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| -rw-r--r-- | compiler/rustc_mir_transform/src/deduce_param_attrs.rs | 248 |
1 files changed, 248 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs new file mode 100644 index 000000000..28b1c5a48 --- /dev/null +++ b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs @@ -0,0 +1,248 @@ +//! 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(_) = *arg { + // ArgumentChecker panics if a direct move of an argument from a caller to a + // callee was detected. + // + // If, in the future, MIR optimizations cause arguments to be moved directly + // from callers to callees, change the panic to instead add the argument in + // question to `mutating_uses`. + ArgumentChecker::new(self.mutable_args.domain_size()) + .visit_operand(arg, location) + } + } + }; + + self.super_terminator(terminator, location); + } +} + +/// A visitor that simply panics if a direct move of an argument from a caller to a callee was +/// detected. +struct ArgumentChecker { + /// The number of arguments to the calling function. + arg_count: usize, +} + +impl ArgumentChecker { + /// Creates a new ArgumentChecker. + fn new(arg_count: usize) -> Self { + Self { arg_count } + } +} + +impl<'tcx> Visitor<'tcx> for ArgumentChecker { + fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) { + // Check to make sure that, if this local is an argument, we didn't move directly from it. + if matches!(context, PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)) + && local != RETURN_PLACE + && local.index() <= self.arg_count + { + // If, in the future, MIR optimizations cause arguments to be moved directly from + // callers to callees, change this panic to instead add the argument in question to + // `mutating_uses`. + panic!("Detected a direct move from a caller's argument to a callee's argument!") + } + } +} + +/// 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 +} |