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Auto merge of #66329 - ktrianta:mir-opt-unreachable-propagation, r=ol…
…i-obk Add unreachable propagation mir optimization pass @oli-obk suggested we create a MIR pass that optimizes away basic blocks that lead only to basic blocks with terminator kind **unreachable**. This is a first take on this, which we started with @gilescope at RustFest Impl Days. The test currently fails when the compiled program runs (undefined behaviour). Is there a way to avoid running the compiled program?
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,108 @@ | ||
| //! 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::transform::simplify; | ||
| use crate::transform::{MirPass, MirSource}; | ||
| use rustc::mir::*; | ||
| use rustc::ty::TyCtxt; | ||
| use rustc_data_structures::fx::{FxHashMap, FxHashSet}; | ||
| use std::borrow::Cow; | ||
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| pub struct UnreachablePropagation; | ||
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| impl MirPass<'_> for UnreachablePropagation { | ||
| fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) { | ||
| if tcx.sess.opts.debugging_opts.mir_opt_level < 3 { | ||
| // Enable only under -Zmir-opt-level=3 as in some cases (check the deeply-nested-opt | ||
| // perf benchmark) LLVM may spend quite a lot of time optimizing the generated code. | ||
| return; | ||
| } | ||
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| let mut unreachable_blocks = FxHashSet::default(); | ||
| let mut replacements = FxHashMap::default(); | ||
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| for (bb, bb_data) in traversal::postorder(body) { | ||
| let terminator = bb_data.terminator(); | ||
| // HACK: If the block contains any asm statement it is not regarded as unreachable. | ||
| // This is a temporary solution that handles possibly diverging asm statements. | ||
| // Accompanying testcases: mir-opt/unreachable_asm.rs and mir-opt/unreachable_asm_2.rs | ||
| let asm_stmt_in_block = || { | ||
| bb_data.statements.iter().any(|stmt: &Statement<'_>| match stmt.kind { | ||
| StatementKind::InlineAsm(..) => true, | ||
| _ => false, | ||
| }) | ||
| }; | ||
|
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| if terminator.kind == TerminatorKind::Unreachable && !asm_stmt_in_block() { | ||
| unreachable_blocks.insert(bb); | ||
| } else { | ||
| let is_unreachable = |succ: BasicBlock| unreachable_blocks.contains(&succ); | ||
| let terminator_kind_opt = remove_successors(&terminator.kind, is_unreachable); | ||
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| if let Some(terminator_kind) = terminator_kind_opt { | ||
| if terminator_kind == TerminatorKind::Unreachable && !asm_stmt_in_block() { | ||
| unreachable_blocks.insert(bb); | ||
| } | ||
| replacements.insert(bb, terminator_kind); | ||
| } | ||
| } | ||
| } | ||
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| let replaced = !replacements.is_empty(); | ||
| for (bb, terminator_kind) in replacements { | ||
| body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind; | ||
| } | ||
|
|
||
| if replaced { | ||
| simplify::remove_dead_blocks(body); | ||
| } | ||
| } | ||
| } | ||
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| fn remove_successors<F>( | ||
| terminator_kind: &TerminatorKind<'tcx>, | ||
| predicate: F, | ||
| ) -> Option<TerminatorKind<'tcx>> | ||
| where | ||
| F: Fn(BasicBlock) -> bool, | ||
| { | ||
| match *terminator_kind { | ||
| TerminatorKind::Goto { target } if predicate(target) => Some(TerminatorKind::Unreachable), | ||
| TerminatorKind::SwitchInt { ref discr, switch_ty, ref values, ref targets } => { | ||
| let original_targets_len = targets.len(); | ||
| let (otherwise, targets) = targets.split_last().unwrap(); | ||
| let retained = values | ||
| .iter() | ||
| .zip(targets.iter()) | ||
| .filter(|(_, &t)| !predicate(t)) | ||
| .collect::<Vec<_>>(); | ||
| let mut values = retained.iter().map(|&(v, _)| *v).collect::<Vec<_>>(); | ||
| let mut targets = retained.iter().map(|&(_, d)| *d).collect::<Vec<_>>(); | ||
|
|
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| if !predicate(*otherwise) { | ||
| targets.push(*otherwise); | ||
| } else { | ||
| values.pop(); | ||
| } | ||
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| let retained_targets_len = targets.len(); | ||
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| if targets.is_empty() { | ||
| Some(TerminatorKind::Unreachable) | ||
| } else if targets.len() == 1 { | ||
| Some(TerminatorKind::Goto { target: targets[0] }) | ||
| } else if original_targets_len != retained_targets_len { | ||
| Some(TerminatorKind::SwitchInt { | ||
| discr: discr.clone(), | ||
| switch_ty, | ||
| values: Cow::from(values), | ||
| targets, | ||
| }) | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
| _ => None, | ||
| } | ||
| } |
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