Auto merge of rust-lang#113970 - cjgillot:assume-all-the-things, r=nikic

Replace switch to unreachable by assume statements

`UnreachablePropagation` currently keeps some switch terminators alive in order to ensure codegen can infer the inequalities on the discriminants.

This PR proposes to encode those inequalities as `Assume` statements.

This allows to simplify MIR further by removing some useless terminators.

Author: bors

Diff for: compiler/rustc_mir_transform/src/lib.rs

@@ -568,10 +568,11 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &[
             &check_alignment::CheckAlignment,
             &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
-            &unreachable_prop::UnreachablePropagation,
+            &inline::Inline,
+            // Substitutions during inlining may introduce switch on enums with uninhabited branches.
             &uninhabited_enum_branching::UninhabitedEnumBranching,
+            &unreachable_prop::UnreachablePropagation,
             &o1(simplify::SimplifyCfg::AfterUninhabitedEnumBranching),
-            &inline::Inline,
             &remove_storage_markers::RemoveStorageMarkers,
             &remove_zsts::RemoveZsts,
             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering

Diff for: compiler/rustc_mir_transform/src/simplify_branches.rs

@@ -16,8 +16,25 @@ impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running SimplifyConstCondition on {:?}", body.source);
         let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
-        for block in body.basic_blocks_mut() {
+        'blocks: for block in body.basic_blocks_mut() {
+            for stmt in block.statements.iter_mut() {
+                if let StatementKind::Intrinsic(box ref intrinsic) = stmt.kind
+                    && let NonDivergingIntrinsic::Assume(discr) = intrinsic
+                    && let Operand::Constant(ref c) = discr
+                    && let Some(constant) = c.const_.try_eval_bool(tcx, param_env)
+                {
+                    if constant {
+                        stmt.make_nop();
+                    } else {
+                        block.statements.clear();
+                        block.terminator_mut().kind = TerminatorKind::Unreachable;
+                        continue 'blocks;
+                    }
+                }
+            }
+
             let terminator = block.terminator_mut();
             terminator.kind = match terminator.kind {
                 TerminatorKind::SwitchInt {

Diff for: compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -3,8 +3,7 @@
 use crate::MirPass;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir::{
-    BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, Terminator,
-    TerminatorKind,
+    BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, Terminator, TerminatorKind,
 };
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{Ty, TyCtxt};
@@ -30,17 +29,20 @@ fn get_switched_on_type<'tcx>(
     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[..]
-        && let StatementKind::Assign(box (l, Rvalue::Discriminant(place))) = stmt_before_term.kind
+    let local = get_discriminant_local(&terminator.kind)?;
+
+    let stmt_before_term = block_data.statements.last()?;
+
+    if let StatementKind::Assign(box (l, Rvalue::Discriminant(place))) = stmt_before_term.kind
         && l.as_local() == Some(local)
-        && let ty = place.ty(body, tcx).ty
-        && ty.is_enum()
     {
-        Some(ty)
-    } else {
-        None
+        let ty = place.ty(body, tcx).ty;
+        if ty.is_enum() {
+            return Some(ty);
+        }
     }
+
+    None
 }
 
 fn variant_discriminants<'tcx>(
@@ -67,28 +69,6 @@ fn variant_discriminants<'tcx>(
     }
 }
 
-/// Ensures that the `otherwise` branch leads to an unreachable bb, returning `None` if so and a new
-/// bb to use as the new target if not.
-fn ensure_otherwise_unreachable<'tcx>(
-    body: &Body<'tcx>,
-    targets: &SwitchTargets,
-) -> Option<BasicBlockData<'tcx>> {
-    let otherwise = targets.otherwise();
-    let bb = &body.basic_blocks[otherwise];
-    if bb.terminator().kind == TerminatorKind::Unreachable
-        && bb.statements.iter().all(|s| matches!(&s.kind, StatementKind::StorageDead(_)))
-    {
-        return None;
-    }
-
-    let mut new_block = BasicBlockData::new(Some(Terminator {
-        source_info: bb.terminator().source_info,
-        kind: TerminatorKind::Unreachable,
-    }));
-    new_block.is_cleanup = bb.is_cleanup;
-    Some(new_block)
-}
-
 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
         sess.mir_opt_level() > 0
@@ -97,13 +77,16 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         trace!("UninhabitedEnumBranching starting for {:?}", body.source);
 
-        for bb in body.basic_blocks.indices() {
+        let mut removable_switchs = Vec::new();
+
+        for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
             trace!("processing block {:?}", bb);
 
-            let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks[bb], tcx, body)
-            else {
+            if bb_data.is_cleanup {
                 continue;
-            };
+            }
+
+            let Some(discriminant_ty) = get_switched_on_type(&bb_data, tcx, body) else { continue };
 
             let layout = tcx.layout_of(
                 tcx.param_env_reveal_all_normalized(body.source.def_id()).and(discriminant_ty),
@@ -117,31 +100,38 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
 
             trace!("allowed_variants = {:?}", allowed_variants);
 
-            if let TerminatorKind::SwitchInt { targets, .. } =
-                &mut body.basic_blocks_mut()[bb].terminator_mut().kind
-            {
-                let mut new_targets = SwitchTargets::new(
-                    targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
-                    targets.otherwise(),
-                );
-
-                if new_targets.iter().count() == allowed_variants.len() {
-                    if let Some(updated) = ensure_otherwise_unreachable(body, &new_targets) {
-                        let new_otherwise = body.basic_blocks_mut().push(updated);
-                        *new_targets.all_targets_mut().last_mut().unwrap() = new_otherwise;
-                    }
-                }
+            let terminator = bb_data.terminator();
+            let TerminatorKind::SwitchInt { targets, .. } = &terminator.kind else { bug!() };
 
-                if let TerminatorKind::SwitchInt { targets, .. } =
-                    &mut body.basic_blocks_mut()[bb].terminator_mut().kind
-                {
-                    *targets = new_targets;
+            let mut reachable_count = 0;
+            for (index, (val, _)) in targets.iter().enumerate() {
+                if allowed_variants.contains(&val) {
+                    reachable_count += 1;
                 } else {
-                    unreachable!()
+                    removable_switchs.push((bb, index));
                 }
-            } else {
-                unreachable!()
             }
+
+            if reachable_count == allowed_variants.len() {
+                removable_switchs.push((bb, targets.iter().count()));
+            }
+        }
+
+        if removable_switchs.is_empty() {
+            return;
+        }
+
+        let new_block = BasicBlockData::new(Some(Terminator {
+            source_info: body.basic_blocks[removable_switchs[0].0].terminator().source_info,
+            kind: TerminatorKind::Unreachable,
+        }));
+        let unreachable_block = body.basic_blocks.as_mut().push(new_block);
+
+        for (bb, index) in removable_switchs {
+            let bb = &mut body.basic_blocks.as_mut()[bb];
+            let terminator = bb.terminator_mut();
+            let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind else { bug!() };
+            targets.all_targets_mut()[index] = unreachable_block;
         }
     }
 }