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Merged
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Jan 15, 2020
Merged

Add unreachable propagation mir optimization pass #66329

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2 changes: 2 additions & 0 deletions src/librustc_mir/transform/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,7 @@ pub mod simplify;
pub mod simplify_branches;
pub mod simplify_try;
pub mod uninhabited_enum_branching;
pub mod unreachable_prop;

pub(crate) fn provide(providers: &mut Providers<'_>) {
self::check_unsafety::provide(providers);
Expand Down Expand Up @@ -299,6 +300,7 @@ fn run_optimization_passes<'tcx>(
// From here on out, regions are gone.
&erase_regions::EraseRegions,
// Optimizations begin.
&unreachable_prop::UnreachablePropagation,
&uninhabited_enum_branching::UninhabitedEnumBranching,
&simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),
&inline::Inline,
Expand Down
108 changes: 108 additions & 0 deletions src/librustc_mir/transform/unreachable_prop.rs
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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;

pub struct UnreachablePropagation;

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;
}

let mut unreachable_blocks = FxHashSet::default();
let mut replacements = FxHashMap::default();

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,
})
};

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);

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);
}
}
}

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);
}
}
}

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<_>>();

if !predicate(*otherwise) {
targets.push(*otherwise);
} else {
values.pop();
}

let retained_targets_len = targets.len();

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,
}
}
26 changes: 10 additions & 16 deletions src/test/mir-opt/simplify_try.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -47,25 +47,22 @@ fn main() {
// }
// bb0: {
// _5 = discriminant(_1);
// switchInt(move _5) -> [0isize: bb4, 1isize: bb2, otherwise: bb1];
// switchInt(move _5) -> [0isize: bb3, otherwise: bb1];
// }
// bb1: {
// unreachable;
// }
// bb2: {
// _6 = ((_1 as Err).0: i32);
// ((_0 as Err).0: i32) = move _6;
// discriminant(_0) = 1;
// goto -> bb3;
// goto -> bb2;
// }
// bb3: {
// bb2: {
// return;
// }
// bb4: {
// bb3: {
// _10 = ((_1 as Ok).0: u32);
// ((_0 as Ok).0: u32) = move _10;
// discriminant(_0) = 0;
// goto -> bb3;
// goto -> bb2;
// }
// }
// END rustc.try_identity.SimplifyArmIdentity.before.mir
Expand Down Expand Up @@ -109,25 +106,22 @@ fn main() {
// }
// bb0: {
// _5 = discriminant(_1);
// switchInt(move _5) -> [0isize: bb4, 1isize: bb2, otherwise: bb1];
// switchInt(move _5) -> [0isize: bb3, otherwise: bb1];
// }
// bb1: {
// unreachable;
// }
// bb2: {
// _0 = move _1;
// nop;
// nop;
// goto -> bb3;
// goto -> bb2;
// }
// bb3: {
// bb2: {
// return;
// }
// bb4: {
// bb3: {
// _0 = move _1;
// nop;
// nop;
// goto -> bb3;
// goto -> bb2;
// }
// }
// END rustc.try_identity.SimplifyArmIdentity.after.mir
Expand Down
81 changes: 30 additions & 51 deletions src/test/mir-opt/uninhabited_enum_branching.rs
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Expand Up @@ -45,53 +45,47 @@ fn main() {
// StorageLive(_2);
// _2 = Test1::C;
// _3 = discriminant(_2);
// switchInt(move _3) -> [0isize: bb3, 1isize: bb4, 2isize: bb1, otherwise: bb2];
// switchInt(move _3) -> [0isize: bb2, 1isize: bb3, otherwise: bb1];
// }
// bb1: {
// StorageLive(_5);
// _5 = const "C";
// _1 = &(*_5);
// StorageDead(_5);
// goto -> bb5;
// goto -> bb4;
// }
// bb2: {
// unreachable;
// }
// bb3: {
// _1 = const "A(Empty)";
// goto -> bb5;
// goto -> bb4;
// }
// bb4: {
// bb3: {
// StorageLive(_4);
// _4 = const "B(Empty)";
// _1 = &(*_4);
// StorageDead(_4);
// goto -> bb5;
// goto -> bb4;
// }
// bb5: {
// bb4: {
// StorageDead(_2);
// StorageDead(_1);
// StorageLive(_6);
// StorageLive(_7);
// _7 = Test2::D;
// _8 = discriminant(_7);
// switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
// switchInt(move _8) -> [4isize: bb6, otherwise: bb5];
// }
// bb6: {
// bb5: {
// StorageLive(_9);
// _9 = const "E";
// _6 = &(*_9);
// StorageDead(_9);
// goto -> bb9;
// }
// bb7: {
// unreachable;
// goto -> bb7;
// }
// bb8: {
// bb6: {
// _6 = const "D";
// goto -> bb9;
// goto -> bb7;
// }
// bb9: {
// bb7: {
// StorageDead(_7);
// StorageDead(_6);
// _0 = ();
Expand All @@ -114,53 +108,47 @@ fn main() {
// StorageLive(_2);
// _2 = Test1::C;
// _3 = discriminant(_2);
// switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
// switchInt(move _3) -> bb1;
// }
// bb1: {
// StorageLive(_5);
// _5 = const "C";
// _1 = &(*_5);
// StorageDead(_5);
// goto -> bb5;
// goto -> bb4;
// }
// bb2: {
// unreachable;
// }
// bb3: {
// _1 = const "A(Empty)";
// goto -> bb5;
// goto -> bb4;
// }
// bb4: {
// bb3: {
// StorageLive(_4);
// _4 = const "B(Empty)";
// _1 = &(*_4);
// StorageDead(_4);
// goto -> bb5;
// goto -> bb4;
// }
// bb5: {
// bb4: {
// StorageDead(_2);
// StorageDead(_1);
// StorageLive(_6);
// StorageLive(_7);
// _7 = Test2::D;
// _8 = discriminant(_7);
// switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
// switchInt(move _8) -> [4isize: bb6, otherwise: bb5];
// }
// bb6: {
// bb5: {
// StorageLive(_9);
// _9 = const "E";
// _6 = &(*_9);
// StorageDead(_9);
// goto -> bb9;
// goto -> bb7;
// }
// bb7: {
// unreachable;
// }
// bb8: {
// bb6: {
// _6 = const "D";
// goto -> bb9;
// goto -> bb7;
// }
// bb9: {
// bb7: {
// StorageDead(_7);
// StorageDead(_6);
// _0 = ();
Expand All @@ -183,9 +171,6 @@ fn main() {
// StorageLive(_2);
// _2 = Test1::C;
// _3 = discriminant(_2);
// switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
// }
// bb1: {
// StorageLive(_5);
// _5 = const "C";
// _1 = &(*_5);
Expand All @@ -196,26 +181,20 @@ fn main() {
// StorageLive(_7);
// _7 = Test2::D;
// _8 = discriminant(_7);
// switchInt(move _8) -> [4isize: bb5, 5isize: bb3, otherwise: bb4];
// }
// bb2: {
// unreachable;
// switchInt(move _8) -> [4isize: bb2, otherwise: bb1];
// }
// bb3: {
// bb1: {
// StorageLive(_9);
// _9 = const "E";
// _6 = &(*_9);
// StorageDead(_9);
// goto -> bb6;
// }
// bb4: {
// unreachable;
// goto -> bb3;
// }
// bb5: {
// bb2: {
// _6 = const "D";
// goto -> bb6;
// goto -> bb3;
// }
// bb6: {
// bb3: {
// StorageDead(_7);
// StorageDead(_6);
// _0 = ();
Expand Down
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