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| 1 | +// Copyright 2016 The Rust Project Developers. See the COPYRIGHT |
| 2 | +// file at the top-level directory of this distribution and at |
| 3 | +// http://rust-lang.org/COPYRIGHT. |
| 4 | +// |
| 5 | +// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| 6 | +// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| 7 | +// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| 8 | +// option. This file may not be copied, modified, or distributed |
| 9 | +// except according to those terms. |
| 10 | + |
| 11 | +//! Identify those variables whose entire value will eventually be |
| 12 | +//! returned from the fn via the RETURN_PLACE. As an optimization, we |
| 13 | +//! can skip computing liveness results for those variables. The idea |
| 14 | +//! is that the return type of the fn only ever contains free |
| 15 | +//! regions. Therefore, the types of those variables are going to |
| 16 | +//! ultimately be contrained to outlive those free regions -- since |
| 17 | +//! free regions are always live for the entire body, this implies |
| 18 | +//! that the liveness results are not important for those regions. |
| 19 | +//! This is most important in the "fns" that we create to represent static |
| 20 | +//! values, since those are often really quite large, and all regions in them |
| 21 | +//! will ultimately be constrained to be `'static`. Two examples: |
| 22 | +//! |
| 23 | +//! ``` |
| 24 | +//! fn foo() -> &'static [u32] { &[] } |
| 25 | +//! static FOO: &[u32] = &[]; |
| 26 | +//! ``` |
| 27 | +//! |
| 28 | +//! In both these cases, the return value will only have static lifetime. |
| 29 | +//! |
| 30 | +//! NB: The simple logic here relies on the fact that outlives |
| 31 | +//! relations in our analysis don't have locations. Otherwise, we |
| 32 | +//! would have to restrict ourselves to values that are |
| 33 | +//! *unconditionally* returned (which would still cover the "big |
| 34 | +//! static value" case). |
| 35 | +//! |
| 36 | +//! The way that this code works is to use union-find -- we iterate |
| 37 | +//! over the MIR and union together two variables X and Y if all |
| 38 | +//! regions in the value of Y are going to be stored into X -- that |
| 39 | +//! is, if `typeof(X): 'a` requires that `typeof(Y): 'a`. This means |
| 40 | +//! that e.g. we can union together `x` and `y` if we have something |
| 41 | +//! like `x = (y, 22)`, but not something like `x = y.f` (since there |
| 42 | +//! may be regions in the type of `y` that do not appear in the field |
| 43 | +//! `f`). |
| 44 | +
|
| 45 | +use rustc::mir::visit::Visitor; |
| 46 | +use rustc::mir::*; |
| 47 | + |
| 48 | +use rustc_data_structures::indexed_vec::Idx; |
| 49 | +use rustc_data_structures::unify as ut; |
| 50 | + |
| 51 | +crate struct EscapingLocals { |
| 52 | + unification_table: ut::UnificationTable<ut::InPlace<AssignedLocal>>, |
| 53 | +} |
| 54 | + |
| 55 | +impl EscapingLocals { |
| 56 | + crate fn compute(mir: &Mir<'tcx>) -> Self { |
| 57 | + let mut visitor = GatherAssignedLocalsVisitor::new(); |
| 58 | + visitor.visit_mir(mir); |
| 59 | + |
| 60 | + EscapingLocals { |
| 61 | + unification_table: visitor.unification_table, |
| 62 | + } |
| 63 | + } |
| 64 | + |
| 65 | + /// True if `local` is known to escape into static |
| 66 | + /// memory. |
| 67 | + crate fn escapes_into_return(&mut self, local: Local) -> bool { |
| 68 | + let return_place = AssignedLocal::from(RETURN_PLACE); |
| 69 | + let other_place = AssignedLocal::from(local); |
| 70 | + self.unification_table.unioned(return_place, other_place) |
| 71 | + } |
| 72 | +} |
| 73 | + |
| 74 | +/// The MIR visitor gathering the union-find of the locals used in |
| 75 | +/// assignments. |
| 76 | +struct GatherAssignedLocalsVisitor { |
| 77 | + unification_table: ut::UnificationTable<ut::InPlace<AssignedLocal>>, |
| 78 | +} |
| 79 | + |
| 80 | +#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)] |
| 81 | +struct AssignedLocal(u32); |
| 82 | + |
| 83 | +impl ut::UnifyKey for AssignedLocal { |
| 84 | + type Value = (); |
| 85 | + |
| 86 | + fn index(&self) -> u32 { |
| 87 | + self.0 |
| 88 | + } |
| 89 | + |
| 90 | + fn from_index(i: u32) -> AssignedLocal { |
| 91 | + AssignedLocal(i) |
| 92 | + } |
| 93 | + |
| 94 | + fn tag() -> &'static str { |
| 95 | + "AssignedLocal" |
| 96 | + } |
| 97 | +} |
| 98 | + |
| 99 | +impl From<Local> for AssignedLocal { |
| 100 | + fn from(item: Local) -> Self { |
| 101 | + // newtype_indexes use usize but are u32s. |
| 102 | + assert!(item.index() < ::std::u32::MAX as usize); |
| 103 | + AssignedLocal(item.index() as u32) |
| 104 | + } |
| 105 | +} |
| 106 | + |
| 107 | +impl GatherAssignedLocalsVisitor { |
| 108 | + fn new() -> Self { |
| 109 | + Self { |
| 110 | + unification_table: ut::UnificationTable::new(), |
| 111 | + } |
| 112 | + } |
| 113 | + |
| 114 | + fn union_locals_if_needed(&mut self, lvalue: Option<Local>, rvalue: Option<Local>) { |
| 115 | + if let Some(lvalue) = lvalue { |
| 116 | + if let Some(rvalue) = rvalue { |
| 117 | + if lvalue != rvalue { |
| 118 | + debug!("EscapingLocals: union {:?} and {:?}", lvalue, rvalue); |
| 119 | + self.unification_table |
| 120 | + .union(AssignedLocal::from(lvalue), AssignedLocal::from(rvalue)); |
| 121 | + } |
| 122 | + } |
| 123 | + } |
| 124 | + } |
| 125 | +} |
| 126 | + |
| 127 | +// Returns the potential `Local` associated to this `Place` or `PlaceProjection` |
| 128 | +fn find_local_in_place(place: &Place) -> Option<Local> { |
| 129 | + match place { |
| 130 | + Place::Local(local) => Some(*local), |
| 131 | + |
| 132 | + // If you do e.g. `x = a.f` then only *part* of the type of |
| 133 | + // `a` escapes into `x` (the part contained in `f`); if `a`'s |
| 134 | + // type has regions that don't appear in `f`, those might not |
| 135 | + // escape. |
| 136 | + Place::Projection(..) => None, |
| 137 | + |
| 138 | + Place::Static { .. } | Place::Promoted { .. } => None, |
| 139 | + } |
| 140 | +} |
| 141 | + |
| 142 | +// Returns the potential `Local` in this `Operand`. |
| 143 | +fn find_local_in_operand(op: &Operand) -> Option<Local> { |
| 144 | + // Conservatively check a subset of `Operand`s we know our |
| 145 | + // benchmarks track, for example `html5ever`. |
| 146 | + match op { |
| 147 | + Operand::Copy(place) | Operand::Move(place) => find_local_in_place(place), |
| 148 | + Operand::Constant(_) => None, |
| 149 | + } |
| 150 | +} |
| 151 | + |
| 152 | +impl Visitor<'tcx> for GatherAssignedLocalsVisitor { |
| 153 | + fn visit_mir(&mut self, mir: &Mir<'tcx>) { |
| 154 | + // We need as many union-find keys as there are locals |
| 155 | + for _ in 0..mir.local_decls.len() { |
| 156 | + self.unification_table.new_key(()); |
| 157 | + } |
| 158 | + |
| 159 | + self.super_mir(mir); |
| 160 | + } |
| 161 | + |
| 162 | + fn visit_assign( |
| 163 | + &mut self, |
| 164 | + block: BasicBlock, |
| 165 | + place: &Place<'tcx>, |
| 166 | + rvalue: &Rvalue<'tcx>, |
| 167 | + location: Location, |
| 168 | + ) { |
| 169 | + let local = find_local_in_place(place); |
| 170 | + |
| 171 | + // Find those cases where there is a `Place` consumed by |
| 172 | + // `rvalue` and we know that all regions in its type will be |
| 173 | + // incorporated into `place`, the `Place` we are assigning to. |
| 174 | + match rvalue { |
| 175 | + // `x = y` is the simplest possible case. |
| 176 | + Rvalue::Use(op) => self.union_locals_if_needed(local, find_local_in_operand(op)), |
| 177 | + |
| 178 | + // `X = &'r P` -- the type of `X` will be `&'r T_P`, where |
| 179 | + // `T_P` is the type of `P`. |
| 180 | + Rvalue::Ref(_, _, place) => { |
| 181 | + // Special case: if you have `X = &*Y` (or `X = &**Y` |
| 182 | + // etc), then the outlives relationships will ensure |
| 183 | + // that all regions in `Y` are constrained by regions |
| 184 | + // in `X` -- this is because the lifetimes of the |
| 185 | + // references we deref through are required to outlive |
| 186 | + // the borrow lifetime `'r` (which appears in `X`). |
| 187 | + // |
| 188 | + // (We don't actually need to check the type of `Y`: |
| 189 | + // since `ProjectionElem::Deref` represents a built-in |
| 190 | + // deref and not an overloaded deref, if the thing we |
| 191 | + // deref through is not a reference, then it must be a |
| 192 | + // `Box` or `*const`, in which case it contains no |
| 193 | + // references.) |
| 194 | + let mut place_ref = place; |
| 195 | + while let Place::Projection(proj) = place_ref { |
| 196 | + if let ProjectionElem::Deref = proj.elem { |
| 197 | + place_ref = &proj.base; |
| 198 | + } else { |
| 199 | + break; |
| 200 | + } |
| 201 | + } |
| 202 | + |
| 203 | + self.union_locals_if_needed(local, find_local_in_place(place_ref)) |
| 204 | + } |
| 205 | + |
| 206 | + Rvalue::Cast(kind, op, _) => match kind { |
| 207 | + CastKind::Unsize => { |
| 208 | + // Casting a `&[T; N]` to `&[T]` or `&Foo` to `&Trait` -- |
| 209 | + // in both cases, no regions are "lost". |
| 210 | + self.union_locals_if_needed(local, find_local_in_operand(op)) |
| 211 | + } |
| 212 | + _ => (), |
| 213 | + }, |
| 214 | + |
| 215 | + // Constructing an aggregate like `(x,)` or `Foo { x }` |
| 216 | + // includes the full type of `x`. |
| 217 | + Rvalue::Aggregate(_, ops) => { |
| 218 | + for rvalue in ops.iter().map(find_local_in_operand) { |
| 219 | + self.union_locals_if_needed(local, rvalue); |
| 220 | + } |
| 221 | + } |
| 222 | + |
| 223 | + // For other things, be conservative and do not union. |
| 224 | + _ => (), |
| 225 | + }; |
| 226 | + |
| 227 | + self.super_assign(block, place, rvalue, location); |
| 228 | + } |
| 229 | +} |
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