diff options
| -rw-r--r-- | compiler/rustc_mir_transform/src/unreachable_prop.rs | 71 |
1 files changed, 48 insertions, 23 deletions
diff --git a/compiler/rustc_mir_transform/src/unreachable_prop.rs b/compiler/rustc_mir_transform/src/unreachable_prop.rs index f916ca36217..a7e522beeab 100644 --- a/compiler/rustc_mir_transform/src/unreachable_prop.rs +++ b/compiler/rustc_mir_transform/src/unreachable_prop.rs @@ -38,7 +38,19 @@ impl MirPass<'_> for UnreachablePropagation { } } + // 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()) @@ -57,42 +69,55 @@ impl MirPass<'_> for UnreachablePropagation { fn remove_successors<'tcx, F>( terminator_kind: &TerminatorKind<'tcx>, - predicate: F, + is_unreachable: F, ) -> Option<TerminatorKind<'tcx>> where F: Fn(BasicBlock) -> bool, { - let terminator = match *terminator_kind { - TerminatorKind::Goto { target } if predicate(target) => TerminatorKind::Unreachable, - TerminatorKind::SwitchInt { ref discr, switch_ty, ref targets } => { + 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(); - let original_targets_len = targets.iter().len() + 1; - let (mut values, mut targets): (Vec<_>, Vec<_>) = - targets.iter().filter(|(_, bb)| !predicate(*bb)).unzip(); + // 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 unrachable, 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)); - if !predicate(otherwise) { - targets.push(otherwise); - } else { - values.pop(); - } + let new_targets = SwitchTargets::new(reachable_iter, otherwise); - let retained_targets_len = targets.len(); + // No unreachable branches were removed. + if new_targets.all_targets().len() == targets.all_targets().len() { + return None; + } - if targets.is_empty() { - TerminatorKind::Unreachable - } else if targets.len() == 1 { - TerminatorKind::Goto { target: targets[0] } - } else if original_targets_len != retained_targets_len { TerminatorKind::SwitchInt { discr: discr.clone(), - switch_ty, - targets: SwitchTargets::new( - values.iter().copied().zip(targets.iter().copied()), - *targets.last().unwrap(), - ), + 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; } } |