1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220
//! Deduces supplementary parameter attributes from MIR.
//!
//! Deduced parameter attributes are those that can only be soundly determined by examining the
//! body of the function instead of just the signature. These can be useful for optimization
//! purposes on a best-effort basis. We compute them here and store them into the crate metadata so
//! dependent crates can use them.
use rustc_hir::def_id::DefId;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::{Body, Local, Location, Operand, Terminator, TerminatorKind, RETURN_PLACE};
use rustc_middle::ty::{self, DeducedParamAttrs, ParamEnv, Ty, TyCtxt};
use rustc_session::config::OptLevel;
/// A visitor that determines which arguments have been mutated. We can't use the mutability field
/// on LocalDecl for this because it has no meaning post-optimization.
struct DeduceReadOnly {
/// Each bit is indexed by argument number, starting at zero (so 0 corresponds to local decl
/// 1). The bit is true if the argument may have been mutated or false if we know it hasn't
/// been up to the point we're at.
mutable_args: BitSet<usize>,
}
impl DeduceReadOnly {
/// Returns a new DeduceReadOnly instance.
fn new(arg_count: usize) -> Self {
Self { mutable_args: BitSet::new_empty(arg_count) }
}
}
impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
fn visit_local(&mut self, local: Local, mut context: PlaceContext, _: Location) {
// We're only interested in arguments.
if local == RETURN_PLACE || local.index() > self.mutable_args.domain_size() {
return;
}
// Replace place contexts that are moves with copies. This is safe in all cases except
// function argument position, which we already handled in `visit_terminator()` by using the
// ArgumentChecker. See the comment in that method for more details.
//
// In the future, we might want to move this out into a separate pass, but for now let's
// just do it on the fly because that's faster.
if matches!(context, PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)) {
context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
}
match context {
PlaceContext::MutatingUse(..)
| PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
// This is a mutation, so mark it as such.
self.mutable_args.insert(local.index() - 1);
}
PlaceContext::NonMutatingUse(..) | PlaceContext::NonUse(..) => {
// Not mutating, so it's fine.
}
}
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
// OK, this is subtle. Suppose that we're trying to deduce whether `x` in `f` is read-only
// and we have the following:
//
// fn f(x: BigStruct) { g(x) }
// fn g(mut y: BigStruct) { y.foo = 1 }
//
// If, at the generated MIR level, `f` turned into something like:
//
// fn f(_1: BigStruct) -> () {
// let mut _0: ();
// bb0: {
// _0 = g(move _1) -> bb1;
// }
// ...
// }
//
// then it would be incorrect to mark `x` (i.e. `_1`) as `readonly`, because `g`'s write to
// its copy of the indirect parameter would actually be a write directly to the pointer that
// `f` passes. Note that function arguments are the only situation in which this problem can
// arise: every other use of `move` in MIR doesn't actually write to the value it moves
// from.
//
// Anyway, right now this situation doesn't actually arise in practice. Instead, the MIR for
// that function looks like this:
//
// fn f(_1: BigStruct) -> () {
// let mut _0: ();
// let mut _2: BigStruct;
// bb0: {
// _2 = move _1;
// _0 = g(move _2) -> bb1;
// }
// ...
// }
//
// Because of that extra move that MIR construction inserts, `x` (i.e. `_1`) can *in
// practice* safely be marked `readonly`.
//
// To handle the possibility that other optimizations (for example, destination propagation)
// might someday generate MIR like the first example above, we panic upon seeing an argument
// to *our* function that is directly moved into *another* function as an argument. Having
// eliminated that problematic case, we can safely treat moves as copies in this analysis.
//
// In the future, if MIR optimizations cause arguments of a caller to be directly moved into
// the argument of a callee, we can just add that argument to `mutated_args` instead of
// panicking.
//
// Note that, because the problematic MIR is never actually generated, we can't add a test
// case for this.
if let TerminatorKind::Call { ref args, .. } = terminator.kind {
for arg in args {
if let Operand::Move(place) = *arg {
let local = place.local;
if place.is_indirect()
|| local == RETURN_PLACE
|| local.index() > self.mutable_args.domain_size()
{
continue;
}
self.mutable_args.insert(local.index() - 1);
}
}
};
self.super_terminator(terminator, location);
}
}
/// Returns true if values of a given type will never be passed indirectly, regardless of ABI.
fn type_will_always_be_passed_directly<'tcx>(ty: Ty<'tcx>) -> bool {
matches!(
ty.kind(),
ty::Bool
| ty::Char
| ty::Float(..)
| ty::Int(..)
| ty::RawPtr(..)
| ty::Ref(..)
| ty::Slice(..)
| ty::Uint(..)
)
}
/// Returns the deduced parameter attributes for a function.
///
/// Deduced parameter attributes are those that can only be soundly determined by examining the
/// body of the function instead of just the signature. These can be useful for optimization
/// purposes on a best-effort basis. We compute them here and store them into the crate metadata so
/// dependent crates can use them.
pub fn deduced_param_attrs<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx [DeducedParamAttrs] {
// This computation is unfortunately rather expensive, so don't do it unless we're optimizing.
// Also skip it in incremental mode.
if tcx.sess.opts.optimize == OptLevel::No || tcx.sess.opts.incremental.is_some() {
return &[];
}
// If the Freeze language item isn't present, then don't bother.
if tcx.lang_items().freeze_trait().is_none() {
return &[];
}
// Codegen won't use this information for anything if all the function parameters are passed
// directly. Detect that and bail, for compilation speed.
let fn_ty = tcx.type_of(def_id);
if matches!(fn_ty.kind(), ty::FnDef(..)) {
if fn_ty
.fn_sig(tcx)
.inputs()
.skip_binder()
.iter()
.cloned()
.all(type_will_always_be_passed_directly)
{
return &[];
}
}
// Don't deduce any attributes for functions that have no MIR.
if !tcx.is_mir_available(def_id) {
return &[];
}
// Deduced attributes for other crates should be read from the metadata instead of via this
// function.
debug_assert!(def_id.is_local());
// Grab the optimized MIR. Analyze it to determine which arguments have been mutated.
let body: &Body<'tcx> = tcx.optimized_mir(def_id);
let mut deduce_read_only = DeduceReadOnly::new(body.arg_count);
deduce_read_only.visit_body(body);
// Set the `readonly` attribute for every argument that we concluded is immutable and that
// contains no UnsafeCells.
//
// FIXME: This is overly conservative around generic parameters: `is_freeze()` will always
// return false for them. For a description of alternatives that could do a better job here,
// see [1].
//
// [1]: https://github.com/rust-lang/rust/pull/103172#discussion_r999139997
let mut deduced_param_attrs = tcx.arena.alloc_from_iter(
body.local_decls.iter().skip(1).take(body.arg_count).enumerate().map(
|(arg_index, local_decl)| DeducedParamAttrs {
read_only: !deduce_read_only.mutable_args.contains(arg_index)
&& local_decl.ty.is_freeze(tcx, ParamEnv::reveal_all()),
},
),
);
// Trailing parameters past the size of the `deduced_param_attrs` array are assumed to have the
// default set of attributes, so we don't have to store them explicitly. Pop them off to save a
// few bytes in metadata.
while deduced_param_attrs.last() == Some(&DeducedParamAttrs::default()) {
let last_index = deduced_param_attrs.len() - 1;
deduced_param_attrs = &mut deduced_param_attrs[0..last_index];
}
deduced_param_attrs
}