Reverts rust-lang/rust#117206

compiler/rustc_mir_transform/src/const_goto.rs

@@ -0,0 +1,128 @@
+//! This pass optimizes the following sequence
+//! ```rust,ignore (example)
+//! bb2: {
+//!     _2 = const true;
+//!     goto -> bb3;
+//! }
+//!
+//! bb3: {
+//!     switchInt(_2) -> [false: bb4, otherwise: bb5];
+//! }
+//! ```
+//! into
+//! ```rust,ignore (example)
+//! bb2: {
+//!     _2 = const true;
+//!     goto -> bb5;
+//! }
+//! ```
+
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_middle::{mir::visit::Visitor, ty::ParamEnv};
+
+use super::simplify::{simplify_cfg, simplify_locals};
+
+pub struct ConstGoto;
+
+impl<'tcx> MirPass<'tcx> for ConstGoto {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        // This pass participates in some as-of-yet untested unsoundness found
+        // in https://github.com/rust-lang/rust/issues/112460
+        sess.mir_opt_level() >= 2 && sess.opts.unstable_opts.unsound_mir_opts
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running ConstGoto on {:?}", body.source);
+        let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
+        let mut opt_finder =
+            ConstGotoOptimizationFinder { tcx, body, optimizations: vec![], param_env };
+        opt_finder.visit_body(body);
+        let should_simplify = !opt_finder.optimizations.is_empty();
+        for opt in opt_finder.optimizations {
+            let block = &mut body.basic_blocks_mut()[opt.bb_with_goto];
+            block.statements.extend(opt.stmts_move_up);
+            let terminator = block.terminator_mut();
+            let new_goto = TerminatorKind::Goto { target: opt.target_to_use_in_goto };
+            debug!("SUCCESS: replacing `{:?}` with `{:?}`", terminator.kind, new_goto);
+            terminator.kind = new_goto;
+        }
+
+        // if we applied optimizations, we potentially have some cfg to cleanup to
+        // make it easier for further passes
+        if should_simplify {
+            simplify_cfg(body);
+            simplify_locals(body, tcx);
+        }
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'_, 'tcx> {
+    fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
+        if data.is_cleanup {
+            // Because of the restrictions around control flow in cleanup blocks, we don't perform
+            // this optimization at all in such blocks.
+            return;
+        }
+        self.super_basic_block_data(block, data);
+    }
+
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
+        let _: Option<_> = try {
+            let target = terminator.kind.as_goto()?;
+            // We only apply this optimization if the last statement is a const assignment
+            let last_statement = self.body.basic_blocks[location.block].statements.last()?;
+
+            if let (place, Rvalue::Use(Operand::Constant(_const))) =
+                last_statement.kind.as_assign()?
+            {
+                // We found a constant being assigned to `place`.
+                // Now check that the target of this Goto switches on this place.
+                let target_bb = &self.body.basic_blocks[target];
+
+                // The `StorageDead(..)` statement does not affect the functionality of mir.
+                // We can move this part of the statement up to the predecessor.
+                let mut stmts_move_up = Vec::new();
+                for stmt in &target_bb.statements {
+                    if let StatementKind::StorageDead(..) = stmt.kind {
+                        stmts_move_up.push(stmt.clone())
+                    } else {
+                        None?;
+                    }
+                }
+
+                let target_bb_terminator = target_bb.terminator();
+                let (discr, targets) = target_bb_terminator.kind.as_switch()?;
+                if discr.place() == Some(*place) {
+                    let switch_ty = place.ty(self.body.local_decls(), self.tcx).ty;
+                    debug_assert_eq!(switch_ty, _const.ty());
+                    // We now know that the Switch matches on the const place, and it is statementless
+                    // Now find which value in the Switch matches the const value.
+                    let const_value = _const.const_.try_eval_bits(self.tcx, self.param_env)?;
+                    let target_to_use_in_goto = targets.target_for_value(const_value);
+                    self.optimizations.push(OptimizationToApply {
+                        bb_with_goto: location.block,
+                        target_to_use_in_goto,
+                        stmts_move_up,
+                    });
+                }
+            }
+            Some(())
+        };
+
+        self.super_terminator(terminator, location);
+    }
+}
+
+struct OptimizationToApply<'tcx> {
+    bb_with_goto: BasicBlock,
+    target_to_use_in_goto: BasicBlock,
+    stmts_move_up: Vec<Statement<'tcx>>,
+}
+
+pub struct ConstGotoOptimizationFinder<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    body: &'a Body<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    optimizations: Vec<OptimizationToApply<'tcx>>,
+}

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -60,7 +60,7 @@ const MAX_PLACES: usize = 100;
 
 impl<'tcx> MirPass<'tcx> for JumpThreading {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 2
+        sess.mir_opt_level() >= 4
     }
 
     #[instrument(skip_all level = "debug")]

compiler/rustc_mir_transform/src/lib.rs

@@ -59,6 +59,7 @@ mod remove_place_mention;
 mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
+mod const_goto;
 mod const_prop;
 mod const_prop_lint;
 mod copy_prop;
@@ -102,6 +103,7 @@ mod remove_unneeded_drops;
 mod remove_zsts;
 mod required_consts;
 mod reveal_all;
+mod separate_const_switch;
 mod shim;
 mod ssa;
 // This pass is public to allow external drivers to perform MIR cleanup
@@ -588,6 +590,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 
             // Has to run after `slice::len` lowering
             &normalize_array_len::NormalizeArrayLen,
+            &const_goto::ConstGoto,
             &ref_prop::ReferencePropagation,
             &sroa::ScalarReplacementOfAggregates,
             &match_branches::MatchBranchSimplification,
@@ -598,6 +601,10 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &dead_store_elimination::DeadStoreElimination::Initial,
             &gvn::GVN,
             &simplify::SimplifyLocals::AfterGVN,
+            // Perform `SeparateConstSwitch` after SSA-based analyses, as cloning blocks may
+            // destroy the SSA property. It should still happen before const-propagation, so the
+            // latter pass will leverage the created opportunities.
+            &separate_const_switch::SeparateConstSwitch,
             &dataflow_const_prop::DataflowConstProp,
             &const_debuginfo::ConstDebugInfo,
             &o1(simplify_branches::SimplifyConstCondition::AfterConstProp),