UnreachableProp: Preserve unreachable branches for multiple targets #99762
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@@ -12,9 +12,8 @@ pub struct UnreachablePropagation; | |
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| 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 | ||
| } | ||
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| fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
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@@ -38,7 +37,19 @@ impl MirPass<'_> for UnreachablePropagation { | |
| } | ||
| } | ||
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| // 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; | ||
| } | ||
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| body.basic_blocks_mut()[bb].statements.clear(); | ||
| } | ||
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| let replaced = !replacements.is_empty(); | ||
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| for (bb, terminator_kind) in replacements { | ||
| if !tcx.consider_optimizing(|| { | ||
| format!("UnreachablePropagation {:?} ", body.source.def_id()) | ||
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@@ -57,42 +68,55 @@ impl MirPass<'_> for UnreachablePropagation { | |
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| 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(); | ||
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| // 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. | ||
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There was a problem hiding this comment. This statement is only true because we don't have enough range information available, right? There was a problem hiding this comment. (Also: please add a codegen test showing this opt) There was a problem hiding this comment.
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Potentially, but IMO it's a bad idea to turn this: match x {
Enum::A => 1,
Enum::B => 2,
Enum::C => 3,
// Not in the user's code, but added for switchInt's "otherwise" target:
// _ => unreachable_unchecked(),
}into: match x {
Enum::A => 1,
Enum::B => 2,
_ => 3,
}It's erasing the refinement that the third case is only reachable when See also #99762 (comment) where I suggest a solution (make
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| // 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)); | ||
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| let new_targets = SwitchTargets::new(reachable_iter, otherwise); | ||
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| // No unreachable branches were removed. | ||
| if new_targets.all_targets().len() == targets.all_targets().len() { | ||
| return None; | ||
| } | ||
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| 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; | ||
| } | ||
| } | ||
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This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
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This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
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*unreachable
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#100907