--- /dev/null
+//! 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::{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:
+///
+/// ```rust
+/// _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:
+///
+/// ```rust
+/// _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<'tcx>(stmt: &Statement<'tcx>) -> 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<'a, 'tcx>(
+ stmt_iter: &mut StmtIter<'a, 'tcx>,
+ 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<'tcx>(stmt: &Statement<'tcx>) -> 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<'a, 'tcx>(
+ stmt_iter: &mut StmtIter<'a, 'tcx>,
+ 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<'tcx>(
+ local: Local,
+ stmts: &[Statement<'tcx>],
+ ) -> 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 assignemnt 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.debugging_opts.unsound_mir_opts {
+ return;
+ }
+
+ let source = body.source;
+ trace!("running SimplifyArmIdentity on {:?}", source);
+
+ let local_uses = LocalUseCounter::get_local_uses(body);
+ let (basic_blocks, local_decls, debug_info) =
+ body.basic_blocks_local_decls_mut_and_var_debug_info();
+ for bb in basic_blocks {
+ if let Some(opt_info) =
+ get_arm_identity_info(&bb.statements, local_decls.len(), debug_info)
+ {
+ trace!("got opt_info = {:#?}", opt_info);
+ if !optimization_applies(&opt_info, local_decls, &local_uses, &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 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<'tcx>(body: &Body<'tcx>) -> IndexVec<Local, usize> {
+ let mut counter = LocalUseCounter { local_uses: IndexVec::from_elem(0, &body.local_decls) };
+ counter.visit_body(body);
+ counter.local_uses
+ }
+}
+
+impl<'tcx> Visitor<'tcx> 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:
+/// ```rust
+/// _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:
+/// ```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::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:
+/// ```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, 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<'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, 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.debugging_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<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, '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
+ // 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| matches!(stmt.kind, StatementKind::LlvmInlineAsm(..)))
+ })
+ .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:
+ /// ```
+ /// 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.variants[*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 { 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 { place, variant_index },
+ StatementKind::Assign(box (_, 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,
+ }
+ }
+}
projects / rustc.git / blobdiff