| //! A pass that duplicates switch-terminated blocks |
| //! into a new copy for each predecessor, provided |
| //! the predecessor sets the value being switched |
| //! over to a constant. |
| //! |
| //! The purpose of this pass is to help constant |
| //! propagation passes to simplify the switch terminator |
| //! of the copied blocks into gotos when some predecessors |
| //! statically determine the output of switches. |
| //! |
| //! ```text |
| //! x = 12 --- ---> something |
| //! \ / 12 |
| //! --> switch x |
| //! / \ otherwise |
| //! x = y --- ---> something else |
| //! ``` |
| //! becomes |
| //! ```text |
| //! x = 12 ---> switch x ------> something |
| //! \ / 12 |
| //! X |
| //! / \ otherwise |
| //! x = y ---> switch x ------> something else |
| //! ``` |
| //! so it can hopefully later be turned by another pass into |
| //! ```text |
| //! x = 12 --------------------> something |
| //! / 12 |
| //! / |
| //! / otherwise |
| //! x = y ---- switch x ------> something else |
| //! ``` |
| //! |
| //! This optimization is meant to cover simple cases |
| //! like `?` desugaring. For now, it thus focuses on |
| //! simplicity rather than completeness (it notably |
| //! sometimes duplicates abusively). |
| |
| use crate::MirPass; |
| use rustc_middle::mir::*; |
| use rustc_middle::ty::TyCtxt; |
| use smallvec::SmallVec; |
| |
| pub struct SeparateConstSwitch; |
| |
| impl<'tcx> MirPass<'tcx> for SeparateConstSwitch { |
| fn is_enabled(&self, sess: &rustc_session::Session) -> bool { |
| // This pass participates in some as-of-yet untested unsoundness found |
| // in https://github.com/rust-lang/rust/issues/112460 |
| sess.mir_opt_level() >= 2 && sess.opts.unstable_opts.unsound_mir_opts |
| } |
| |
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| // If execution did something, applying a simplification layer |
| // helps later passes optimize the copy away. |
| if separate_const_switch(body) > 0 { |
| super::simplify::simplify_cfg(tcx, body); |
| } |
| } |
| } |
| |
| /// Returns the amount of blocks that were duplicated |
| pub fn separate_const_switch(body: &mut Body<'_>) -> usize { |
| let mut new_blocks: SmallVec<[(BasicBlock, BasicBlock); 6]> = SmallVec::new(); |
| let predecessors = body.basic_blocks.predecessors(); |
| 'block_iter: for (block_id, block) in body.basic_blocks.iter_enumerated() { |
| if let TerminatorKind::SwitchInt { |
| discr: Operand::Copy(switch_place) | Operand::Move(switch_place), |
| .. |
| } = block.terminator().kind |
| { |
| // If the block is on an unwind path, do not |
| // apply the optimization as unwind paths |
| // rely on a unique parent invariant |
| if block.is_cleanup { |
| continue 'block_iter; |
| } |
| |
| // If the block has fewer than 2 predecessors, ignore it |
| // we could maybe chain blocks that have exactly one |
| // predecessor, but for now we ignore that |
| if predecessors[block_id].len() < 2 { |
| continue 'block_iter; |
| } |
| |
| // First, let's find a non-const place |
| // that determines the result of the switch |
| if let Some(switch_place) = find_determining_place(switch_place, block) { |
| // We now have an input place for which it would |
| // be interesting if predecessors assigned it from a const |
| |
| let mut predecessors_left = predecessors[block_id].len(); |
| 'predec_iter: for predecessor_id in predecessors[block_id].iter().copied() { |
| let predecessor = &body.basic_blocks[predecessor_id]; |
| |
| // First we make sure the predecessor jumps |
| // in a reasonable way |
| match &predecessor.terminator().kind { |
| // The following terminators are |
| // unconditionally valid |
| TerminatorKind::Goto { .. } | TerminatorKind::SwitchInt { .. } => {} |
| |
| TerminatorKind::FalseEdge { real_target, .. } => { |
| if *real_target != block_id { |
| continue 'predec_iter; |
| } |
| } |
| |
| // The following terminators are not allowed |
| TerminatorKind::UnwindResume |
| | TerminatorKind::Drop { .. } |
| | TerminatorKind::Call { .. } |
| | TerminatorKind::Assert { .. } |
| | TerminatorKind::FalseUnwind { .. } |
| | TerminatorKind::Yield { .. } |
| | TerminatorKind::UnwindTerminate(_) |
| | TerminatorKind::Return |
| | TerminatorKind::Unreachable |
| | TerminatorKind::InlineAsm { .. } |
| | TerminatorKind::CoroutineDrop => { |
| continue 'predec_iter; |
| } |
| } |
| |
| if is_likely_const(switch_place, predecessor) { |
| new_blocks.push((predecessor_id, block_id)); |
| predecessors_left -= 1; |
| if predecessors_left < 2 { |
| // If the original block only has one predecessor left, |
| // we have nothing left to do |
| break 'predec_iter; |
| } |
| } |
| } |
| } |
| } |
| } |
| |
| // Once the analysis is done, perform the duplication |
| let body_span = body.span; |
| let copied_blocks = new_blocks.len(); |
| let blocks = body.basic_blocks_mut(); |
| for (pred_id, target_id) in new_blocks { |
| let new_block = blocks[target_id].clone(); |
| let new_block_id = blocks.push(new_block); |
| let terminator = blocks[pred_id].terminator_mut(); |
| |
| match terminator.kind { |
| TerminatorKind::Goto { ref mut target } => { |
| *target = new_block_id; |
| } |
| |
| TerminatorKind::FalseEdge { ref mut real_target, .. } => { |
| if *real_target == target_id { |
| *real_target = new_block_id; |
| } |
| } |
| |
| TerminatorKind::SwitchInt { ref mut targets, .. } => { |
| targets.all_targets_mut().iter_mut().for_each(|x| { |
| if *x == target_id { |
| *x = new_block_id; |
| } |
| }); |
| } |
| |
| TerminatorKind::UnwindResume |
| | TerminatorKind::UnwindTerminate(_) |
| | TerminatorKind::Return |
| | TerminatorKind::Unreachable |
| | TerminatorKind::CoroutineDrop |
| | TerminatorKind::Assert { .. } |
| | TerminatorKind::FalseUnwind { .. } |
| | TerminatorKind::Drop { .. } |
| | TerminatorKind::Call { .. } |
| | TerminatorKind::InlineAsm { .. } |
| | TerminatorKind::Yield { .. } => { |
| span_bug!( |
| body_span, |
| "basic block terminator had unexpected kind {:?}", |
| &terminator.kind |
| ) |
| } |
| } |
| } |
| |
| copied_blocks |
| } |
| |
| /// This function describes a rough heuristic guessing |
| /// whether a place is last set with a const within the block. |
| /// Notably, it will be overly pessimistic in cases that are already |
| /// not handled by `separate_const_switch`. |
| fn is_likely_const<'tcx>(mut tracked_place: Place<'tcx>, block: &BasicBlockData<'tcx>) -> bool { |
| for statement in block.statements.iter().rev() { |
| match &statement.kind { |
| StatementKind::Assign(assign) => { |
| if assign.0 == tracked_place { |
| match assign.1 { |
| // These rvalues are definitely constant |
| Rvalue::Use(Operand::Constant(_)) |
| | Rvalue::Ref(_, _, _) |
| | Rvalue::AddressOf(_, _) |
| | Rvalue::Cast(_, Operand::Constant(_), _) |
| | Rvalue::NullaryOp(_, _) |
| | Rvalue::ShallowInitBox(_, _) |
| | Rvalue::UnaryOp(_, Operand::Constant(_)) => return true, |
| |
| // These rvalues make things ambiguous |
| Rvalue::Repeat(_, _) |
| | Rvalue::ThreadLocalRef(_) |
| | Rvalue::Len(_) |
| | Rvalue::BinaryOp(_, _) |
| | Rvalue::CheckedBinaryOp(_, _) |
| | Rvalue::Aggregate(_, _) => return false, |
| |
| // These rvalues move the place to track |
| Rvalue::Cast(_, Operand::Copy(place) | Operand::Move(place), _) |
| | Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) |
| | Rvalue::CopyForDeref(place) |
| | Rvalue::UnaryOp(_, Operand::Copy(place) | Operand::Move(place)) |
| | Rvalue::Discriminant(place) => tracked_place = place, |
| } |
| } |
| } |
| |
| // If the discriminant is set, it is always set |
| // as a constant, so the job is done. |
| // As we are **ignoring projections**, if the place |
| // we are tracking sees its discriminant be set, |
| // that means we had to be tracking the discriminant |
| // specifically (as it is impossible to switch over |
| // an enum directly, and if we were switching over |
| // its content, we would have had to at least cast it to |
| // some variant first) |
| StatementKind::SetDiscriminant { place, .. } => { |
| if **place == tracked_place { |
| return true; |
| } |
| } |
| |
| // These statements have no influence on the place |
| // we are interested in |
| StatementKind::FakeRead(_) |
| | StatementKind::Deinit(_) |
| | StatementKind::StorageLive(_) |
| | StatementKind::Retag(_, _) |
| | StatementKind::AscribeUserType(_, _) |
| | StatementKind::PlaceMention(..) |
| | StatementKind::Coverage(_) |
| | StatementKind::StorageDead(_) |
| | StatementKind::Intrinsic(_) |
| | StatementKind::ConstEvalCounter |
| | StatementKind::Nop => {} |
| } |
| } |
| |
| // If no good reason for the place to be const is found, |
| // give up. We could maybe go up predecessors, but in |
| // most cases giving up now should be sufficient. |
| false |
| } |
| |
| /// Finds a unique place that entirely determines the value |
| /// of `switch_place`, if it exists. This is only a heuristic. |
| /// Ideally we would like to track multiple determining places |
| /// for some edge cases, but one is enough for a lot of situations. |
| fn find_determining_place<'tcx>( |
| mut switch_place: Place<'tcx>, |
| block: &BasicBlockData<'tcx>, |
| ) -> Option<Place<'tcx>> { |
| for statement in block.statements.iter().rev() { |
| match &statement.kind { |
| StatementKind::Assign(op) => { |
| if op.0 != switch_place { |
| continue; |
| } |
| |
| match op.1 { |
| // The following rvalues move the place |
| // that may be const in the predecessor |
| Rvalue::Use(Operand::Move(new) | Operand::Copy(new)) |
| | Rvalue::UnaryOp(_, Operand::Copy(new) | Operand::Move(new)) |
| | Rvalue::CopyForDeref(new) |
| | Rvalue::Cast(_, Operand::Move(new) | Operand::Copy(new), _) |
| | Rvalue::Repeat(Operand::Move(new) | Operand::Copy(new), _) |
| | Rvalue::Discriminant(new) |
| => switch_place = new, |
| |
| // The following rvalues might still make the block |
| // be valid but for now we reject them |
| Rvalue::Len(_) |
| | Rvalue::Ref(_, _, _) |
| | Rvalue::BinaryOp(_, _) |
| | Rvalue::CheckedBinaryOp(_, _) |
| | Rvalue::Aggregate(_, _) |
| |
| // The following rvalues definitely mean we cannot |
| // or should not apply this optimization |
| | Rvalue::Use(Operand::Constant(_)) |
| | Rvalue::Repeat(Operand::Constant(_), _) |
| | Rvalue::ThreadLocalRef(_) |
| | Rvalue::AddressOf(_, _) |
| | Rvalue::NullaryOp(_, _) |
| | Rvalue::ShallowInitBox(_, _) |
| | Rvalue::UnaryOp(_, Operand::Constant(_)) |
| | Rvalue::Cast(_, Operand::Constant(_), _) => return None, |
| } |
| } |
| |
| // These statements have no influence on the place |
| // we are interested in |
| StatementKind::FakeRead(_) |
| | StatementKind::Deinit(_) |
| | StatementKind::StorageLive(_) |
| | StatementKind::StorageDead(_) |
| | StatementKind::Retag(_, _) |
| | StatementKind::AscribeUserType(_, _) |
| | StatementKind::PlaceMention(..) |
| | StatementKind::Coverage(_) |
| | StatementKind::Intrinsic(_) |
| | StatementKind::ConstEvalCounter |
| | StatementKind::Nop => {} |
| |
| // If the discriminant is set, it is always set |
| // as a constant, so the job is already done. |
| // As we are **ignoring projections**, if the place |
| // we are tracking sees its discriminant be set, |
| // that means we had to be tracking the discriminant |
| // specifically (as it is impossible to switch over |
| // an enum directly, and if we were switching over |
| // its content, we would have had to at least cast it to |
| // some variant first) |
| StatementKind::SetDiscriminant { place, .. } => { |
| if **place == switch_place { |
| return None; |
| } |
| } |
| } |
| } |
| |
| Some(switch_place) |
| } |
