diff options
Diffstat (limited to 'compiler/rustc_mir/src/transform/promote_consts.rs')
| -rw-r--r-- | compiler/rustc_mir/src/transform/promote_consts.rs | 1258 |
1 files changed, 1258 insertions, 0 deletions
diff --git a/compiler/rustc_mir/src/transform/promote_consts.rs b/compiler/rustc_mir/src/transform/promote_consts.rs new file mode 100644 index 00000000000..b2dda1caa54 --- /dev/null +++ b/compiler/rustc_mir/src/transform/promote_consts.rs @@ -0,0 +1,1258 @@ +//! A pass that promotes borrows of constant rvalues. +//! +//! The rvalues considered constant are trees of temps, +//! each with exactly one initialization, and holding +//! a constant value with no interior mutability. +//! They are placed into a new MIR constant body in +//! `promoted` and the borrow rvalue is replaced with +//! a `Literal::Promoted` using the index into `promoted` +//! of that constant MIR. +//! +//! This pass assumes that every use is dominated by an +//! initialization and can otherwise silence errors, if +//! move analysis runs after promotion on broken MIR. + +use rustc_ast::LitKind; +use rustc_hir as hir; +use rustc_hir::def_id::DefId; +use rustc_middle::mir::traversal::ReversePostorder; +use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor}; +use rustc_middle::mir::*; +use rustc_middle::ty::cast::CastTy; +use rustc_middle::ty::subst::InternalSubsts; +use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable}; +use rustc_span::symbol::sym; +use rustc_span::{Span, DUMMY_SP}; + +use rustc_index::vec::{Idx, IndexVec}; +use rustc_target::spec::abi::Abi; + +use std::cell::Cell; +use std::{cmp, iter, mem}; + +use crate::const_eval::{is_const_fn, is_unstable_const_fn}; +use crate::transform::check_consts::{is_lang_panic_fn, qualifs, ConstCx}; +use crate::transform::{MirPass, MirSource}; + +/// A `MirPass` for promotion. +/// +/// Promotion is the extraction of promotable temps into separate MIR bodies. This pass also emits +/// errors when promotion of `#[rustc_args_required_const]` arguments fails. +/// +/// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each +/// newly created `Constant`. +#[derive(Default)] +pub struct PromoteTemps<'tcx> { + pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>, +} + +impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> { + fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) { + // There's not really any point in promoting errorful MIR. + // + // This does not include MIR that failed const-checking, which we still try to promote. + if body.return_ty().references_error() { + tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors"); + return; + } + + if src.promoted.is_some() { + return; + } + + let def = src.with_opt_param().expect_local(); + + let mut rpo = traversal::reverse_postorder(body); + let ccx = ConstCx::new(tcx, def.did, body); + let (temps, all_candidates) = collect_temps_and_candidates(&ccx, &mut rpo); + + let promotable_candidates = validate_candidates(&ccx, &temps, &all_candidates); + + let promoted = promote_candidates(def.to_global(), body, tcx, temps, promotable_candidates); + self.promoted_fragments.set(promoted); + } +} + +/// State of a temporary during collection and promotion. +#[derive(Copy, Clone, PartialEq, Eq, Debug)] +pub enum TempState { + /// No references to this temp. + Undefined, + /// One direct assignment and any number of direct uses. + /// A borrow of this temp is promotable if the assigned + /// value is qualified as constant. + Defined { location: Location, uses: usize }, + /// Any other combination of assignments/uses. + Unpromotable, + /// This temp was part of an rvalue which got extracted + /// during promotion and needs cleanup. + PromotedOut, +} + +impl TempState { + pub fn is_promotable(&self) -> bool { + debug!("is_promotable: self={:?}", self); + if let TempState::Defined { .. } = *self { true } else { false } + } +} + +/// A "root candidate" for promotion, which will become the +/// returned value in a promoted MIR, unless it's a subset +/// of a larger candidate. +#[derive(Copy, Clone, PartialEq, Eq, Debug)] +pub enum Candidate { + /// Borrow of a constant temporary, candidate for lifetime extension. + Ref(Location), + + /// Promotion of the `x` in `[x; 32]`. + Repeat(Location), + + /// Currently applied to function calls where the callee has the unstable + /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle + /// intrinsic. The intrinsic requires the arguments are indeed constant and + /// the attribute currently provides the semantic requirement that arguments + /// must be constant. + Argument { bb: BasicBlock, index: usize }, + + /// `const` operand in asm!. + InlineAsm { bb: BasicBlock, index: usize }, +} + +impl Candidate { + /// Returns `true` if we should use the "explicit" rules for promotability for this `Candidate`. + fn forces_explicit_promotion(&self) -> bool { + match self { + Candidate::Ref(_) | Candidate::Repeat(_) => false, + Candidate::Argument { .. } | Candidate::InlineAsm { .. } => true, + } + } +} + +fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Vec<usize>> { + let attrs = tcx.get_attrs(def_id); + let attr = attrs.iter().find(|a| tcx.sess.check_name(a, sym::rustc_args_required_const))?; + let mut ret = vec![]; + for meta in attr.meta_item_list()? { + match meta.literal()?.kind { + LitKind::Int(a, _) => { + ret.push(a as usize); + } + _ => return None, + } + } + Some(ret) +} + +struct Collector<'a, 'tcx> { + ccx: &'a ConstCx<'a, 'tcx>, + temps: IndexVec<Local, TempState>, + candidates: Vec<Candidate>, +} + +impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> { + fn visit_local(&mut self, &index: &Local, context: PlaceContext, location: Location) { + debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location); + // We're only interested in temporaries and the return place + match self.ccx.body.local_kind(index) { + LocalKind::Temp | LocalKind::ReturnPointer => {} + LocalKind::Arg | LocalKind::Var => return, + } + + // Ignore drops, if the temp gets promoted, + // then it's constant and thus drop is noop. + // Non-uses are also irrelevant. + if context.is_drop() || !context.is_use() { + debug!( + "visit_local: context.is_drop={:?} context.is_use={:?}", + context.is_drop(), + context.is_use(), + ); + return; + } + + let temp = &mut self.temps[index]; + debug!("visit_local: temp={:?}", temp); + if *temp == TempState::Undefined { + match context { + PlaceContext::MutatingUse(MutatingUseContext::Store) + | PlaceContext::MutatingUse(MutatingUseContext::Call) => { + *temp = TempState::Defined { location, uses: 0 }; + return; + } + _ => { /* mark as unpromotable below */ } + } + } else if let TempState::Defined { ref mut uses, .. } = *temp { + // We always allow borrows, even mutable ones, as we need + // to promote mutable borrows of some ZSTs e.g., `&mut []`. + let allowed_use = match context { + PlaceContext::MutatingUse(MutatingUseContext::Borrow) + | PlaceContext::NonMutatingUse(_) => true, + PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false, + }; + debug!("visit_local: allowed_use={:?}", allowed_use); + if allowed_use { + *uses += 1; + return; + } + /* mark as unpromotable below */ + } + *temp = TempState::Unpromotable; + } + + fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) { + self.super_rvalue(rvalue, location); + + match *rvalue { + Rvalue::Ref(..) => { + self.candidates.push(Candidate::Ref(location)); + } + Rvalue::Repeat(..) if self.ccx.tcx.features().const_in_array_repeat_expressions => { + // FIXME(#49147) only promote the element when it isn't `Copy` + // (so that code that can copy it at runtime is unaffected). + self.candidates.push(Candidate::Repeat(location)); + } + _ => {} + } + } + + fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) { + self.super_terminator(terminator, location); + + match terminator.kind { + TerminatorKind::Call { ref func, .. } => { + if let ty::FnDef(def_id, _) = func.ty(self.ccx.body, self.ccx.tcx).kind { + let fn_sig = self.ccx.tcx.fn_sig(def_id); + if let Abi::RustIntrinsic | Abi::PlatformIntrinsic = fn_sig.abi() { + let name = self.ccx.tcx.item_name(def_id); + // FIXME(eddyb) use `#[rustc_args_required_const(2)]` for shuffles. + if name.as_str().starts_with("simd_shuffle") { + self.candidates + .push(Candidate::Argument { bb: location.block, index: 2 }); + + return; // Don't double count `simd_shuffle` candidates + } + } + + if let Some(constant_args) = args_required_const(self.ccx.tcx, def_id) { + for index in constant_args { + self.candidates.push(Candidate::Argument { bb: location.block, index }); + } + } + } + } + TerminatorKind::InlineAsm { ref operands, .. } => { + for (index, op) in operands.iter().enumerate() { + match op { + InlineAsmOperand::Const { .. } => { + self.candidates.push(Candidate::InlineAsm { bb: location.block, index }) + } + _ => {} + } + } + } + _ => {} + } + } +} + +pub fn collect_temps_and_candidates( + ccx: &ConstCx<'mir, 'tcx>, + rpo: &mut ReversePostorder<'_, 'tcx>, +) -> (IndexVec<Local, TempState>, Vec<Candidate>) { + let mut collector = Collector { + temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls), + candidates: vec![], + ccx, + }; + for (bb, data) in rpo { + collector.visit_basic_block_data(bb, data); + } + (collector.temps, collector.candidates) +} + +/// Checks whether locals that appear in a promotion context (`Candidate`) are actually promotable. +/// +/// This wraps an `Item`, and has access to all fields of that `Item` via `Deref` coercion. +struct Validator<'a, 'tcx> { + ccx: &'a ConstCx<'a, 'tcx>, + temps: &'a IndexVec<Local, TempState>, + + /// Explicit promotion happens e.g. for constant arguments declared via + /// `rustc_args_required_const`. + /// Implicit promotion has almost the same rules, except that disallows `const fn` + /// except for those marked `#[rustc_promotable]`. This is to avoid changing + /// a legitimate run-time operation into a failing compile-time operation + /// e.g. due to addresses being compared inside the function. + explicit: bool, +} + +impl std::ops::Deref for Validator<'a, 'tcx> { + type Target = ConstCx<'a, 'tcx>; + + fn deref(&self) -> &Self::Target { + &self.ccx + } +} + +struct Unpromotable; + +impl<'tcx> Validator<'_, 'tcx> { + fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> { + match candidate { + Candidate::Ref(loc) => { + assert!(!self.explicit); + + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => { + match kind { + BorrowKind::Shared | BorrowKind::Mut { .. } => {} + + // FIXME(eddyb) these aren't promoted here but *could* + // be promoted as part of a larger value because + // `validate_rvalue` doesn't check them, need to + // figure out what is the intended behavior. + BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable), + } + + // We can only promote interior borrows of promotable temps (non-temps + // don't get promoted anyway). + self.validate_local(place.local)?; + + if place.projection.contains(&ProjectionElem::Deref) { + return Err(Unpromotable); + } + + let mut has_mut_interior = + self.qualif_local::<qualifs::HasMutInterior>(place.local); + // HACK(eddyb) this should compute the same thing as + // `<HasMutInterior as Qualif>::in_projection` from + // `check_consts::qualifs` but without recursion. + if has_mut_interior { + // This allows borrowing fields which don't have + // `HasMutInterior`, from a type that does, e.g.: + // `let _: &'static _ = &(Cell::new(1), 2).1;` + let mut place_projection = &place.projection[..]; + // FIXME(eddyb) use a forward loop instead of a reverse one. + while let &[ref proj_base @ .., elem] = place_projection { + // FIXME(eddyb) this is probably excessive, with + // the exception of `union` member accesses. + let ty = + Place::ty_from(place.local, proj_base, self.body, self.tcx) + .projection_ty(self.tcx, elem) + .ty; + if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) { + has_mut_interior = false; + break; + } + + place_projection = proj_base; + } + } + + // FIXME(eddyb) this duplicates part of `validate_rvalue`. + if has_mut_interior { + return Err(Unpromotable); + } + if self.qualif_local::<qualifs::NeedsDrop>(place.local) { + return Err(Unpromotable); + } + + if let BorrowKind::Mut { .. } = kind { + let ty = place.ty(self.body, self.tcx).ty; + + // In theory, any zero-sized value could be borrowed + // mutably without consequences. However, only &mut [] + // is allowed right now, and only in functions. + if self.const_kind + == Some(hir::ConstContext::Static(hir::Mutability::Mut)) + { + // Inside a `static mut`, &mut [...] is also allowed. + match ty.kind { + ty::Array(..) | ty::Slice(_) => {} + _ => return Err(Unpromotable), + } + } else if let ty::Array(_, len) = ty.kind { + // FIXME(eddyb) the `self.is_non_const_fn` condition + // seems unnecessary, given that this is merely a ZST. + match len.try_eval_usize(self.tcx, self.param_env) { + Some(0) if self.const_kind.is_none() => {} + _ => return Err(Unpromotable), + } + } else { + return Err(Unpromotable); + } + } + + Ok(()) + } + _ => bug!(), + } + } + Candidate::Repeat(loc) => { + assert!(!self.explicit); + + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, Rvalue::Repeat(ref operand, _))) => { + if !self.tcx.features().const_in_array_repeat_expressions { + return Err(Unpromotable); + } + + self.validate_operand(operand) + } + _ => bug!(), + } + } + Candidate::Argument { bb, index } => { + assert!(self.explicit); + + let terminator = self.body[bb].terminator(); + match &terminator.kind { + TerminatorKind::Call { args, .. } => self.validate_operand(&args[index]), + _ => bug!(), + } + } + Candidate::InlineAsm { bb, index } => { + assert!(self.explicit); + + let terminator = self.body[bb].terminator(); + match &terminator.kind { + TerminatorKind::InlineAsm { operands, .. } => match &operands[index] { + InlineAsmOperand::Const { value } => self.validate_operand(value), + _ => bug!(), + }, + _ => bug!(), + } + } + } + } + + // FIXME(eddyb) maybe cache this? + fn qualif_local<Q: qualifs::Qualif>(&self, local: Local) -> bool { + if let TempState::Defined { location: loc, .. } = self.temps[local] { + let num_stmts = self.body[loc.block].statements.len(); + + if loc.statement_index < num_stmts { + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, rhs)) => qualifs::in_rvalue::<Q, _>( + &self.ccx, + &mut |l| self.qualif_local::<Q>(l), + rhs, + ), + _ => { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + } + } + } else { + let terminator = self.body[loc.block].terminator(); + match &terminator.kind { + TerminatorKind::Call { .. } => { + let return_ty = self.body.local_decls[local].ty; + Q::in_any_value_of_ty(&self.ccx, return_ty) + } + kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + } + } + } else { + let span = self.body.local_decls[local].source_info.span; + span_bug!(span, "{:?} not promotable, qualif_local shouldn't have been called", local); + } + } + + // FIXME(eddyb) maybe cache this? + fn validate_local(&self, local: Local) -> Result<(), Unpromotable> { + if let TempState::Defined { location: loc, .. } = self.temps[local] { + let num_stmts = self.body[loc.block].statements.len(); + + if loc.statement_index < num_stmts { + let statement = &self.body[loc.block].statements[loc.statement_index]; + match &statement.kind { + StatementKind::Assign(box (_, rhs)) => self.validate_rvalue(rhs), + _ => { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + } + } + } else { + let terminator = self.body[loc.block].terminator(); + match &terminator.kind { + TerminatorKind::Call { func, args, .. } => self.validate_call(func, args), + TerminatorKind::Yield { .. } => Err(Unpromotable), + kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + } + } + } else { + Err(Unpromotable) + } + } + + fn validate_place(&self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable> { + match place { + PlaceRef { local, projection: [] } => self.validate_local(local), + PlaceRef { local, projection: [proj_base @ .., elem] } => { + match *elem { + ProjectionElem::Deref => { + let mut not_promotable = true; + // This is a special treatment for cases like *&STATIC where STATIC is a + // global static variable. + // This pattern is generated only when global static variables are directly + // accessed and is qualified for promotion safely. + if let TempState::Defined { location, .. } = self.temps[local] { + let def_stmt = + self.body[location.block].statements.get(location.statement_index); + if let Some(Statement { + kind: + StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(c)))), + .. + }) = def_stmt + { + if let Some(did) = c.check_static_ptr(self.tcx) { + if let Some(hir::ConstContext::Static(..)) = self.const_kind { + // The `is_empty` predicate is introduced to exclude the case + // where the projection operations are [ .field, * ]. + // The reason is because promotion will be illegal if field + // accesses precede the dereferencing. + // Discussion can be found at + // https://github.com/rust-lang/rust/pull/74945#discussion_r463063247 + // There may be opportunity for generalization, but this needs to be + // accounted for. + if proj_base.is_empty() + && !self.tcx.is_thread_local_static(did) + { + not_promotable = false; + } + } + } + } + } + if not_promotable { + return Err(Unpromotable); + } + } + ProjectionElem::Downcast(..) => { + return Err(Unpromotable); + } + + ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {} + + ProjectionElem::Index(local) => { + self.validate_local(local)?; + } + + ProjectionElem::Field(..) => { + if self.const_kind.is_none() { + let base_ty = + Place::ty_from(place.local, proj_base, self.body, self.tcx).ty; + if let Some(def) = base_ty.ty_adt_def() { + // No promotion of union field accesses. + if def.is_union() { + return Err(Unpromotable); + } + } + } + } + } + + self.validate_place(PlaceRef { local: place.local, projection: proj_base }) + } + } + } + + fn validate_operand(&self, operand: &Operand<'tcx>) -> Result<(), Unpromotable> { + match operand { + Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()), + + // The qualifs for a constant (e.g. `HasMutInterior`) are checked in + // `validate_rvalue` upon access. + Operand::Constant(c) => { + if let Some(def_id) = c.check_static_ptr(self.tcx) { + // Only allow statics (not consts) to refer to other statics. + // FIXME(eddyb) does this matter at all for promotion? + let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_))); + if !is_static { + return Err(Unpromotable); + } + + let is_thread_local = self.tcx.is_thread_local_static(def_id); + if is_thread_local { + return Err(Unpromotable); + } + } + + Ok(()) + } + } + } + + fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> { + match *rvalue { + Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => { + let operand_ty = operand.ty(self.body, self.tcx); + let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast"); + let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast"); + match (cast_in, cast_out) { + (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => { + // in normal functions, mark such casts as not promotable + return Err(Unpromotable); + } + _ => {} + } + } + + Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => { + if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind { + assert!( + op == BinOp::Eq + || op == BinOp::Ne + || op == BinOp::Le + || op == BinOp::Lt + || op == BinOp::Ge + || op == BinOp::Gt + || op == BinOp::Offset + ); + + // raw pointer operations are not allowed inside promoteds + return Err(Unpromotable); + } + } + + Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable), + + _ => {} + } + + match rvalue { + Rvalue::ThreadLocalRef(_) => Err(Unpromotable), + + Rvalue::NullaryOp(..) => Ok(()), + + Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()), + + Rvalue::Use(operand) + | Rvalue::Repeat(operand, _) + | Rvalue::UnaryOp(_, operand) + | Rvalue::Cast(_, operand, _) => self.validate_operand(operand), + + Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => { + self.validate_operand(lhs)?; + self.validate_operand(rhs) + } + + Rvalue::AddressOf(_, place) => { + // Raw reborrows can come from reference to pointer coercions, + // so are allowed. + if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() { + let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty; + if let ty::Ref(..) = base_ty.kind { + return self.validate_place(PlaceRef { + local: place.local, + projection: proj_base, + }); + } + } + Err(Unpromotable) + } + + Rvalue::Ref(_, kind, place) => { + if let BorrowKind::Mut { .. } = kind { + let ty = place.ty(self.body, self.tcx).ty; + + // In theory, any zero-sized value could be borrowed + // mutably without consequences. However, only &mut [] + // is allowed right now, and only in functions. + if self.const_kind == Some(hir::ConstContext::Static(hir::Mutability::Mut)) { + // Inside a `static mut`, &mut [...] is also allowed. + match ty.kind { + ty::Array(..) | ty::Slice(_) => {} + _ => return Err(Unpromotable), + } + } else if let ty::Array(_, len) = ty.kind { + // FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a + // const context which seems unnecessary given that this is merely a ZST. + match len.try_eval_usize(self.tcx, self.param_env) { + Some(0) if self.const_kind.is_none() => {} + _ => return Err(Unpromotable), + } + } else { + return Err(Unpromotable); + } + } + + // Special-case reborrows to be more like a copy of the reference. + let mut place = place.as_ref(); + if let [proj_base @ .., ProjectionElem::Deref] = &place.projection { + let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty; + if let ty::Ref(..) = base_ty.kind { + place = PlaceRef { local: place.local, projection: proj_base }; + } + } + + self.validate_place(place)?; + + // HACK(eddyb) this should compute the same thing as + // `<HasMutInterior as Qualif>::in_projection` from + // `check_consts::qualifs` but without recursion. + let mut has_mut_interior = + self.qualif_local::<qualifs::HasMutInterior>(place.local); + if has_mut_interior { + let mut place_projection = place.projection; + // FIXME(eddyb) use a forward loop instead of a reverse one. + while let &[ref proj_base @ .., elem] = place_projection { + // FIXME(eddyb) this is probably excessive, with + // the exception of `union` member accesses. + let ty = Place::ty_from(place.local, proj_base, self.body, self.tcx) + .projection_ty(self.tcx, elem) + .ty; + if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) { + has_mut_interior = false; + break; + } + + place_projection = proj_base; + } + } + if has_mut_interior { + return Err(Unpromotable); + } + + Ok(()) + } + + Rvalue::Aggregate(_, ref operands) => { + for o in operands { + self.validate_operand(o)?; + } + + Ok(()) + } + } + } + + fn validate_call( + &self, + callee: &Operand<'tcx>, + args: &[Operand<'tcx>], + ) -> Result<(), Unpromotable> { + let fn_ty = callee.ty(self.body, self.tcx); + + if !self.explicit && self.const_kind.is_none() { + if let ty::FnDef(def_id, _) = fn_ty.kind { + // Never promote runtime `const fn` calls of + // functions without `#[rustc_promotable]`. + if !self.tcx.is_promotable_const_fn(def_id) { + return Err(Unpromotable); + } + } + } + + let is_const_fn = match fn_ty.kind { + ty::FnDef(def_id, _) => { + is_const_fn(self.tcx, def_id) + || is_unstable_const_fn(self.tcx, def_id).is_some() + || is_lang_panic_fn(self.tcx, self.def_id.to_def_id()) + } + _ => false, + }; + if !is_const_fn { + return Err(Unpromotable); + } + + self.validate_operand(callee)?; + for arg in args { + self.validate_operand(arg)?; + } + + Ok(()) + } +} + +// FIXME(eddyb) remove the differences for promotability in `static`, `const`, `const fn`. +pub fn validate_candidates( + ccx: &ConstCx<'_, '_>, + temps: &IndexVec<Local, TempState>, + candidates: &[Candidate], +) -> Vec<Candidate> { + let mut validator = Validator { ccx, temps, explicit: false }; + + candidates + .iter() + .copied() + .filter(|&candidate| { + validator.explicit = candidate.forces_explicit_promotion(); + + // FIXME(eddyb) also emit the errors for shuffle indices + // and `#[rustc_args_required_const]` arguments here. + + let is_promotable = validator.validate_candidate(candidate).is_ok(); + + // If we use explicit validation, we carry the risk of turning a legitimate run-time + // operation into a failing compile-time operation. Make sure that does not happen + // by asserting that there is no possible run-time behavior here in case promotion + // fails. + if validator.explicit && !is_promotable { + ccx.tcx.sess.delay_span_bug( + ccx.body.span, + "Explicit promotion requested, but failed to promote", + ); + } + + match candidate { + Candidate::Argument { bb, index } | Candidate::InlineAsm { bb, index } + if !is_promotable => + { + let span = ccx.body[bb].terminator().source_info.span; + let msg = format!("argument {} is required to be a constant", index + 1); + ccx.tcx.sess.span_err(span, &msg); + } + _ => (), + } + + is_promotable + }) + .collect() +} + +struct Promoter<'a, 'tcx> { + tcx: TyCtxt<'tcx>, + source: &'a mut Body<'tcx>, + promoted: Body<'tcx>, + temps: &'a mut IndexVec<Local, TempState>, + extra_statements: &'a mut Vec<(Location, Statement<'tcx>)>, + + /// If true, all nested temps are also kept in the + /// source MIR, not moved to the promoted MIR. + keep_original: bool, +} + +impl<'a, 'tcx> Promoter<'a, 'tcx> { + fn new_block(&mut self) -> BasicBlock { + let span = self.promoted.span; + self.promoted.basic_blocks_mut().push(BasicBlockData { + statements: vec![], + terminator: Some(Terminator { + source_info: SourceInfo::outermost(span), + kind: TerminatorKind::Return, + }), + is_cleanup: false, + }) + } + + fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) { + let last = self.promoted.basic_blocks().last().unwrap(); + let data = &mut self.promoted[last]; + data.statements.push(Statement { + source_info: SourceInfo::outermost(span), + kind: StatementKind::Assign(box (Place::from(dest), rvalue)), + }); + } + + fn is_temp_kind(&self, local: Local) -> bool { + self.source.local_kind(local) == LocalKind::Temp + } + + /// Copies the initialization of this temp to the + /// promoted MIR, recursing through temps. + fn promote_temp(&mut self, temp: Local) -> Local { + let old_keep_original = self.keep_original; + let loc = match self.temps[temp] { + TempState::Defined { location, uses } if uses > 0 => { + if uses > 1 { + self.keep_original = true; + } + location + } + state => { + span_bug!(self.promoted.span, "{:?} not promotable: {:?}", temp, state); + } + }; + if !self.keep_original { + self.temps[temp] = TempState::PromotedOut; + } + + let num_stmts = self.source[loc.block].statements.len(); + let new_temp = self.promoted.local_decls.push(LocalDecl::new( + self.source.local_decls[temp].ty, + self.source.local_decls[temp].source_info.span, + )); + + debug!("promote({:?} @ {:?}/{:?}, {:?})", temp, loc, num_stmts, self.keep_original); + + // First, take the Rvalue or Call out of the source MIR, + // or duplicate it, depending on keep_original. + if loc.statement_index < num_stmts { + let (mut rvalue, source_info) = { + let statement = &mut self.source[loc.block].statements[loc.statement_index]; + let rhs = match statement.kind { + StatementKind::Assign(box (_, ref mut rhs)) => rhs, + _ => { + span_bug!( + statement.source_info.span, + "{:?} is not an assignment", + statement + ); + } + }; + + ( + if self.keep_original { + rhs.clone() + } else { + let unit = Rvalue::Use(Operand::Constant(box Constant { + span: statement.source_info.span, + user_ty: None, + literal: ty::Const::zero_sized(self.tcx, self.tcx.types.unit), + })); + mem::replace(rhs, unit) + }, + statement.source_info, + ) + }; + + self.visit_rvalue(&mut rvalue, loc); + self.assign(new_temp, rvalue, source_info.span); + } else { + let terminator = if self.keep_original { + self.source[loc.block].terminator().clone() + } else { + let terminator = self.source[loc.block].terminator_mut(); + let target = match terminator.kind { + TerminatorKind::Call { destination: Some((_, target)), .. } => target, + ref kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + }; + Terminator { + source_info: terminator.source_info, + kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { target }), + } + }; + + match terminator.kind { + TerminatorKind::Call { mut func, mut args, from_hir_call, fn_span, .. } => { + self.visit_operand(&mut func, loc); + for arg in &mut args { + self.visit_operand(arg, loc); + } + + let last = self.promoted.basic_blocks().last().unwrap(); + let new_target = self.new_block(); + + *self.promoted[last].terminator_mut() = Terminator { + kind: TerminatorKind::Call { + func, + args, + cleanup: None, + destination: Some((Place::from(new_temp), new_target)), + from_hir_call, + fn_span, + }, + ..terminator + }; + } + ref kind => { + span_bug!(terminator.source_info.span, "{:?} not promotable", kind); + } + }; + }; + + self.keep_original = old_keep_original; + new_temp + } + + fn promote_candidate( + mut self, + def: ty::WithOptConstParam<DefId>, + candidate: Candidate, + next_promoted_id: usize, + ) -> Option<Body<'tcx>> { + let mut rvalue = { + let promoted = &mut self.promoted; + let promoted_id = Promoted::new(next_promoted_id); + let tcx = self.tcx; + let mut promoted_operand = |ty, span| { + promoted.span = span; + promoted.local_decls[RETURN_PLACE] = LocalDecl::new(ty, span); + + Operand::Constant(Box::new(Constant { + span, + user_ty: None, + literal: tcx.mk_const(ty::Const { + ty, + val: ty::ConstKind::Unevaluated( + def, + InternalSubsts::for_item(tcx, def.did, |param, _| { + if let ty::GenericParamDefKind::Lifetime = param.kind { + tcx.lifetimes.re_erased.into() + } else { + tcx.mk_param_from_def(param) + } + }), + Some(promoted_id), + ), + }), + })) + }; + let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut(); + match candidate { + Candidate::Ref(loc) => { + let statement = &mut blocks[loc.block].statements[loc.statement_index]; + match statement.kind { + StatementKind::Assign(box ( + _, + Rvalue::Ref(ref mut region, borrow_kind, ref mut place), + )) => { + // Use the underlying local for this (necessarily interior) borrow. + let ty = local_decls.local_decls()[place.local].ty; + let span = statement.source_info.span; + + let ref_ty = tcx.mk_ref( + tcx.lifetimes.re_erased, + ty::TypeAndMut { ty, mutbl: borrow_kind.to_mutbl_lossy() }, + ); + + *region = tcx.lifetimes.re_erased; + + let mut projection = vec![PlaceElem::Deref]; + projection.extend(place.projection); + place.projection = tcx.intern_place_elems(&projection); + + // Create a temp to hold the promoted reference. + // This is because `*r` requires `r` to be a local, + // otherwise we would use the `promoted` directly. + let mut promoted_ref = LocalDecl::new(ref_ty, span); + promoted_ref.source_info = statement.source_info; + let promoted_ref = local_decls.push(promoted_ref); + assert_eq!(self.temps.push(TempState::Unpromotable), promoted_ref); + + let promoted_ref_statement = Statement { + source_info: statement.source_info, + kind: StatementKind::Assign(Box::new(( + Place::from(promoted_ref), + Rvalue::Use(promoted_operand(ref_ty, span)), + ))), + }; + self.extra_statements.push((loc, promoted_ref_statement)); + + Rvalue::Ref( + tcx.lifetimes.re_erased, + borrow_kind, + Place { + local: mem::replace(&mut place.local, promoted_ref), + projection: List::empty(), + }, + ) + } + _ => bug!(), + } + } + Candidate::Repeat(loc) => { + let statement = &mut blocks[loc.block].statements[loc.statement_index]; + match statement.kind { + StatementKind::Assign(box (_, Rvalue::Repeat(ref mut operand, _))) => { + let ty = operand.ty(local_decls, self.tcx); + let span = statement.source_info.span; + + Rvalue::Use(mem::replace(operand, promoted_operand(ty, span))) + } + _ => bug!(), + } + } + Candidate::Argument { bb, index } => { + let terminator = blocks[bb].terminator_mut(); + match terminator.kind { + TerminatorKind::Call { ref mut args, .. } => { + let ty = args[index].ty(local_decls, self.tcx); + let span = terminator.source_info.span; + + Rvalue::Use(mem::replace(&mut args[index], promoted_operand(ty, span))) + } + // We expected a `TerminatorKind::Call` for which we'd like to promote an + // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but + // we are seeing a `Goto`. That means that the `promote_temps` method + // already promoted this call away entirely. This case occurs when calling + // a function requiring a constant argument and as that constant value + // providing a value whose computation contains another call to a function + // requiring a constant argument. + TerminatorKind::Goto { .. } => return None, + _ => bug!(), + } + } + Candidate::InlineAsm { bb, index } => { + let terminator = blocks[bb].terminator_mut(); + match terminator.kind { + TerminatorKind::InlineAsm { ref mut operands, .. } => { + match &mut operands[index] { + InlineAsmOperand::Const { ref mut value } => { + let ty = value.ty(local_decls, self.tcx); + let span = terminator.source_info.span; + + Rvalue::Use(mem::replace(value, promoted_operand(ty, span))) + } + _ => bug!(), + } + } + + _ => bug!(), + } + } + } + }; + + assert_eq!(self.new_block(), START_BLOCK); + self.visit_rvalue( + &mut rvalue, + Location { block: BasicBlock::new(0), statement_index: usize::MAX }, + ); + + let span = self.promoted.span; + self.assign(RETURN_PLACE, rvalue, span); + Some(self.promoted) + } +} + +/// Replaces all temporaries with their promoted counterparts. +impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> { + fn tcx(&self) -> TyCtxt<'tcx> { + self.tcx + } + + fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { + if self.is_temp_kind(*local) { + *local = self.promote_temp(*local); + } + } +} + +pub fn promote_candidates<'tcx>( + def: ty::WithOptConstParam<DefId>, + body: &mut Body<'tcx>, + tcx: TyCtxt<'tcx>, + mut temps: IndexVec<Local, TempState>, + candidates: Vec<Candidate>, +) -> IndexVec<Promoted, Body<'tcx>> { + // Visit candidates in reverse, in case they're nested. + debug!("promote_candidates({:?})", candidates); + + let mut promotions = IndexVec::new(); + + let mut extra_statements = vec![]; + for candidate in candidates.into_iter().rev() { + match candidate { + Candidate::Repeat(Location { block, statement_index }) + | Candidate::Ref(Location { block, statement_index }) => { + if let StatementKind::Assign(box (place, _)) = + &body[block].statements[statement_index].kind + { + if let Some(local) = place.as_local() { + if temps[local] == TempState::PromotedOut { + // Already promoted. + continue; + } + } + } + } + Candidate::Argument { .. } | Candidate::InlineAsm { .. } => {} + } + + // Declare return place local so that `mir::Body::new` doesn't complain. + let initial_locals = iter::once(LocalDecl::new(tcx.types.never, body.span)).collect(); + + let mut promoted = Body::new( + IndexVec::new(), + // FIXME: maybe try to filter this to avoid blowing up + // memory usage? + body.source_scopes.clone(), + initial_locals, + IndexVec::new(), + 0, + vec![], + body.span, + body.generator_kind, + ); + promoted.ignore_interior_mut_in_const_validation = true; + + let promoter = Promoter { + promoted, + tcx, + source: body, + temps: &mut temps, + extra_statements: &mut extra_statements, + keep_original: false, + }; + + //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice + if let Some(promoted) = promoter.promote_candidate(def, candidate, promotions.len()) { + promotions.push(promoted); + } + } + + // Insert each of `extra_statements` before its indicated location, which + // has to be done in reverse location order, to not invalidate the rest. + extra_statements.sort_by_key(|&(loc, _)| cmp::Reverse(loc)); + for (loc, statement) in extra_statements { + body[loc.block].statements.insert(loc.statement_index, statement); + } + + // Eliminate assignments to, and drops of promoted temps. + let promoted = |index: Local| temps[index] == TempState::PromotedOut; + for block in body.basic_blocks_mut() { + block.statements.retain(|statement| match &statement.kind { + StatementKind::Assign(box (place, _)) => { + if let Some(index) = place.as_local() { + !promoted(index) + } else { + true + } + } + StatementKind::StorageLive(index) | StatementKind::StorageDead(index) => { + !promoted(*index) + } + _ => true, + }); + let terminator = block.terminator_mut(); + if let TerminatorKind::Drop { place, target, .. } = &terminator.kind { + if let Some(index) = place.as_local() { + if promoted(index) { + terminator.kind = TerminatorKind::Goto { target: *target }; + } + } + } + } + + promotions +} + +/// This function returns `true` if the `const_in_array_repeat_expressions` feature attribute should +/// be suggested. This function is probably quite expensive, it shouldn't be run in the happy path. +/// Feature attribute should be suggested if `operand` can be promoted and the feature is not +/// enabled. +crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>( + ccx: &ConstCx<'_, 'tcx>, + operand: &Operand<'tcx>, +) -> bool { + let mut rpo = traversal::reverse_postorder(&ccx.body); + let (temps, _) = collect_temps_and_candidates(&ccx, &mut rpo); + let validator = Validator { ccx, temps: &temps, explicit: false }; + + let should_promote = validator.validate_operand(operand).is_ok(); + let feature_flag = validator.ccx.tcx.features().const_in_array_repeat_expressions; + debug!( + "should_suggest_const_in_array_repeat_expressions_flag: def_id={:?} \ + should_promote={:?} feature_flag={:?}", + validator.ccx.def_id, should_promote, feature_flag + ); + should_promote && !feature_flag +} |