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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_mir_transform/src/const_goto.rs | 117 |
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>>, +} |