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add a pass to remove no-op landing pads
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,137 @@ | ||
| // Copyright 2017 The Rust Project Developers. See the COPYRIGHT | ||
| // file at the top-level directory of this distribution and at | ||
| // http://rust-lang.org/COPYRIGHT. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
|
|
||
| use rustc::ty::TyCtxt; | ||
| use rustc::mir::*; | ||
| use rustc_data_structures::bitvec::BitVector; | ||
| use rustc_data_structures::indexed_vec::Idx; | ||
| use transform::{MirPass, MirSource}; | ||
| use util::patch::MirPatch; | ||
|
|
||
| /// A pass that removes no-op landing pads and replaces jumps to them with | ||
| /// `None`. This is important because otherwise LLVM generates terrible | ||
| /// code for these. | ||
| pub struct RemoveNoopLandingPads; | ||
|
|
||
| impl MirPass for RemoveNoopLandingPads { | ||
| fn run_pass<'a, 'tcx>(&self, | ||
| tcx: TyCtxt<'a, 'tcx, 'tcx>, | ||
| _src: MirSource, | ||
| mir: &mut Mir<'tcx>) { | ||
| if tcx.sess.no_landing_pads() { | ||
| return | ||
| } | ||
|
|
||
| debug!("remove_noop_landing_pads({:?})", mir); | ||
| self.remove_nop_landing_pads(mir); | ||
| } | ||
| } | ||
|
|
||
| impl RemoveNoopLandingPads { | ||
| fn is_nop_landing_pad(&self, bb: BasicBlock, mir: &Mir, nop_landing_pads: &BitVector) | ||
| -> bool | ||
| { | ||
| for stmt in &mir[bb].statements { | ||
| match stmt.kind { | ||
| StatementKind::StorageLive(_) | | ||
| StatementKind::StorageDead(_) | | ||
| StatementKind::EndRegion(_) | | ||
| StatementKind::Nop => { | ||
| // These are all nops in a landing pad (there's some | ||
| // borrowck interaction between EndRegion and storage | ||
| // instructions, but this should all run after borrowck). | ||
| } | ||
|
|
||
| StatementKind::Assign(Place::Local(_), Rvalue::Use(_)) => { | ||
| // Writing to a local (e.g. a drop flag) does not | ||
| // turn a landing pad to a non-nop | ||
| } | ||
|
|
||
| StatementKind::Assign(_, _) | | ||
| StatementKind::SetDiscriminant { .. } | | ||
| StatementKind::InlineAsm { .. } | | ||
| StatementKind::Validate { .. } => { | ||
| return false; | ||
| } | ||
| } | ||
| } | ||
|
|
||
| let terminator = mir[bb].terminator(); | ||
| match terminator.kind { | ||
| TerminatorKind::Goto { .. } | | ||
| TerminatorKind::Resume | | ||
| TerminatorKind::SwitchInt { .. } | | ||
| TerminatorKind::FalseEdges { .. } => { | ||
| terminator.successors().iter().all(|succ| { | ||
| nop_landing_pads.contains(succ.index()) | ||
| }) | ||
| }, | ||
| TerminatorKind::GeneratorDrop | | ||
| TerminatorKind::Yield { .. } | | ||
| TerminatorKind::Return | | ||
| TerminatorKind::Unreachable | | ||
| TerminatorKind::Call { .. } | | ||
| TerminatorKind::Assert { .. } | | ||
| TerminatorKind::DropAndReplace { .. } | | ||
| TerminatorKind::Drop { .. } => { | ||
| false | ||
| } | ||
| } | ||
| } | ||
|
|
||
| fn remove_nop_landing_pads(&self, mir: &mut Mir) { | ||
| // make sure there's a single resume block | ||
| let resume_block = { | ||
| let patch = MirPatch::new(mir); | ||
| let resume_block = patch.resume_block(); | ||
| patch.apply(mir); | ||
| resume_block | ||
| }; | ||
| debug!("remove_noop_landing_pads: resume block is {:?}", resume_block); | ||
|
|
||
| let mut jumps_folded = 0; | ||
| let mut landing_pads_removed = 0; | ||
| let mut nop_landing_pads = BitVector::new(mir.basic_blocks().len()); | ||
|
|
||
| // This is a post-order traversal, so that if A post-dominates B | ||
| // then A will be visited before B. | ||
| let postorder: Vec<_> = traversal::postorder(mir).map(|(bb, _)| bb).collect(); | ||
| for bb in postorder { | ||
| debug!(" processing {:?}", bb); | ||
| for target in mir[bb].terminator_mut().successors_mut() { | ||
| if *target != resume_block && nop_landing_pads.contains(target.index()) { | ||
| debug!(" folding noop jump to {:?} to resume block", target); | ||
| *target = resume_block; | ||
| jumps_folded += 1; | ||
| } | ||
| } | ||
|
|
||
| match mir[bb].terminator_mut().unwind_mut() { | ||
| Some(unwind) => { | ||
| if *unwind == Some(resume_block) { | ||
| debug!(" removing noop landing pad"); | ||
| jumps_folded -= 1; | ||
| landing_pads_removed += 1; | ||
| *unwind = None; | ||
| } | ||
| } | ||
| _ => {} | ||
| } | ||
|
|
||
| let is_nop_landing_pad = self.is_nop_landing_pad(bb, mir, &nop_landing_pads); | ||
| if is_nop_landing_pad { | ||
| nop_landing_pads.insert(bb.index()); | ||
| } | ||
| debug!(" is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad); | ||
| } | ||
|
|
||
| debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed); | ||
| } | ||
| } |