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|
//! A pass that propagates the unreachable terminator of a block to its predecessors
//! when all of their successors are unreachable. This is achieved through a
//! post-order traversal of the blocks.
use crate::simplify;
use crate::MirPass;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
pub struct UnreachablePropagation;
impl MirPass<'_> for UnreachablePropagation {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
// Enable only under -Zmir-opt-level=2 as this can make programs less debuggable.
sess.mir_opt_level() >= 2
}
fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let mut unreachable_blocks = FxHashSet::default();
let mut replacements = FxHashMap::default();
for (bb, bb_data) in traversal::postorder(body) {
let terminator = bb_data.terminator();
if terminator.kind == TerminatorKind::Unreachable {
unreachable_blocks.insert(bb);
} else {
let is_unreachable = |succ: BasicBlock| unreachable_blocks.contains(&succ);
let terminator_kind_opt = remove_successors(&terminator.kind, is_unreachable);
if let Some(terminator_kind) = terminator_kind_opt {
if terminator_kind == TerminatorKind::Unreachable {
unreachable_blocks.insert(bb);
}
replacements.insert(bb, terminator_kind);
}
}
}
// We do want do keep some unreachable blocks, but make them empty.
for bb in unreachable_blocks {
if !tcx.consider_optimizing(|| {
format!("UnreachablePropagation {:?} ", body.source.def_id())
}) {
break;
}
body.basic_blocks_mut()[bb].statements.clear();
}
let replaced = !replacements.is_empty();
for (bb, terminator_kind) in replacements {
if !tcx.consider_optimizing(|| {
format!("UnreachablePropagation {:?} ", body.source.def_id())
}) {
break;
}
body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind;
}
if replaced {
simplify::remove_dead_blocks(tcx, body);
}
}
}
fn remove_successors<'tcx, F>(
terminator_kind: &TerminatorKind<'tcx>,
is_unreachable: F,
) -> Option<TerminatorKind<'tcx>>
where
F: Fn(BasicBlock) -> bool,
{
let terminator = match terminator_kind {
// This will unconditionally run into an unreachable and is therefore unreachable as well.
TerminatorKind::Goto { target } if is_unreachable(*target) => TerminatorKind::Unreachable,
TerminatorKind::SwitchInt { targets, discr, switch_ty } => {
let otherwise = targets.otherwise();
// If all targets are unreachable, we can be unreachable as well.
if targets.all_targets().iter().all(|bb| is_unreachable(*bb)) {
TerminatorKind::Unreachable
} else if is_unreachable(otherwise) {
// If there are multiple targets, don't delete unreachable branches (like an unreachable otherwise)
// unless otherwise is unreachable, in which case deleting a normal branch causes it to be merged with
// the otherwise, keeping its unreachable.
// This looses information about reachability causing worse codegen.
// For example (see src/test/codegen/match-optimizes-away.rs)
//
// pub enum Two { A, B }
// pub fn identity(x: Two) -> Two {
// match x {
// Two::A => Two::A,
// Two::B => Two::B,
// }
// }
//
// This generates a `switchInt() -> [0: 0, 1: 1, otherwise: unreachable]`, which allows us or LLVM to
// turn it into just `x` later. Without the unreachable, such a transformation would be illegal.
// If the otherwise branch is unreachable, we can delete all other unreacahble targets, as they will
// still point to the unreachable and therefore not lose reachability information.
let reachable_iter = targets.iter().filter(|(_, bb)| !is_unreachable(*bb));
let new_targets = SwitchTargets::new(reachable_iter, otherwise);
// No unreachable branches were removed.
if new_targets.all_targets().len() == targets.all_targets().len() {
return None;
}
TerminatorKind::SwitchInt {
discr: discr.clone(),
switch_ty: *switch_ty,
targets: new_targets,
}
} else {
// If the otherwise branch is reachable, we don't want to delete any unreachable branches.
return None;
}
}
_ => return None,
};
Some(terminator)
}
|
