Auto merge of #65694 - wesleywiser:uninhabited_enum_variants_pass, r=…

…<try>

[mir-opt] Implement pass to remove branches on uninhabited variants

Based on discussion [here](#64890 (comment)), this is a pass to eliminate dead code that is caused by branching on an enum with uninhabited variants.

r? @oli-obk

src/librustc_mir/lib.rs

@@ -26,6 +26,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(associated_type_bounds)]
 #![feature(range_is_empty)]
 #![feature(stmt_expr_attributes)]
+#![feature(bool_to_option)]
 
 #![recursion_limit="256"]
 

src/librustc_mir/transform/mod.rs

@@ -36,6 +36,7 @@ pub mod const_prop;
 pub mod generator;
 pub mod inline;
 pub mod uniform_array_move_out;
+pub mod uninhabited_enum_branching;
 
 pub(crate) fn provide(providers: &mut Providers<'_>) {
     self::qualify_consts::provide(providers);
@@ -257,6 +258,8 @@ fn run_optimization_passes<'tcx>(
 
 
         // Optimizations begin.
+        &uninhabited_enum_branching::UninhabitedEnumBranching,
+        &simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),
         &uniform_array_move_out::RestoreSubsliceArrayMoveOut::new(tcx),
         &inline::Inline,
 

src/librustc_mir/transform/uninhabited_enum_branching.rs

@@ -0,0 +1,119 @@
+//! A pass that eliminates branches on uninhabited enum variants.
+
+use crate::transform::{MirPass, MirSource};
+use rustc::mir::{BasicBlock, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind};
+use rustc::ty::layout::{Abi, TyLayout, Variants};
+use rustc::ty::{Ty, TyCtxt};
+
+pub struct UninhabitedEnumBranching;
+
+fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
+    if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
+        p.as_local()
+    } else {
+        None
+    }
+}
+
+/// Finds all basic blocks which terminate by switching on a discriminant.
+/// Returns a `Vec` of block ids and the `Ty` the discriminant is read from.
+fn find_eligible_blocks<'tcx>(body: &Body<'tcx>) -> Vec<(BasicBlock, Ty<'tcx>)> {
+    let mut blocks_to_update = Vec::new();
+
+    for (bb, block_data) in body.basic_blocks().iter_enumerated() {
+        let terminator = block_data.terminator();
+
+        // Only bother checking blocks which terminate by switching on a local.
+        if let Some(local) = get_discriminant_local(&terminator.kind) {
+            let stmt_before_term = (block_data.statements.len() > 0)
+                .then_with(|| &block_data.statements[block_data.statements.len() - 1].kind);
+
+            if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) =
+                stmt_before_term
+            {
+                if l.as_local() == Some(local) {
+                    if let Some(r_local) = place.as_local() {
+                        let ty = body.local_decls[r_local].ty;
+
+                        if ty.is_enum() {
+                            blocks_to_update.push((bb, ty));
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    blocks_to_update
+}
+
+fn variant_discriminants<'tcx>(
+    layout: &TyLayout<'tcx>,
+    ty: Ty<'tcx>,
+    tcx: TyCtxt<'tcx>,
+) -> Vec<u128> {
+    match &layout.details.variants {
+        Variants::Single { index } => vec![index.as_u32() as u128],
+        Variants::Multiple { variants, .. } => variants
+            .iter_enumerated()
+            .filter_map(|(idx, layout)| {
+                (layout.abi != Abi::Uninhabited)
+                    .then_with(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
+            })
+            .collect(),
+    }
+}
+
+impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        if source.promoted.is_some() {
+            return;
+        }
+
+        trace!("UninhabitedEnumBranching starting for {:?}", source);
+
+        let blocks_to_update = find_eligible_blocks(&body);
+
+        for (bb, discriminant_ty) in blocks_to_update {
+            trace!("processing block {:?}", bb);
+            let block_data = &mut body[bb];
+
+            let layout = tcx.layout_of(tcx.param_env(source.def_id()).and(discriminant_ty));
+
+            let allowed_variants = if let Ok(layout) = layout {
+                variant_discriminants(&layout, discriminant_ty, tcx)
+            } else {
+                continue;
+            };
+
+            trace!("allowed_variants = {:?}", allowed_variants);
+
+            if let TerminatorKind::SwitchInt { values, targets, .. } =
+                &mut block_data.terminator_mut().kind
+            {
+                let vals = &*values;
+                let zipped = vals.iter().zip(targets.into_iter());
+
+                let mut matched_values = Vec::with_capacity(allowed_variants.len());
+                let mut matched_targets = Vec::with_capacity(allowed_variants.len() + 1);
+
+                for (val, target) in zipped {
+                    if allowed_variants.contains(val) {
+                        matched_values.push(*val);
+                        matched_targets.push(*target);
+                    } else {
+                        trace!("eliminating {:?} -> {:?}", val, target);
+                    }
+                }
+
+                // handle the "otherwise" branch
+                matched_targets.push(targets.pop().unwrap());
+
+                *values = matched_values.into();
+                *targets = matched_targets;
+            } else {
+                unreachable!()
+            }
+        }
+    }
+}

src/test/mir-opt/uninhabited_enum_branching.rs

@@ -0,0 +1,224 @@
+enum Empty { }
+
+// test matching an enum with uninhabited variants
+enum Test1 {
+    A(Empty),
+    B(Empty),
+    C
+}
+
+// test an enum where the discriminants don't match the variant indexes
+// (the optimization should do nothing here)
+enum Test2 {
+    D = 4,
+    E = 5,
+}
+
+fn main() {
+    match Test1::C {
+        Test1::A(_) => "A(Empty)",
+        Test1::B(_) => "B(Empty)",
+        Test1::C => "C",
+    };
+
+    match Test2::D {
+        Test2::D => "D",
+        Test2::E => "E",
+    };
+}
+
+// END RUST SOURCE
+//
+// START rustc.main.UninhabitedEnumBranching.before.mir
+// let mut _0: ();
+// let _1: &str;
+// let mut _2: Test1;
+// let mut _3: isize;
+// let mut _4: &str;
+// let mut _5: &str;
+// let _6: &str;
+// let mut _7: Test2;
+// let mut _8: isize;
+// let mut _9: &str;
+// bb0: {
+//   StorageLive(_1);
+//   StorageLive(_2);
+//   _2 = Test1::C;
+//   _3 = discriminant(_2);
+//   switchInt(move _3) -> [0isize: bb3, 1isize: bb4, 2isize: bb1, otherwise: bb2];
+// }
+// bb1: {
+//   StorageLive(_5);
+//   _5 = const "C";
+//   _1 = &(*_5);
+//   StorageDead(_5);
+//   goto -> bb5;
+// }
+// bb2: {
+//   unreachable;
+// }
+// bb3: {
+//   _1 = const "A(Empty)";
+//   goto -> bb5;
+// }
+// bb4: {
+//   StorageLive(_4);
+//   _4 = const "B(Empty)";
+//   _1 = &(*_4);
+//   StorageDead(_4);
+//   goto -> bb5;
+// }
+// bb5: {
+//   StorageDead(_2);
+//   StorageDead(_1);
+//   StorageLive(_6);
+//   StorageLive(_7);
+//   _7 = Test2::D;
+//   _8 = discriminant(_7);
+//   switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
+// }
+// bb6: {
+//   StorageLive(_9);
+//   _9 = const "E";
+//   _6 = &(*_9);
+//   StorageDead(_9);
+//   goto -> bb9;
+// }
+// bb7: {
+//   unreachable;
+// }
+// bb8: {
+//   _6 = const "D";
+//   goto -> bb9;
+// }
+// bb9: {
+//   StorageDead(_7);
+//   StorageDead(_6);
+//   _0 = ();
+//   return;
+// }
+// END rustc.main.UninhabitedEnumBranching.before.mir
+// START rustc.main.UninhabitedEnumBranching.after.mir
+// let mut _0: ();
+// let _1: &str;
+// let mut _2: Test1;
+// let mut _3: isize;
+// let mut _4: &str;
+// let mut _5: &str;
+// let _6: &str;
+// let mut _7: Test2;
+// let mut _8: isize;
+// let mut _9: &str;
+// bb0: {
+//   StorageLive(_1);
+//   StorageLive(_2);
+//   _2 = Test1::C;
+//   _3 = discriminant(_2);
+//   switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
+// }
+// bb1: {
+//   StorageLive(_5);
+//   _5 = const "C";
+//   _1 = &(*_5);
+//   StorageDead(_5);
+//   goto -> bb5;
+// }
+// bb2: {
+//   unreachable;
+// }
+// bb3: {
+//   _1 = const "A(Empty)";
+//   goto -> bb5;
+// }
+// bb4: {
+//   StorageLive(_4);
+//   _4 = const "B(Empty)";
+//   _1 = &(*_4);
+//   StorageDead(_4);
+//   goto -> bb5;
+// }
+// bb5: {
+//   StorageDead(_2);
+//   StorageDead(_1);
+//   StorageLive(_6);
+//   StorageLive(_7);
+//   _7 = Test2::D;
+//   _8 = discriminant(_7);
+//   switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
+// }
+// bb6: {
+//   StorageLive(_9);
+//   _9 = const "E";
+//   _6 = &(*_9);
+//   StorageDead(_9);
+//   goto -> bb9;
+// }
+// bb7: {
+//   unreachable;
+// }
+// bb8: {
+//   _6 = const "D";
+//   goto -> bb9;
+// }
+// bb9: {
+//   StorageDead(_7);
+//   StorageDead(_6);
+//   _0 = ();
+//   return;
+// }
+// END rustc.main.UninhabitedEnumBranching.after.mir
+// START rustc.main.SimplifyCfg-after-uninhabited-enum-branching.after.mir
+// let mut _0: ();
+// let _1: &str;
+// let mut _2: Test1;
+// let mut _3: isize;
+// let mut _4: &str;
+// let mut _5: &str;
+// let _6: &str;
+// let mut _7: Test2;
+// let mut _8: isize;
+// let mut _9: &str;
+// bb0: {
+//   StorageLive(_1);
+//   StorageLive(_2);
+//   _2 = Test1::C;
+//   _3 = discriminant(_2);
+//   switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
+// }
+// bb1: {
+//   StorageLive(_5);
+//   _5 = const "C";
+//   _1 = &(*_5);
+//   StorageDead(_5);
+//   StorageDead(_2);
+//   StorageDead(_1);
+//   StorageLive(_6);
+//   StorageLive(_7);
+//   _7 = Test2::D;
+//   _8 = discriminant(_7);
+//   switchInt(move _8) -> [4isize: bb5, 5isize: bb3, otherwise: bb4];
+// }
+// bb2: {
+//   unreachable;
+// }
+// bb3: {
+//   StorageLive(_9);
+//   _9 = const "E";
+//   _6 = &(*_9);
+//   StorageDead(_9);
+//   goto -> bb6;
+// }
+// bb4: {
+//   unreachable;
+// }
+// bb5: {
+//   _6 = const "D";
+//   goto -> bb6;
+// }
+// bb6: {
+//   StorageDead(_7);
+//   StorageDead(_6);
+//   _0 = ();
+//   return;
+// }
+// END rustc.main.SimplifyCfg-after-uninhabited-enum-branching.after.mir