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Auto merge of rust-lang#107009 - cjgillot:jump-threading, r=pnkfelix
Implement jump threading MIR opt This pass is an attempt to generalize `ConstGoto` and `SeparateConstSwitch` passes into a more complete jump threading pass. This pass is rather heavy, as it performs a truncated backwards DFS on MIR starting from each `SwitchInt` terminator. This backwards DFS remains very limited, as it only walks through `Goto` terminators. It is build to support constants and discriminants, and a propagating through a very limited set of operations. The pass successfully manages to disentangle the `Some(x?)` use case and the DFA use case. It still needs a few tests before being ready.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,98 @@ | ||
| use rustc_middle::mir::visit::*; | ||
| use rustc_middle::mir::*; | ||
| use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt}; | ||
|
|
||
| const INSTR_COST: usize = 5; | ||
| const CALL_PENALTY: usize = 25; | ||
| const LANDINGPAD_PENALTY: usize = 50; | ||
| const RESUME_PENALTY: usize = 45; | ||
|
|
||
| /// Verify that the callee body is compatible with the caller. | ||
| #[derive(Clone)] | ||
| pub(crate) struct CostChecker<'b, 'tcx> { | ||
| tcx: TyCtxt<'tcx>, | ||
| param_env: ParamEnv<'tcx>, | ||
| cost: usize, | ||
| callee_body: &'b Body<'tcx>, | ||
| instance: Option<ty::Instance<'tcx>>, | ||
| } | ||
|
|
||
| impl<'b, 'tcx> CostChecker<'b, 'tcx> { | ||
| pub fn new( | ||
| tcx: TyCtxt<'tcx>, | ||
| param_env: ParamEnv<'tcx>, | ||
| instance: Option<ty::Instance<'tcx>>, | ||
| callee_body: &'b Body<'tcx>, | ||
| ) -> CostChecker<'b, 'tcx> { | ||
| CostChecker { tcx, param_env, callee_body, instance, cost: 0 } | ||
| } | ||
|
|
||
| pub fn cost(&self) -> usize { | ||
| self.cost | ||
| } | ||
|
|
||
| fn instantiate_ty(&self, v: Ty<'tcx>) -> Ty<'tcx> { | ||
| if let Some(instance) = self.instance { | ||
| instance.instantiate_mir(self.tcx, ty::EarlyBinder::bind(&v)) | ||
| } else { | ||
| v | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> { | ||
| fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) { | ||
| // Don't count StorageLive/StorageDead in the inlining cost. | ||
| match statement.kind { | ||
| StatementKind::StorageLive(_) | ||
| | StatementKind::StorageDead(_) | ||
| | StatementKind::Deinit(_) | ||
| | StatementKind::Nop => {} | ||
| _ => self.cost += INSTR_COST, | ||
| } | ||
| } | ||
|
|
||
| fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, _: Location) { | ||
| let tcx = self.tcx; | ||
| match terminator.kind { | ||
| TerminatorKind::Drop { ref place, unwind, .. } => { | ||
| // If the place doesn't actually need dropping, treat it like a regular goto. | ||
| let ty = self.instantiate_ty(place.ty(self.callee_body, tcx).ty); | ||
| if ty.needs_drop(tcx, self.param_env) { | ||
| self.cost += CALL_PENALTY; | ||
| if let UnwindAction::Cleanup(_) = unwind { | ||
| self.cost += LANDINGPAD_PENALTY; | ||
| } | ||
| } else { | ||
| self.cost += INSTR_COST; | ||
| } | ||
| } | ||
| TerminatorKind::Call { func: Operand::Constant(ref f), unwind, .. } => { | ||
| let fn_ty = self.instantiate_ty(f.const_.ty()); | ||
| self.cost += if let ty::FnDef(def_id, _) = *fn_ty.kind() && tcx.is_intrinsic(def_id) { | ||
| // Don't give intrinsics the extra penalty for calls | ||
| INSTR_COST | ||
| } else { | ||
| CALL_PENALTY | ||
| }; | ||
| if let UnwindAction::Cleanup(_) = unwind { | ||
| self.cost += LANDINGPAD_PENALTY; | ||
| } | ||
| } | ||
| TerminatorKind::Assert { unwind, .. } => { | ||
| self.cost += CALL_PENALTY; | ||
| if let UnwindAction::Cleanup(_) = unwind { | ||
| self.cost += LANDINGPAD_PENALTY; | ||
| } | ||
| } | ||
| TerminatorKind::UnwindResume => self.cost += RESUME_PENALTY, | ||
| TerminatorKind::InlineAsm { unwind, .. } => { | ||
| self.cost += INSTR_COST; | ||
| if let UnwindAction::Cleanup(_) = unwind { | ||
| self.cost += LANDINGPAD_PENALTY; | ||
| } | ||
| } | ||
| _ => self.cost += INSTR_COST, | ||
| } | ||
| } | ||
| } |
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