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|
use crate::MirPass;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use rustc_target::spec::PanicStrategy;
/// A pass that removes noop landing pads and replaces jumps to them with
/// `None`. This is important because otherwise LLVM generates terrible
/// code for these.
pub struct RemoveNoopLandingPads;
impl<'tcx> MirPass<'tcx> for RemoveNoopLandingPads {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.panic_strategy() != PanicStrategy::Abort
}
fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
debug!("remove_noop_landing_pads({:?})", body);
self.remove_nop_landing_pads(body)
}
}
impl RemoveNoopLandingPads {
fn is_nop_landing_pad(
&self,
bb: BasicBlock,
body: &Body<'_>,
nop_landing_pads: &BitSet<BasicBlock>,
) -> bool {
for stmt in &body[bb].statements {
match &stmt.kind {
StatementKind::FakeRead(..)
| StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::AscribeUserType(..)
| StatementKind::Coverage(..)
| StatementKind::Nop => {
// These are all noops in a landing pad
}
StatementKind::Assign(box (place, Rvalue::Use(_) | Rvalue::Discriminant(_))) => {
if place.as_local().is_some() {
// Writing to a local (e.g., a drop flag) does not
// turn a landing pad to a non-nop
} else {
return false;
}
}
StatementKind::Assign { .. }
| StatementKind::SetDiscriminant { .. }
| StatementKind::Deinit(..)
| StatementKind::Intrinsic(..)
| StatementKind::Retag { .. } => {
return false;
}
}
}
let terminator = body[bb].terminator();
match terminator.kind {
TerminatorKind::Goto { .. }
| TerminatorKind::Resume
| TerminatorKind::SwitchInt { .. }
| TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. } => {
terminator.successors().all(|succ| nop_landing_pads.contains(succ))
}
TerminatorKind::GeneratorDrop
| TerminatorKind::Yield { .. }
| TerminatorKind::Return
| TerminatorKind::Abort
| TerminatorKind::Unreachable
| TerminatorKind::Call { .. }
| TerminatorKind::Assert { .. }
| TerminatorKind::DropAndReplace { .. }
| TerminatorKind::Drop { .. }
| TerminatorKind::InlineAsm { .. } => false,
}
}
fn remove_nop_landing_pads(&self, body: &mut Body<'_>) {
debug!("body: {:#?}", body);
// make sure there's a resume block
let resume_block = {
let mut patch = MirPatch::new(body);
let resume_block = patch.resume_block();
patch.apply(body);
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 = BitSet::new_empty(body.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(body).map(|(bb, _)| bb).collect();
for bb in postorder {
debug!(" processing {:?}", bb);
if let Some(unwind) = body[bb].terminator_mut().unwind_mut() {
if let Some(unwind_bb) = *unwind {
if nop_landing_pads.contains(unwind_bb) {
debug!(" removing noop landing pad");
landing_pads_removed += 1;
*unwind = None;
}
}
}
for target in body[bb].terminator_mut().successors_mut() {
if *target != resume_block && nop_landing_pads.contains(*target) {
debug!(" folding noop jump to {:?} to resume block", target);
*target = resume_block;
jumps_folded += 1;
}
}
let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
if is_nop_landing_pad {
nop_landing_pads.insert(bb);
}
debug!(" is_nop_landing_pad({:?}) = {}", bb, is_nop_landing_pad);
}
debug!("removed {:?} jumps and {:?} landing pads", jumps_folded, landing_pads_removed);
}
}
|
