<del> //! The general point of the optimizations provided here is to simplify something like: //! //! ```rust //! # fn foo<T, E>(x: Result<T, E>) -> Result<T, E> { //! match x { //!     Ok(x) => Ok(x), //!     Err(x) => Err(x) //! } //! # } //! ``` //! //! into just `x`. use crate::{simplify, MirPass}; use itertools::Itertools as _; use rustc_index::{bit_set::BitSet, vec::IndexVec}; use rustc_middle::mir::visit::{NonUseContext, PlaceContext, Visitor}; use rustc_middle::mir::*; use rustc_middle::ty::{self, List, Ty, TyCtxt}; use rustc_target::abi::VariantIdx; use std::iter::{once, Enumerate, Peekable}; use std::slice::Iter; /// Simplifies arms of form `Variant(x) => Variant(x)` to just a move. /// /// This is done by transforming basic blocks where the statements match: /// /// ```ignore (MIR) /// _LOCAL_TMP = ((_LOCAL_1 as Variant ).FIELD: TY ); /// _TMP_2 = _LOCAL_TMP; /// ((_LOCAL_0 as Variant).FIELD: TY) = move _TMP_2; /// discriminant(_LOCAL_0) = VAR_IDX; /// ``` /// /// into: /// /// ```ignore (MIR) /// _LOCAL_0 = move _LOCAL_1 /// ``` pub struct SimplifyArmIdentity; #[derive(Debug)] struct ArmIdentityInfo<'tcx> { /// Storage location for the variant's field local_temp_0: Local, /// Storage location holding the variant being read from local_1: Local, /// The variant field being read from vf_s0: VarField<'tcx>, /// Index of the statement which loads the variant being read get_variant_field_stmt: usize, /// Tracks each assignment to a temporary of the variant's field field_tmp_assignments: Vec<(Local, Local)>, /// Storage location holding the variant's field that was read from local_tmp_s1: Local, /// Storage location holding the enum that we are writing to local_0: Local, /// The variant field being written to vf_s1: VarField<'tcx>, /// Storage location that the discriminant is being written to set_discr_local: Local, /// The variant being written set_discr_var_idx: VariantIdx, /// Index of the statement that should be overwritten as a move stmt_to_overwrite: usize, /// SourceInfo for the new move source_info: SourceInfo, /// Indices of matching Storage{Live,Dead} statements encountered. /// (StorageLive index,, StorageDead index, Local) storage_stmts: Vec<(usize, usize, Local)>, /// The statements that should be removed (turned into nops) stmts_to_remove: Vec<usize>, /// Indices of debug variables that need to be adjusted to point to // `{local_0}.{dbg_projection}`. dbg_info_to_adjust: Vec<usize>, /// The projection used to rewrite debug info. dbg_projection: &'tcx List<PlaceElem<'tcx>>, } fn get_arm_identity_info<'a, 'tcx>( stmts: &'a [Statement<'tcx>], locals_count: usize, debug_info: &'a [VarDebugInfo<'tcx>], ) -> Option<ArmIdentityInfo<'tcx>> { // This can't possibly match unless there are at least 3 statements in the block // so fail fast on tiny blocks. if stmts.len() < 3 { return None; } let mut tmp_assigns = Vec::new(); let mut nop_stmts = Vec::new(); let mut storage_stmts = Vec::new(); let mut storage_live_stmts = Vec::new(); let mut storage_dead_stmts = Vec::new(); type StmtIter<'a, 'tcx> = Peekable<Enumerate<Iter<'a, Statement<'tcx>>>>; fn is_storage_stmt(stmt: &Statement<'_>) -> bool { matches!(stmt.kind, StatementKind::StorageLive(_) | StatementKind::StorageDead(_)) } /// Eats consecutive Statements which match `test`, performing the specified `action` for each. /// The iterator `stmt_iter` is not advanced if none were matched. fn try_eat<'a, 'tcx>( stmt_iter: &mut StmtIter<'a, 'tcx>, test: impl Fn(&'a Statement<'tcx>) -> bool, mut action: impl FnMut(usize, &'a Statement<'tcx>), ) { while stmt_iter.peek().map_or(false, |(_, stmt)| test(stmt)) { let (idx, stmt) = stmt_iter.next().unwrap(); action(idx, stmt); } } /// Eats consecutive `StorageLive` and `StorageDead` Statements. /// The iterator `stmt_iter` is not advanced if none were found. fn try_eat_storage_stmts( stmt_iter: &mut StmtIter<'_, '_>, storage_live_stmts: &mut Vec<(usize, Local)>, storage_dead_stmts: &mut Vec<(usize, Local)>, ) { try_eat(stmt_iter, is_storage_stmt, |idx, stmt| { if let StatementKind::StorageLive(l) = stmt.kind { storage_live_stmts.push((idx, l)); } else if let StatementKind::StorageDead(l) = stmt.kind { storage_dead_stmts.push((idx, l)); } }) } fn is_tmp_storage_stmt(stmt: &Statement<'_>) -> bool { use rustc_middle::mir::StatementKind::Assign; if let Assign(box (place, Rvalue::Use(Operand::Copy(p) | Operand::Move(p)))) = &stmt.kind { place.as_local().is_some() && p.as_local().is_some() } else { false } } /// Eats consecutive `Assign` Statements. // The iterator `stmt_iter` is not advanced if none were found. fn try_eat_assign_tmp_stmts( stmt_iter: &mut StmtIter<'_, '_>, tmp_assigns: &mut Vec<(Local, Local)>, nop_stmts: &mut Vec<usize>, ) { try_eat(stmt_iter, is_tmp_storage_stmt, |idx, stmt| { use rustc_middle::mir::StatementKind::Assign; if let Assign(box (place, Rvalue::Use(Operand::Copy(p) | Operand::Move(p)))) = &stmt.kind { tmp_assigns.push((place.as_local().unwrap(), p.as_local().unwrap())); nop_stmts.push(idx); } }) } fn find_storage_live_dead_stmts_for_local( local: Local, stmts: &[Statement<'_>], ) -> Option<(usize, usize)> { trace!("looking for {:?}", local); let mut storage_live_stmt = None; let mut storage_dead_stmt = None; for (idx, stmt) in stmts.iter().enumerate() { if stmt.kind == StatementKind::StorageLive(local) { storage_live_stmt = Some(idx); } else if stmt.kind == StatementKind::StorageDead(local) { storage_dead_stmt = Some(idx); } } Some((storage_live_stmt?, storage_dead_stmt.unwrap_or(usize::MAX))) } // Try to match the expected MIR structure with the basic block we're processing. // We want to see something that looks like: // ``` // (StorageLive(_) | StorageDead(_));* // _LOCAL_INTO = ((_LOCAL_FROM as Variant).FIELD: TY); // (StorageLive(_) | StorageDead(_));* // (tmp_n+1 = tmp_n);* // (StorageLive(_) | StorageDead(_));* // (tmp_n+1 = tmp_n);* // ((LOCAL_FROM as Variant).FIELD: TY) = move tmp; // discriminant(LOCAL_FROM) = VariantIdx; // (StorageLive(_) | StorageDead(_));* // ``` let mut stmt_iter = stmts.iter().enumerate().peekable(); try_eat_storage_stmts(&mut stmt_iter, &mut storage_live_stmts, &mut storage_dead_stmts); let (get_variant_field_stmt, stmt) = stmt_iter.next()?; let (local_tmp_s0, local_1, vf_s0, dbg_projection) = match_get_variant_field(stmt)?; try_eat_storage_stmts(&mut stmt_iter, &mut storage_live_stmts, &mut storage_dead_stmts); try_eat_assign_tmp_stmts(&mut stmt_iter, &mut tmp_assigns, &mut nop_stmts); try_eat_storage_stmts(&mut stmt_iter, &mut storage_live_stmts, &mut storage_dead_stmts); try_eat_assign_tmp_stmts(&mut stmt_iter, &mut tmp_assigns, &mut nop_stmts); let (idx, stmt) = stmt_iter.next()?; let (local_tmp_s1, local_0, vf_s1) = match_set_variant_field(stmt)?; nop_stmts.push(idx); let (idx, stmt) = stmt_iter.next()?; let (set_discr_local, set_discr_var_idx) = match_set_discr(stmt)?; let discr_stmt_source_info = stmt.source_info; nop_stmts.push(idx); try_eat_storage_stmts(&mut stmt_iter, &mut storage_live_stmts, &mut storage_dead_stmts); for (live_idx, live_local) in storage_live_stmts { if let Some(i) = storage_dead_stmts.iter().rposition(|(_, l)| *l == live_local) { let (dead_idx, _) = storage_dead_stmts.swap_remove(i); storage_stmts.push((live_idx, dead_idx, live_local)); if live_local == local_tmp_s0 { nop_stmts.push(get_variant_field_stmt); } } } // We sort primitive usize here so we can use unstable sort nop_stmts.sort_unstable(); // Use one of the statements we're going to discard between the point // where the storage location for the variant field becomes live and // is killed. let (live_idx, dead_idx) = find_storage_live_dead_stmts_for_local(local_tmp_s0, stmts)?; let stmt_to_overwrite = nop_stmts.iter().find(|stmt_idx| live_idx < **stmt_idx && **stmt_idx < dead_idx); let mut tmp_assigned_vars = BitSet::new_empty(locals_count); for (l, r) in &tmp_assigns { tmp_assigned_vars.insert(*l); tmp_assigned_vars.insert(*r); } let dbg_info_to_adjust: Vec<_> = debug_info .iter() .enumerate() .filter_map(|(i, var_info)| { if let VarDebugInfoContents::Place(p) = var_info.value { if tmp_assigned_vars.contains(p.local) { return Some(i); } } None }) .collect(); Some(ArmIdentityInfo { local_temp_0: local_tmp_s0, local_1, vf_s0, get_variant_field_stmt, field_tmp_assignments: tmp_assigns, local_tmp_s1, local_0, vf_s1, set_discr_local, set_discr_var_idx, stmt_to_overwrite: *stmt_to_overwrite?, source_info: discr_stmt_source_info, storage_stmts, stmts_to_remove: nop_stmts, dbg_info_to_adjust, dbg_projection, }) } fn optimization_applies<'tcx>( opt_info: &ArmIdentityInfo<'tcx>, local_decls: &IndexVec<Local, LocalDecl<'tcx>>, local_uses: &IndexVec<Local, usize>, var_debug_info: &[VarDebugInfo<'tcx>], ) -> bool { trace!("testing if optimization applies..."); // FIXME(wesleywiser): possibly relax this restriction? if opt_info.local_0 == opt_info.local_1 { trace!("NO: moving into ourselves"); return false; } else if opt_info.vf_s0 != opt_info.vf_s1 { trace!("NO: the field-and-variant information do not match"); return false; } else if local_decls[opt_info.local_0].ty != local_decls[opt_info.local_1].ty { // FIXME(Centril,oli-obk): possibly relax to same layout? trace!("NO: source and target locals have different types"); return false; } else if (opt_info.local_0, opt_info.vf_s0.var_idx) != (opt_info.set_discr_local, opt_info.set_discr_var_idx) { trace!("NO: the discriminants do not match"); return false; } // Verify the assignment chain consists of the form b = a; c = b; d = c; etc... if opt_info.field_tmp_assignments.is_empty() { trace!("NO: no assignments found"); return false; } let mut last_assigned_to = opt_info.field_tmp_assignments[0].1; let source_local = last_assigned_to; for (l, r) in &opt_info.field_tmp_assignments { if *r != last_assigned_to { trace!("NO: found unexpected assignment {:?} = {:?}", l, r); return false; } last_assigned_to = *l; } // Check that the first and last used locals are only used twice // since they are of the form: // // ``` // _first = ((_x as Variant).n: ty); // _n = _first; // ... // ((_y as Variant).n: ty) = _n; // discriminant(_y) = z; // ``` for (l, r) in &opt_info.field_tmp_assignments { if local_uses[*l] != 2 { warn!("NO: FAILED assignment chain local {:?} was used more than twice", l); return false; } else if local_uses[*r] != 2 { warn!("NO: FAILED assignment chain local {:?} was used more than twice", r); return false; } } // Check that debug info only points to full Locals and not projections. for dbg_idx in &opt_info.dbg_info_to_adjust { let dbg_info = &var_debug_info[*dbg_idx]; if let VarDebugInfoContents::Place(p) = dbg_info.value { if !p.projection.is_empty() { trace!("NO: debug info for {:?} had a projection {:?}", dbg_info.name, p); return false; } } } if source_local != opt_info.local_temp_0 { trace!( "NO: start of assignment chain does not match enum variant temp: {:?} != {:?}", source_local, opt_info.local_temp_0 ); return false; } else if last_assigned_to != opt_info.local_tmp_s1 { trace!( "NO: end of assignment chain does not match written enum temp: {:?} != {:?}", last_assigned_to, opt_info.local_tmp_s1 ); return false; } trace!("SUCCESS: optimization applies!"); true } impl<'tcx> MirPass<'tcx> for SimplifyArmIdentity { fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { // FIXME(77359): This optimization can result in unsoundness. if !tcx.sess.opts.unstable_opts.unsound_mir_opts { return; } let source = body.source; trace!("running SimplifyArmIdentity on {:?}", source); let local_uses = LocalUseCounter::get_local_uses(body); for bb in body.basic_blocks.as_mut() { if let Some(opt_info) = get_arm_identity_info(&bb.statements, body.local_decls.len(), &body.var_debug_info) { trace!("got opt_info = {:#?}", opt_info); if !optimization_applies( &opt_info, &body.local_decls, &local_uses, &body.var_debug_info, ) { debug!("optimization skipped for {:?}", source); continue; } // Also remove unused Storage{Live,Dead} statements which correspond // to temps used previously. for (live_idx, dead_idx, local) in &opt_info.storage_stmts { // The temporary that we've read the variant field into is scoped to this block, // so we can remove the assignment. if *local == opt_info.local_temp_0 { bb.statements[opt_info.get_variant_field_stmt].make_nop(); } for (left, right) in &opt_info.field_tmp_assignments { if local == left || local == right { bb.statements[*live_idx].make_nop(); bb.statements[*dead_idx].make_nop(); } } } // Right shape; transform for stmt_idx in opt_info.stmts_to_remove { bb.statements[stmt_idx].make_nop(); } let stmt = &mut bb.statements[opt_info.stmt_to_overwrite]; stmt.source_info = opt_info.source_info; stmt.kind = StatementKind::Assign(Box::new(( opt_info.local_0.into(), Rvalue::Use(Operand::Move(opt_info.local_1.into())), ))); bb.statements.retain(|stmt| stmt.kind != StatementKind::Nop); // Fix the debug info to point to the right local for dbg_index in opt_info.dbg_info_to_adjust { let dbg_info = &mut body.var_debug_info[dbg_index]; assert!( matches!(dbg_info.value, VarDebugInfoContents::Place(_)), "value was not a Place" ); if let VarDebugInfoContents::Place(p) = &mut dbg_info.value { assert!(p.projection.is_empty()); p.local = opt_info.local_0; p.projection = opt_info.dbg_projection; } } trace!("block is now {:?}", bb.statements); } } } } struct LocalUseCounter { local_uses: IndexVec<Local, usize>, } impl LocalUseCounter { fn get_local_uses(body: &Body<'_>) -> IndexVec<Local, usize> { let mut counter = LocalUseCounter { local_uses: IndexVec::from_elem(0, &body.local_decls) }; counter.visit_body(body); counter.local_uses } } impl Visitor<'_> for LocalUseCounter { fn visit_local(&mut self, local: Local, context: PlaceContext, _location: Location) { if context.is_storage_marker() || context == PlaceContext::NonUse(NonUseContext::VarDebugInfo) { return; } self.local_uses[local] += 1; } } /// Match on: /// ```ignore (MIR) /// _LOCAL_INTO = ((_LOCAL_FROM as Variant).FIELD: TY); /// ``` fn match_get_variant_field<'tcx>( stmt: &Statement<'tcx>, ) -> Option<(Local, Local, VarField<'tcx>, &'tcx List<PlaceElem<'tcx>>)> { match &stmt.kind { StatementKind::Assign(box ( place_into, Rvalue::Use(Operand::Copy(pf) | 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, pf.projection)) } _ => None, } } /// Match on: /// ```ignore (MIR) /// ((_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::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, } } /// Match on: /// ```ignore (MIR) /// discriminant(_LOCAL_TO_SET) = VAR_IDX; /// ``` fn match_set_discr(stmt: &Statement<'_>) -> Option<(Local, VariantIdx)> { match &stmt.kind { StatementKind::SetDiscriminant { place, variant_index } => { Some((place.as_local()?, *variant_index)) } _ => None, } } #[derive(PartialEq, Debug)] struct VarField<'tcx> { field: Field, field_ty: Ty<'tcx>, var_idx: VariantIdx, } /// Match on `((_LOCAL as Variant).FIELD: TY)`. fn match_variant_field_place(place: Place<'_>) -> Option<(Local, VarField<'_>)> { 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, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { // This optimization is disabled by default for now due to // soundness concerns; see issue #89485 and PR #89489. if !tcx.sess.opts.unstable_opts.unsound_mir_opts { return; } trace!("Running SimplifyBranchSame on {:?}", body.source); let finder = SimplifyBranchSameOptimizationFinder { body, tcx }; let opts = finder.find(); let did_remove_blocks = opts.len() > 0; for opt in opts.iter() { trace!("SUCCESS: Applying optimization {:?}", opt); // Replace `SwitchInt(..) -> [bb_first, ..];` with a `goto -> bb_first;`. body.basic_blocks_mut()[opt.bb_to_opt_terminator].terminator_mut().kind = TerminatorKind::Goto { target: opt.bb_to_goto }; } if did_remove_blocks { // We have dead blocks now, so remove those. simplify::remove_dead_blocks(tcx, body); } } } #[derive(Debug)] struct SimplifyBranchSameOptimization { /// All basic blocks are equal so go to this one bb_to_goto: BasicBlock, /// Basic block where the terminator can be simplified to a goto bb_to_opt_terminator: BasicBlock, } struct SwitchTargetAndValue { target: BasicBlock, // None in case of the `otherwise` case value: Option<u128>, } struct SimplifyBranchSameOptimizationFinder<'a, 'tcx> { body: &'a Body<'tcx>, tcx: TyCtxt<'tcx>, } impl<'tcx> SimplifyBranchSameOptimizationFinder<'_, 'tcx> { fn find(&self) -> Vec<SimplifyBranchSameOptimization> { self.body .basic_blocks .iter_enumerated() .filter_map(|(bb_idx, bb)| { let (discr_switched_on, targets_and_values) = match &bb.terminator().kind { TerminatorKind::SwitchInt { targets, discr, .. } => { let targets_and_values: Vec<_> = targets.iter() .map(|(val, target)| SwitchTargetAndValue { target, value: Some(val) }) .chain(once(SwitchTargetAndValue { target: targets.otherwise(), value: None })) .collect(); (discr, targets_and_values) }, _ => return None, }; // find the adt that has its discriminant read // assuming this must be the last statement of the block let adt_matched_on = match &bb.statements.last()?.kind { StatementKind::Assign(box (place, rhs)) if Some(*place) == discr_switched_on.place() => { match rhs { Rvalue::Discriminant(adt_place) if adt_place.ty(self.body, self.tcx).ty.is_enum() => adt_place, _ => { trace!("NO: expected a discriminant read of an enum instead of: {:?}", rhs); return None; } } } other => { trace!("NO: expected an assignment of a discriminant read to a place. Found: {:?}", other); return None }, }; let mut iter_bbs_reachable = targets_and_values .iter() .map(|target_and_value| (target_and_value, &self.body.basic_blocks[target_and_value.target])) .filter(|(_, bb)| { // Reaching `unreachable` is UB so assume it doesn't happen. bb.terminator().kind != TerminatorKind::Unreachable }) .peekable(); let bb_first = iter_bbs_reachable.peek().map_or(&targets_and_values[0], |(idx, _)| *idx); let mut all_successors_equivalent = StatementEquality::TrivialEqual; // All successor basic blocks must be equal or contain statements that are pairwise considered equal. for ((target_and_value_l,bb_l), (target_and_value_r,bb_r)) in iter_bbs_reachable.tuple_windows() { let trivial_checks = bb_l.is_cleanup == bb_r.is_cleanup && bb_l.terminator().kind == bb_r.terminator().kind && bb_l.statements.len() == bb_r.statements.len(); let statement_check = || { bb_l.statements.iter().zip(&bb_r.statements).try_fold(StatementEquality::TrivialEqual, |acc,(l,r)| { let stmt_equality = self.statement_equality(*adt_matched_on, &l, target_and_value_l, &r, target_and_value_r); if matches!(stmt_equality, StatementEquality::NotEqual) { // short circuit None } else { Some(acc.combine(&stmt_equality)) } }) .unwrap_or(StatementEquality::NotEqual) }; if !trivial_checks { all_successors_equivalent = StatementEquality::NotEqual; break; } all_successors_equivalent = all_successors_equivalent.combine(&statement_check()); }; match all_successors_equivalent{ StatementEquality::TrivialEqual => { // statements are trivially equal, so just take first trace!("Statements are trivially equal"); Some(SimplifyBranchSameOptimization { bb_to_goto: bb_first.target, bb_to_opt_terminator: bb_idx, }) } StatementEquality::ConsideredEqual(bb_to_choose) => { trace!("Statements are considered equal"); Some(SimplifyBranchSameOptimization { bb_to_goto: bb_to_choose, bb_to_opt_terminator: bb_idx, }) } StatementEquality::NotEqual => { trace!("NO: not all successors of basic block {:?} were equivalent", bb_idx); None } } }) .collect() } /// Tests if two statements can be considered equal /// /// Statements can be trivially equal if the kinds match. /// But they can also be considered equal in the following case A: /// ```ignore (MIR) /// discriminant(_0) = 0;   // bb1 /// _0 = move _1;           // bb2 /// ``` /// In this case the two statements are equal iff /// - `_0` is an enum where the variant index 0 is fieldless, and /// -  bb1 was targeted by a switch where the discriminant of `_1` was switched on fn statement_equality( &self, adt_matched_on: Place<'tcx>, x: &Statement<'tcx>, x_target_and_value: &SwitchTargetAndValue, y: &Statement<'tcx>, y_target_and_value: &SwitchTargetAndValue, ) -> StatementEquality { let helper = |rhs: &Rvalue<'tcx>, place: &Place<'tcx>, variant_index: VariantIdx, switch_value: u128, side_to_choose| { let place_type = place.ty(self.body, self.tcx).ty; let adt = match *place_type.kind() { ty::Adt(adt, _) if adt.is_enum() => adt, _ => return StatementEquality::NotEqual, }; // We need to make sure that the switch value that targets the bb with // SetDiscriminant is the same as the variant discriminant. let variant_discr = adt.discriminant_for_variant(self.tcx, variant_index).val; if variant_discr != switch_value { trace!( "NO: variant discriminant {} does not equal switch value {}", variant_discr, switch_value ); return StatementEquality::NotEqual; } let variant_is_fieldless = adt.variant(variant_index).fields.is_empty(); if !variant_is_fieldless { trace!("NO: variant {:?} was not fieldless", variant_index); return StatementEquality::NotEqual; } match rhs { Rvalue::Use(operand) if operand.place() == Some(adt_matched_on) => { StatementEquality::ConsideredEqual(side_to_choose) } _ => { trace!( "NO: RHS of assignment was {:?}, but expected it to match the adt being matched on in the switch, which is {:?}", rhs, adt_matched_on ); StatementEquality::NotEqual } } }; match (&x.kind, &y.kind) { // trivial case (x, y) if x == y => StatementEquality::TrivialEqual, // check for case A ( StatementKind::Assign(box (_, rhs)), &StatementKind::SetDiscriminant { ref place, variant_index }, ) if y_target_and_value.value.is_some() => { // choose basic block of x, as that has the assign helper( rhs, place, variant_index, y_target_and_value.value.unwrap(), x_target_and_value.target, ) } ( &StatementKind::SetDiscriminant { ref place, variant_index }, &StatementKind::Assign(box (_, ref rhs)), ) if x_target_and_value.value.is_some() => { // choose basic block of y, as that has the assign helper( rhs, place, variant_index, x_target_and_value.value.unwrap(), y_target_and_value.target, ) } _ => { trace!("NO: statements `{:?}` and `{:?}` not considered equal", x, y); StatementEquality::NotEqual } } } } #[derive(Copy, Clone, Eq, PartialEq)] enum StatementEquality { /// The two statements are trivially equal; same kind TrivialEqual, /// The two statements are considered equal, but may be of different kinds. The BasicBlock field is the basic block to jump to when performing the branch-same optimization. /// For example, `_0 = _1` and `discriminant(_0) = discriminant(0)` are considered equal if 0 is a fieldless variant of an enum. But we don't want to jump to the basic block with the SetDiscriminant, as that is not legal if _1 is not the 0 variant index ConsideredEqual(BasicBlock), /// The two statements are not equal NotEqual, } impl StatementEquality { fn combine(&self, other: &StatementEquality) -> StatementEquality { use StatementEquality::*; match (self, other) { (TrivialEqual, TrivialEqual) => TrivialEqual, (TrivialEqual, ConsideredEqual(b)) | (ConsideredEqual(b), TrivialEqual) => { ConsideredEqual(*b) } (ConsideredEqual(b1), ConsideredEqual(b2)) => { if b1 == b2 { ConsideredEqual(*b1) } else { NotEqual } } (_, NotEqual) | (NotEqual, _) => NotEqual, } } } </del>