1 files changed, 117 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/const_goto.rs b/compiler/rustc_mir_transform/src/const_goto.rs
new file mode 100644
index 000000000..5acf939f0
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/const_goto.rs
@@ -0,0 +1,117 @@
+//! 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 crate::MirPass;
+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 {
+        sess.mir_opt_level() >= 4
+    }
+
+    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(tcx, body);
+            simplify_locals(body, tcx);
+        }
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'_, 'tcx> {
+    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, switch_ty, targets) = target_bb_terminator.kind.as_switch()?;
+                if discr.place() == Some(*place) {
+                    // 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.literal.try_eval_bits(self.tcx, self.param_env, switch_ty)?;
+                    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>>,
+}