL_1 as Variant ).FIELD: TY );
/// ((_LOCAL_0 as Variant).FIELD: TY) = move _LOCAL_TMP;
/// discriminant(_LOCAL_0) = VAR_IDX;
/// ```
///
/// into:
///
/// ```rust
/// _LOCAL_0 = move _LOCAL_1
/// ```
pub struct SimplifyArmIdentity;

impl<'tcx> MirPass<'tcx> for SimplifyArmIdentity {
    fn run_pass(&self, _: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
        let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
        for bb in basic_blocks {
            // Need 3 statements:
            let (s0, s1, s2) = match &mut *bb.statements {
                [s0, s1, s2] => (s0, s1, s2),
                _ => continue,
            };

            // Pattern match on the form we want:
            let (local_tmp_s0, local_1, vf_s0) = match match_get_variant_field(s0) {
                None => continue,
                Some(x) => x,
            };
            let (local_tmp_s1, local_0, vf_s1) = match match_set_variant_field(s1) {
                None => continue,
                Some(x) => x,
            };
            if local_tmp_s0 != local_tmp_s1
                // The field-and-variant information match up.
                || vf_s0 != vf_s1
                // Source and target locals have the same type.
                // FIXME(Centril | oli-obk): possibly relax to same layout?
                || local_decls[local_0].ty != local_decls[local_1].ty
                // We're setting the discriminant of `local_0` to this variant.
                || Some((local_0, vf_s0.var_idx)) != match_set_discr(s2)
            {
                continue;
            }

            // Right shape; transform!
            match &mut s0.kind {
                StatementKind::Assign(box (place, rvalue)) => {
                    *place = local_0.into();
                    *rvalue = Rvalue::Use(Operand::Move(local_1.into()));
                }
                _ => unreachable!(),
            }
            s1.make_nop();
            s2.make_nop();
        }
    }
}

/// Match on:
/// ```rust
/// _LOCAL_INTO = ((_LOCAL_FROM as Variant).FIELD: TY);
/// ```
fn match_get_variant_field<'tcx>(stmt: &Statement<'tcx>) -> Option<(Local, Local, VarField<'tcx>)> {
    match &stmt.kind {
        StatementKind::Assign(box (place_into, rvalue_from)) => match rvalue_from {
            Rvalue::Use(Operand::Copy(pf)) | Rvalue::Use(Operand::Move(pf)) => {
                let local_into = place_into.as_local()?;
                let (local_from, vf) = match_variant_field_place(&pf)?;
                Some((local_into, local_from, vf))
            }
            _ => None,
        },
        _ => None,
    }
}

/// Match on:
/// ```rust
/// ((_LOCAL_FROM as Variant).FIELD: TY) = move _LOCAL_INTO;
/// ```
fn match_set_variant_field<'tcx>(stmt: &Statement<'tcx>) -> Option<(Local, Local, VarField<'tcx>)> {
    match &stmt.kind {
        StatementKind::Assign(box (place_from, rvalue_into)) => match rvalue_into {
            Rvalue::Use(Operand::Move(place_into)) => {
                let local_into = place_into.as_local()?;
                let (local_from, vf) = match_variant_field_place(&place_from)?;
                Some((local_into, local_from, vf))
            }
            _ => None,
        },
        _ => None,
    }
}

/// Match on:
/// ```rust
/// discriminant(_LOCAL_TO_SET) = VAR_IDX;
/// ```
fn match_set_discr<'tcx>(stmt: &Statement<'tcx>) -> Option<(Local, VariantIdx)> {
    match &stmt.kind {
        StatementKind::SetDiscriminant { place, variant_index } => Some((
            place.as_local()?,
            *variant_index
        )),
        _ => None,
    }
}

#[derive(PartialEq)]
struct VarField<'tcx> {
    field: Field,
    field_ty: Ty<'tcx>,
    var_idx: VariantIdx,
}

/// Match on `((_LOCAL as Variant).FIELD: TY)`.
fn match_variant_field_place<'tcx>(place: &Place<'tcx>) -> Option<(Local, VarField<'tcx>)> {
    match place.as_ref() {
        PlaceRef {
            base: &PlaceBase::Local(local),
            projection: &[ProjectionElem::Downcast(_, var_idx), ProjectionElem::Field(field, ty)],
        } => Some((local, VarField { field, field_ty: ty, var_idx })),
        _ => None,
    }
}

/// Simplifies `SwitchInt(_) -> [targets]`,
/// where all the `targets` have the same form,
/// into `goto -> target_first`.
pub struct SimplifyBranchSame;

impl<'tcx> MirPass<'tcx> for SimplifyBranchSame {
    fn run_pass(&self, _: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
        let mut did_remove_blocks = false;
        let bbs = body.basic_blocks_mut();
        for bb_idx in bbs.indices() {
            let targets = match &bbs[bb_idx].terminator().kind {
                TerminatorKind::SwitchInt { targets, .. } => targets,
                _ => continue,
            };

            let mut iter_bbs_reachable = targets
                .iter()
                .map(|idx| (*idx, &bbs[*idx]))
                .filter(|(_, bb)| {
                    // Reaching `unreachable` is UB so assume it doesn't happen.
                    bb.terminator().kind != TerminatorKind::Unreachable
                    // But `asm!(...)` could abort the program,
                    // so we cannot assume that the `unreachable` terminator itself is reachable.
                    // FIXME(Centril): use a normalization pass instead of a check.
                    || bb.statements.iter().any(|stmt| match stmt.kind {
                        StatementKind::InlineAsm(..) => true,
                        _ => false,
                    })
                })
                .peekable();

            // We want to `goto -> bb_first`.
            let bb_first = iter_bbs_reachable
                .peek()
                .map(|(idx, _)| *idx)
                .unwrap_or(targets[0]);

            // All successor basic blocks should have the exact same form.
            let all_successors_equivalent = iter_bbs_reachable
                .map(|(_, bb)| bb)
                .tuple_windows()
                .all(|(bb_l, bb_r)| {
                    bb_l.is_cleanup == bb_r.is_cleanup
                    && bb_l.terminator().kind == bb_r.terminator().kind
                    && bb_l.statements.iter().eq_by(&bb_r.statements, |x, y| x.kind == y.kind)
                });

            if all_successors_equivalent {
                // Replace `SwitchInt(..) -> [bb_first, ..];` with a `goto -> bb_first;`.
                bbs[bb_idx].terminator_mut().kind = TerminatorKind::Goto { target: bb_first };
                did_remove_blocks = true;
            }
        }

        if did_remove_blocks {
            // We have dead blocks now, so remove those.
            simplify::remove_dead_blocks(body);
        }
    }
}