diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs index 22aed9a9e66..3c37eccc184 100644 --- a/src/librustc_mir/transform/mod.rs +++ b/src/librustc_mir/transform/mod.rs @@ -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); @@ -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, diff --git a/src/librustc_mir/transform/unreachable_prop.rs b/src/librustc_mir/transform/unreachable_prop.rs new file mode 100644 index 00000000000..27173e0c171 --- /dev/null +++ b/src/librustc_mir/transform/unreachable_prop.rs @@ -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( + terminator_kind: &TerminatorKind<'tcx>, + predicate: F, +) -> Option> +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::>(); + let mut values = retained.iter().map(|&(v, _)| *v).collect::>(); + let mut targets = retained.iter().map(|&(_, d)| *d).collect::>(); + + 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, + } +} diff --git a/src/test/mir-opt/simplify_try.rs b/src/test/mir-opt/simplify_try.rs index 656b405ef34..d85eff45b49 100644 --- a/src/test/mir-opt/simplify_try.rs +++ b/src/test/mir-opt/simplify_try.rs @@ -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 @@ -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; +// _0 = move _1; +// nop; +// nop; +// goto -> bb2; // } // bb2: { -// _0 = move _1; -// nop; -// nop; -// goto -> bb3; -// } -// bb3: { // return; // } -// bb4: { +// bb3: { // _0 = move _1; // nop; // nop; -// goto -> bb3; +// goto -> bb2; // } // } // END rustc.try_identity.SimplifyArmIdentity.after.mir diff --git a/src/test/mir-opt/uninhabited_enum_branching.rs b/src/test/mir-opt/uninhabited_enum_branching.rs index aa56918a9b8..dda5fd4fb75 100644 --- a/src/test/mir-opt/uninhabited_enum_branching.rs +++ b/src/test/mir-opt/uninhabited_enum_branching.rs @@ -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; +// _1 = const "A(Empty)"; +// goto -> bb4; // } // bb3: { -// _1 = const "A(Empty)"; -// goto -> bb5; -// } -// bb4: { // 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; +// } +// bb6: { +// _6 = const "D"; +// goto -> bb7; // } // bb7: { -// unreachable; -// } -// bb8: { -// _6 = const "D"; -// goto -> bb9; -// } -// bb9: { // StorageDead(_7); // StorageDead(_6); // _0 = (); @@ -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; +// _1 = const "A(Empty)"; +// goto -> bb4; // } // bb3: { -// _1 = const "A(Empty)"; -// goto -> bb5; -// } -// bb4: { // 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; +// } +// bb6: { +// _6 = const "D"; +// goto -> bb7; // } // bb7: { -// unreachable; -// } -// bb8: { -// _6 = const "D"; -// goto -> bb9; -// } -// bb9: { // StorageDead(_7); // StorageDead(_6); // _0 = (); @@ -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); @@ -196,26 +181,20 @@ fn main() { // StorageLive(_7); // _7 = Test2::D; // _8 = discriminant(_7); -// switchInt(move _8) -> [4isize: bb5, 5isize: bb3, otherwise: bb4]; +// switchInt(move _8) -> [4isize: bb2, otherwise: bb1]; // } -// bb2: { -// unreachable; -// } -// bb3: { +// bb1: { // StorageLive(_9); // _9 = const "E"; // _6 = &(*_9); // StorageDead(_9); -// goto -> bb6; +// goto -> bb3; // } -// bb4: { -// unreachable; -// } -// bb5: { +// bb2: { // _6 = const "D"; -// goto -> bb6; +// goto -> bb3; // } -// bb6: { +// bb3: { // StorageDead(_7); // StorageDead(_6); // _0 = (); diff --git a/src/test/mir-opt/unreachable.rs b/src/test/mir-opt/unreachable.rs new file mode 100644 index 00000000000..fa5c1a074ee --- /dev/null +++ b/src/test/mir-opt/unreachable.rs @@ -0,0 +1,78 @@ +enum Empty {} + +fn empty() -> Option { + None +} + +fn main() { + if let Some(_x) = empty() { + let mut _y; + + if true { + _y = 21; + } else { + _y = 42; + } + + match _x { } + } +} + +// END RUST SOURCE +// START rustc.main.UnreachablePropagation.before.mir +// bb0: { +// StorageLive(_1); +// _1 = const empty() -> bb1; +// } +// bb1: { +// _2 = discriminant(_1); +// switchInt(move _2) -> [1isize: bb3, otherwise: bb2]; +// } +// bb2: { +// _0 = (); +// StorageDead(_1); +// return; +// } +// bb3: { +// StorageLive(_3); +// _3 = move ((_1 as Some).0: Empty); +// StorageLive(_4); +// StorageLive(_5); +// StorageLive(_6); +// _6 = const true; +// switchInt(_6) -> [false: bb4, otherwise: bb5]; +// } +// bb4: { +// _4 = const 42i32; +// _5 = (); +// goto -> bb6; +// } +// bb5: { +// _4 = const 21i32; +// _5 = (); +// goto -> bb6; +// } +// bb6: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_7); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.before.mir +// START rustc.main.UnreachablePropagation.after.mir +// bb0: { +// StorageLive(_1); +// _1 = const empty() -> bb1; +// } +// bb1: { +// _2 = discriminant(_1); +// goto -> bb2; +// } +// bb2: { +// _0 = (); +// StorageDead(_1); +// return; +// } +// } +// END rustc.main.UnreachablePropagation.after.mir diff --git a/src/test/mir-opt/unreachable_asm.rs b/src/test/mir-opt/unreachable_asm.rs new file mode 100644 index 00000000000..ca614ac32b7 --- /dev/null +++ b/src/test/mir-opt/unreachable_asm.rs @@ -0,0 +1,72 @@ +// ignore-tidy-linelength +#![feature(asm)] + +enum Empty {} + +fn empty() -> Option { + None +} + +fn main() { + if let Some(_x) = empty() { + let mut _y; + + if true { + _y = 21; + } else { + _y = 42; + } + + // asm instruction stops unreachable propagation to if else blocks bb4 and bb5. + unsafe { asm!("NOP"); } + match _x { } + } +} + +// END RUST SOURCE +// START rustc.main.UnreachablePropagation.before.mir +// bb4: { +// _4 = const 42i32; +// _5 = (); +// goto -> bb6; +// } +// bb5: { +// _4 = const 21i32; +// _5 = (); +// goto -> bb6; +// } +// bb6: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_7); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _7 = (); +// StorageDead(_7); +// StorageLive(_8); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.before.mir +// START rustc.main.UnreachablePropagation.after.mir +// bb4: { +// _4 = const 42i32; +// _5 = (); +// goto -> bb6; +// } +// bb5: { +// _4 = const 21i32; +// _5 = (); +// goto -> bb6; +// } +// bb6: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_7); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _7 = (); +// StorageDead(_7); +// StorageLive(_8); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.after.mir diff --git a/src/test/mir-opt/unreachable_asm_2.rs b/src/test/mir-opt/unreachable_asm_2.rs new file mode 100644 index 00000000000..8fdbcfb5cab --- /dev/null +++ b/src/test/mir-opt/unreachable_asm_2.rs @@ -0,0 +1,84 @@ +// ignore-tidy-linelength +#![feature(asm)] + +enum Empty {} + +fn empty() -> Option { + None +} + +fn main() { + if let Some(_x) = empty() { + let mut _y; + + if true { + // asm instruction stops unreachable propagation to block bb3. + unsafe { asm!("NOP"); } + _y = 21; + } else { + // asm instruction stops unreachable propagation to block bb3. + unsafe { asm!("NOP"); } + _y = 42; + } + + match _x { } + } +} + +// END RUST SOURCE +// START rustc.main.UnreachablePropagation.before.mir +// bb3: { +// ... +// switchInt(_6) -> [false: bb4, otherwise: bb5]; +// } +// bb4: { +// StorageLive(_8); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _8 = (); +// StorageDead(_8); +// _4 = const 42i32; +// _5 = (); +// goto -> bb6; +// } +// bb5: { +// StorageLive(_7); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _7 = (); +// StorageDead(_7); +// _4 = const 21i32; +// _5 = (); +// goto -> bb6; +// } +// bb6: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_9); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.before.mir +// START rustc.main.UnreachablePropagation.after.mir +// bb3: { +// ... +// switchInt(_6) -> [false: bb4, otherwise: bb5]; +// } +// bb4: { +// StorageLive(_8); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _8 = (); +// StorageDead(_8); +// _4 = const 42i32; +// _5 = (); +// unreachable; +// } +// bb5: { +// StorageLive(_7); +// asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []); +// _7 = (); +// StorageDead(_7); +// _4 = const 21i32; +// _5 = (); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.after.mir diff --git a/src/test/mir-opt/unreachable_diverging.rs b/src/test/mir-opt/unreachable_diverging.rs new file mode 100644 index 00000000000..bf05019d5ce --- /dev/null +++ b/src/test/mir-opt/unreachable_diverging.rs @@ -0,0 +1,65 @@ +pub enum Empty {} + +fn empty() -> Option { + None +} + +fn loop_forever() { + loop {} +} + +fn main() { + let x = true; + if let Some(bomb) = empty() { + if x { + loop_forever() + } + match bomb {} + } +} + +// END RUST SOURCE +// START rustc.main.UnreachablePropagation.before.mir +// bb3: { +// StorageLive(_4); +// _4 = move ((_2 as Some).0: Empty); +// StorageLive(_5); +// StorageLive(_6); +// _6 = _1; +// switchInt(_6) -> [false: bb4, otherwise: bb5]; +// } +// bb4: { +// _5 = (); +// goto -> bb6; +// } +// bb5: { +// _5 = const loop_forever() -> bb6; +// } +// bb6: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_7); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.before.mir +// START rustc.main.UnreachablePropagation.after.mir +// bb3: { +// StorageLive(_4); +// _4 = move ((_2 as Some).0: Empty); +// StorageLive(_5); +// StorageLive(_6); +// _6 = _1; +// goto -> bb4; +// } +// bb4: { +// _5 = const loop_forever() -> bb5; +// } +// bb5: { +// StorageDead(_6); +// StorageDead(_5); +// StorageLive(_7); +// unreachable; +// } +// } +// END rustc.main.UnreachablePropagation.after.mir