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|
//! 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::{simplify, MirPass, MirSource};
use itertools::Itertools as _;
use rustc_middle::mir::*;
use rustc_middle::ty::{Ty, TyCtxt};
use rustc_target::abi::VariantIdx;
/// 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
// Avoid moving into ourselves.
|| local_0 == local_1
// 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!
s0.source_info = s2.source_info;
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 {
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::LlvmInlineAsm(..) => 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);
}
}
}
|
