Auto merge of #65694 - wesleywiser:uninhabited_enum_variants_pass, r=oli-obk
[mir-opt] Implement pass to remove branches on uninhabited variants Based on discussion [here](https://github.com/rust-lang/rust/pull/64890#discussion_r333612125), this is a pass to eliminate dead code that is caused by branching on an enum with uninhabited variants. r? @oli-obk
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commit
c296b2d830
@ -26,6 +26,7 @@
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#![feature(associated_type_bounds)]
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#![feature(range_is_empty)]
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#![feature(stmt_expr_attributes)]
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#![feature(bool_to_option)]
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#![recursion_limit="256"]
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@ -36,6 +36,7 @@
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pub mod generator;
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pub mod inline;
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pub mod uniform_array_move_out;
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pub mod uninhabited_enum_branching;
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pub(crate) fn provide(providers: &mut Providers<'_>) {
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self::qualify_consts::provide(providers);
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@ -258,6 +259,8 @@ fn run_optimization_passes<'tcx>(
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// Optimizations begin.
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&uninhabited_enum_branching::UninhabitedEnumBranching,
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&simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),
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&uniform_array_move_out::RestoreSubsliceArrayMoveOut::new(tcx),
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&inline::Inline,
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126
src/librustc_mir/transform/uninhabited_enum_branching.rs
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126
src/librustc_mir/transform/uninhabited_enum_branching.rs
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@ -0,0 +1,126 @@
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//! A pass that eliminates branches on uninhabited enum variants.
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use crate::transform::{MirPass, MirSource};
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use rustc::mir::{
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BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind,
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};
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use rustc::ty::layout::{Abi, TyLayout, Variants};
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use rustc::ty::{Ty, TyCtxt};
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pub struct UninhabitedEnumBranching;
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fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
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if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
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p.as_local()
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} else {
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None
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}
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}
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/// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
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/// discriminant is read from. Otherwise, returns None.
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fn get_switched_on_type<'tcx>(
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block_data: &BasicBlockData<'tcx>,
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body: &Body<'tcx>,
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) -> Option<Ty<'tcx>> {
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let terminator = block_data.terminator();
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// Only bother checking blocks which terminate by switching on a local.
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if let Some(local) = get_discriminant_local(&terminator.kind) {
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let stmt_before_term = (block_data.statements.len() > 0)
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.then_with(|| &block_data.statements[block_data.statements.len() - 1].kind);
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if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
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{
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if l.as_local() == Some(local) {
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if let Some(r_local) = place.as_local() {
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let ty = body.local_decls[r_local].ty;
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if ty.is_enum() {
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return Some(ty);
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}
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}
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}
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}
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}
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None
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}
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fn variant_discriminants<'tcx>(
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layout: &TyLayout<'tcx>,
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ty: Ty<'tcx>,
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tcx: TyCtxt<'tcx>,
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) -> Vec<u128> {
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match &layout.details.variants {
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Variants::Single { index } => vec![index.as_u32() as u128],
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Variants::Multiple { variants, .. } => variants
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.iter_enumerated()
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.filter_map(|(idx, layout)| {
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(layout.abi != Abi::Uninhabited)
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.then_with(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
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})
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.collect(),
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}
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}
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impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
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fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
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if source.promoted.is_some() {
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return;
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}
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trace!("UninhabitedEnumBranching starting for {:?}", source);
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let basic_block_count = body.basic_blocks().len();
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for bb in 0..basic_block_count {
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let bb = BasicBlock::from_usize(bb);
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trace!("processing block {:?}", bb);
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let discriminant_ty =
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if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], body) {
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ty
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} else {
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continue;
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};
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let layout = tcx.layout_of(tcx.param_env(source.def_id()).and(discriminant_ty));
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let allowed_variants = if let Ok(layout) = layout {
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variant_discriminants(&layout, discriminant_ty, tcx)
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} else {
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continue;
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};
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trace!("allowed_variants = {:?}", allowed_variants);
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if let TerminatorKind::SwitchInt { values, targets, .. } =
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&mut body.basic_blocks_mut()[bb].terminator_mut().kind
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{
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let vals = &*values;
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let zipped = vals.iter().zip(targets.into_iter());
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let mut matched_values = Vec::with_capacity(allowed_variants.len());
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let mut matched_targets = Vec::with_capacity(allowed_variants.len() + 1);
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for (val, target) in zipped {
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if allowed_variants.contains(val) {
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matched_values.push(*val);
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matched_targets.push(*target);
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} else {
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trace!("eliminating {:?} -> {:?}", val, target);
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}
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}
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// handle the "otherwise" branch
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matched_targets.push(targets.pop().unwrap());
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*values = matched_values.into();
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*targets = matched_targets;
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} else {
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unreachable!()
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}
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}
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}
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}
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224
src/test/mir-opt/uninhabited_enum_branching.rs
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224
src/test/mir-opt/uninhabited_enum_branching.rs
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@ -0,0 +1,224 @@
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enum Empty { }
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// test matching an enum with uninhabited variants
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enum Test1 {
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A(Empty),
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B(Empty),
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C
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}
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// test an enum where the discriminants don't match the variant indexes
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// (the optimization should do nothing here)
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enum Test2 {
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D = 4,
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E = 5,
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}
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fn main() {
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match Test1::C {
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Test1::A(_) => "A(Empty)",
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Test1::B(_) => "B(Empty)",
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Test1::C => "C",
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};
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match Test2::D {
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Test2::D => "D",
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Test2::E => "E",
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};
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}
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// END RUST SOURCE
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//
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// START rustc.main.UninhabitedEnumBranching.before.mir
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// let mut _0: ();
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// let _1: &str;
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// let mut _2: Test1;
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// let mut _3: isize;
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// let mut _4: &str;
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// let mut _5: &str;
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// let _6: &str;
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// let mut _7: Test2;
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// let mut _8: isize;
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// let mut _9: &str;
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// bb0: {
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// StorageLive(_1);
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// StorageLive(_2);
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// _2 = Test1::C;
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// _3 = discriminant(_2);
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// switchInt(move _3) -> [0isize: bb3, 1isize: bb4, 2isize: bb1, otherwise: bb2];
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// }
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// bb1: {
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// StorageLive(_5);
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// _5 = const "C";
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// _1 = &(*_5);
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// StorageDead(_5);
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// goto -> bb5;
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// }
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// bb2: {
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// unreachable;
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// }
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// bb3: {
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// _1 = const "A(Empty)";
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// goto -> bb5;
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// }
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// bb4: {
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// StorageLive(_4);
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// _4 = const "B(Empty)";
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// _1 = &(*_4);
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// StorageDead(_4);
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// goto -> bb5;
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// }
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// bb5: {
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// StorageDead(_2);
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// StorageDead(_1);
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// StorageLive(_6);
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// StorageLive(_7);
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// _7 = Test2::D;
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// _8 = discriminant(_7);
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// switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
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// }
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// bb6: {
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// StorageLive(_9);
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// _9 = const "E";
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// _6 = &(*_9);
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// StorageDead(_9);
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// goto -> bb9;
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// }
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// bb7: {
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// unreachable;
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// }
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// bb8: {
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// _6 = const "D";
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// goto -> bb9;
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// }
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// bb9: {
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// StorageDead(_7);
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// StorageDead(_6);
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// _0 = ();
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// return;
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// }
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// END rustc.main.UninhabitedEnumBranching.before.mir
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// START rustc.main.UninhabitedEnumBranching.after.mir
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// let mut _0: ();
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// let _1: &str;
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// let mut _2: Test1;
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// let mut _3: isize;
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// let mut _4: &str;
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// let mut _5: &str;
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// let _6: &str;
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// let mut _7: Test2;
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// let mut _8: isize;
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// let mut _9: &str;
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// bb0: {
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// StorageLive(_1);
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// StorageLive(_2);
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// _2 = Test1::C;
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// _3 = discriminant(_2);
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// switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
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// }
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// bb1: {
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// StorageLive(_5);
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// _5 = const "C";
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// _1 = &(*_5);
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// StorageDead(_5);
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// goto -> bb5;
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// }
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// bb2: {
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// unreachable;
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// }
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// bb3: {
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// _1 = const "A(Empty)";
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// goto -> bb5;
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// }
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// bb4: {
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// StorageLive(_4);
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// _4 = const "B(Empty)";
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// _1 = &(*_4);
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// StorageDead(_4);
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// goto -> bb5;
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// }
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// bb5: {
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// StorageDead(_2);
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// StorageDead(_1);
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// StorageLive(_6);
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// StorageLive(_7);
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// _7 = Test2::D;
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// _8 = discriminant(_7);
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// switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
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// }
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// bb6: {
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// StorageLive(_9);
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// _9 = const "E";
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// _6 = &(*_9);
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// StorageDead(_9);
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// goto -> bb9;
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// }
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// bb7: {
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// unreachable;
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// }
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// bb8: {
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// _6 = const "D";
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// goto -> bb9;
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// }
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// bb9: {
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// StorageDead(_7);
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// StorageDead(_6);
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// _0 = ();
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// return;
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// }
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// END rustc.main.UninhabitedEnumBranching.after.mir
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// START rustc.main.SimplifyCfg-after-uninhabited-enum-branching.after.mir
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// let mut _0: ();
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// let _1: &str;
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// let mut _2: Test1;
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// let mut _3: isize;
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// let mut _4: &str;
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// let mut _5: &str;
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// let _6: &str;
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// let mut _7: Test2;
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// let mut _8: isize;
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// let mut _9: &str;
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// bb0: {
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// StorageLive(_1);
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// StorageLive(_2);
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// _2 = Test1::C;
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// _3 = discriminant(_2);
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// switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
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// }
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// bb1: {
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// StorageLive(_5);
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// _5 = const "C";
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// _1 = &(*_5);
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// StorageDead(_5);
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// StorageDead(_2);
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// StorageDead(_1);
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// StorageLive(_6);
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// StorageLive(_7);
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// _7 = Test2::D;
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// _8 = discriminant(_7);
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// switchInt(move _8) -> [4isize: bb5, 5isize: bb3, otherwise: bb4];
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// }
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// bb2: {
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// unreachable;
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// }
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// bb3: {
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// StorageLive(_9);
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// _9 = const "E";
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// _6 = &(*_9);
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// StorageDead(_9);
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// goto -> bb6;
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// }
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// bb4: {
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// unreachable;
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// }
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// bb5: {
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// _6 = const "D";
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// goto -> bb6;
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// }
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// bb6: {
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// StorageDead(_7);
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// StorageDead(_6);
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// _0 = ();
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// return;
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// }
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// END rustc.main.SimplifyCfg-after-uninhabited-enum-branching.after.mir
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