rustc_mir_transform/deduce_param_attrs.rs
1//! Deduces supplementary parameter attributes from MIR.
2//!
3//! Deduced parameter attributes are those that can only be soundly determined by examining the
4//! body of the function instead of just the signature. These can be useful for optimization
5//! purposes on a best-effort basis. We compute them here and store them into the crate metadata so
6//! dependent crates can use them.
7
8use rustc_hir::def_id::LocalDefId;
9use rustc_index::bit_set::DenseBitSet;
10use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
11use rustc_middle::mir::{Body, Location, Operand, Place, RETURN_PLACE, Terminator, TerminatorKind};
12use rustc_middle::ty::{self, DeducedParamAttrs, Ty, TyCtxt};
13use rustc_session::config::OptLevel;
14
15/// A visitor that determines which arguments have been mutated. We can't use the mutability field
16/// on LocalDecl for this because it has no meaning post-optimization.
17struct DeduceReadOnly {
18 /// Each bit is indexed by argument number, starting at zero (so 0 corresponds to local decl
19 /// 1). The bit is true if the argument may have been mutated or false if we know it hasn't
20 /// been up to the point we're at.
21 mutable_args: DenseBitSet<usize>,
22}
23
24impl DeduceReadOnly {
25 /// Returns a new DeduceReadOnly instance.
26 fn new(arg_count: usize) -> Self {
27 Self { mutable_args: DenseBitSet::new_empty(arg_count) }
28 }
29}
30
31impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
32 fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
33 // We're only interested in arguments.
34 if place.local == RETURN_PLACE || place.local.index() > self.mutable_args.domain_size() {
35 return;
36 }
37
38 let mark_as_mutable = match context {
39 PlaceContext::MutatingUse(..) => {
40 // This is a mutation, so mark it as such.
41 true
42 }
43 PlaceContext::NonMutatingUse(NonMutatingUseContext::RawBorrow) => {
44 // Whether mutating though a `&raw const` is allowed is still undecided, so we
45 // disable any sketchy `readonly` optimizations for now. But we only need to do
46 // this if the pointer would point into the argument. IOW: for indirect places,
47 // like `&raw (*local).field`, this surely cannot mutate `local`.
48 !place.is_indirect()
49 }
50 PlaceContext::NonMutatingUse(..) | PlaceContext::NonUse(..) => {
51 // Not mutating, so it's fine.
52 false
53 }
54 };
55
56 if mark_as_mutable {
57 self.mutable_args.insert(place.local.index() - 1);
58 }
59 }
60
61 fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
62 // OK, this is subtle. Suppose that we're trying to deduce whether `x` in `f` is read-only
63 // and we have the following:
64 //
65 // fn f(x: BigStruct) { g(x) }
66 // fn g(mut y: BigStruct) { y.foo = 1 }
67 //
68 // If, at the generated MIR level, `f` turned into something like:
69 //
70 // fn f(_1: BigStruct) -> () {
71 // let mut _0: ();
72 // bb0: {
73 // _0 = g(move _1) -> bb1;
74 // }
75 // ...
76 // }
77 //
78 // then it would be incorrect to mark `x` (i.e. `_1`) as `readonly`, because `g`'s write to
79 // its copy of the indirect parameter would actually be a write directly to the pointer that
80 // `f` passes. Note that function arguments are the only situation in which this problem can
81 // arise: every other use of `move` in MIR doesn't actually write to the value it moves
82 // from.
83 if let TerminatorKind::Call { ref args, .. } = terminator.kind {
84 for arg in args {
85 if let Operand::Move(place) = arg.node {
86 let local = place.local;
87 if place.is_indirect()
88 || local == RETURN_PLACE
89 || local.index() > self.mutable_args.domain_size()
90 {
91 continue;
92 }
93
94 self.mutable_args.insert(local.index() - 1);
95 }
96 }
97 };
98
99 self.super_terminator(terminator, location);
100 }
101}
102
103/// Returns true if values of a given type will never be passed indirectly, regardless of ABI.
104fn type_will_always_be_passed_directly(ty: Ty<'_>) -> bool {
105 matches!(
106 ty.kind(),
107 ty::Bool
108 | ty::Char
109 | ty::Float(..)
110 | ty::Int(..)
111 | ty::RawPtr(..)
112 | ty::Ref(..)
113 | ty::Slice(..)
114 | ty::Uint(..)
115 )
116}
117
118/// Returns the deduced parameter attributes for a function.
119///
120/// Deduced parameter attributes are those that can only be soundly determined by examining the
121/// body of the function instead of just the signature. These can be useful for optimization
122/// purposes on a best-effort basis. We compute them here and store them into the crate metadata so
123/// dependent crates can use them.
124pub(super) fn deduced_param_attrs<'tcx>(
125 tcx: TyCtxt<'tcx>,
126 def_id: LocalDefId,
127) -> &'tcx [DeducedParamAttrs] {
128 // This computation is unfortunately rather expensive, so don't do it unless we're optimizing.
129 // Also skip it in incremental mode.
130 if tcx.sess.opts.optimize == OptLevel::No || tcx.sess.opts.incremental.is_some() {
131 return &[];
132 }
133
134 // If the Freeze lang item isn't present, then don't bother.
135 if tcx.lang_items().freeze_trait().is_none() {
136 return &[];
137 }
138
139 // Codegen won't use this information for anything if all the function parameters are passed
140 // directly. Detect that and bail, for compilation speed.
141 let fn_ty = tcx.type_of(def_id).instantiate_identity();
142 if matches!(fn_ty.kind(), ty::FnDef(..))
143 && fn_ty
144 .fn_sig(tcx)
145 .inputs()
146 .skip_binder()
147 .iter()
148 .cloned()
149 .all(type_will_always_be_passed_directly)
150 {
151 return &[];
152 }
153
154 // Don't deduce any attributes for functions that have no MIR.
155 if !tcx.is_mir_available(def_id) {
156 return &[];
157 }
158
159 // Grab the optimized MIR. Analyze it to determine which arguments have been mutated.
160 let body: &Body<'tcx> = tcx.optimized_mir(def_id);
161 let mut deduce_read_only = DeduceReadOnly::new(body.arg_count);
162 deduce_read_only.visit_body(body);
163
164 // Set the `readonly` attribute for every argument that we concluded is immutable and that
165 // contains no UnsafeCells.
166 //
167 // FIXME: This is overly conservative around generic parameters: `is_freeze()` will always
168 // return false for them. For a description of alternatives that could do a better job here,
169 // see [1].
170 //
171 // [1]: https://github.com/rust-lang/rust/pull/103172#discussion_r999139997
172 let typing_env = body.typing_env(tcx);
173 let mut deduced_param_attrs = tcx.arena.alloc_from_iter(
174 body.local_decls.iter().skip(1).take(body.arg_count).enumerate().map(
175 |(arg_index, local_decl)| DeducedParamAttrs {
176 read_only: !deduce_read_only.mutable_args.contains(arg_index)
177 // We must normalize here to reveal opaques and normalize
178 // their generic parameters, otherwise we'll see exponential
179 // blow-up in compile times: #113372
180 && tcx
181 .normalize_erasing_regions(typing_env, local_decl.ty)
182 .is_freeze(tcx, typing_env),
183 },
184 ),
185 );
186
187 // Trailing parameters past the size of the `deduced_param_attrs` array are assumed to have the
188 // default set of attributes, so we don't have to store them explicitly. Pop them off to save a
189 // few bytes in metadata.
190 while deduced_param_attrs.last() == Some(&DeducedParamAttrs::default()) {
191 let last_index = deduced_param_attrs.len() - 1;
192 deduced_param_attrs = &mut deduced_param_attrs[0..last_index];
193 }
194
195 deduced_param_attrs
196}