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Create promoted MIR fragments in const and statics

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The previous strategy of removing `Drop` and `StorageDead` for promoted
locals only worked for rvalue lifetime extension. We now use the same
implementation for promotion across all kinds of items.
This commit is contained in:
Dylan MacKenzie 2019-11-22 10:58:58 -08:00
parent fcf4bee7d3
commit f9ed2199ff
1 changed files with 4 additions and 100 deletions
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104
src/librustc_mir/transform/promote_consts.rs
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@ -24,7 +24,6 @@ use syntax::symbol::sym;
use syntax_pos::{Span, DUMMY_SP};
use rustc_index::vec::{IndexVec, Idx};
use rustc_index::bit_set::HybridBitSet;
use rustc_target::spec::abi::Abi;
use std::cell::Cell;
@ -35,10 +34,8 @@ use crate::transform::check_consts::{qualifs, Item, ConstKind, is_lang_panic_fn}
/// A `MirPass` for promotion.
///
/// In this case, "promotion" entails the following:
/// - Extract promotable temps in `fn` and `const fn` into their own MIR bodies.
/// - Extend lifetimes in `const` and `static` by removing `Drop` and `StorageDead`.
/// - Emit errors if the requirements of `#[rustc_args_required_const]` are not met.
/// 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 `StaticKind::Promoted`.
@ -63,26 +60,13 @@ impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
let def_id = src.def_id();
let item = Item::new(tcx, def_id, body);
let mut rpo = traversal::reverse_postorder(body);
let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
let promotable_candidates = validate_candidates(tcx, body, def_id, &temps, &all_candidates);
// For now, lifetime extension is done in `const` and `static`s without creating promoted
// MIR fragments by removing `Drop` and `StorageDead` for each referent. However, this will
// not work inside loops when they are allowed in `const`s.
//
// FIXME: use promoted MIR fragments everywhere?
let promoted_fragments = if should_create_promoted_mir_fragments(item.const_kind) {
promote_candidates(def_id, body, tcx, temps, promotable_candidates)
} else {
// FIXME: promote const array initializers in consts.
remove_drop_and_storage_dead_on_promoted_locals(tcx, body, &promotable_candidates);
IndexVec::new()
};
self.promoted_fragments.set(promoted_fragments);
let promoted = promote_candidates(def_id, body, tcx, temps, promotable_candidates);
self.promoted_fragments.set(promoted);
}
}
@ -1188,83 +1172,3 @@ crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
should_promote={:?} feature_flag={:?}", mir_def_id, should_promote, feature_flag);
should_promote && !feature_flag
}
fn should_create_promoted_mir_fragments(const_kind: Option<ConstKind>) -> bool {
match const_kind {
Some(ConstKind::ConstFn) | None => true,
Some(ConstKind::Const) | Some(ConstKind::Static) | Some(ConstKind::StaticMut) => false,
}
}
/// In `const` and `static` everything without `StorageDead`
/// is `'static`, we don't have to create promoted MIR fragments,
/// just remove `Drop` and `StorageDead` on "promoted" locals.
fn remove_drop_and_storage_dead_on_promoted_locals(
tcx: TyCtxt<'tcx>,
body: &mut Body<'tcx>,
promotable_candidates: &[Candidate],
) {
debug!("run_pass: promotable_candidates={:?}", promotable_candidates);
// Removing `StorageDead` will cause errors for temps declared inside a loop body. For now we
// simply skip promotion if a loop exists, since loops are not yet allowed in a `const`.
//
// FIXME: Just create MIR fragments for `const`s instead of using this hackish approach?
if body.is_cfg_cyclic() {
tcx.sess.delay_span_bug(body.span, "Control-flow cycle detected in `const`");
return;
}
// The underlying local for promotion contexts like `&temp` and `&(temp.proj)`.
let mut requires_lifetime_extension = HybridBitSet::new_empty(body.local_decls.len());
promotable_candidates
.iter()
.filter_map(|c| {
match c {
Candidate::Ref(loc) => Some(loc),
Candidate::Repeat(_) | Candidate::Argument { .. } => None,
}
})
.map(|&Location { block, statement_index }| {
// FIXME: store the `Local` for each `Candidate` when it is created.
let place = match &body[block].statements[statement_index].kind {
StatementKind::Assign(box ( _, Rvalue::Ref(_, _, place))) => place,
_ => bug!("`Candidate::Ref` without corresponding assignment"),
};
match place.base {
PlaceBase::Local(local) => local,
PlaceBase::Static(_) => bug!("`Candidate::Ref` for a non-local"),
}
})
.for_each(|local| {
requires_lifetime_extension.insert(local);
});
// Remove `Drop` terminators and `StorageDead` statements for all promotable temps that require
// lifetime extension.
for block in body.basic_blocks_mut() {
block.statements.retain(|statement| {
match statement.kind {
StatementKind::StorageDead(index) => !requires_lifetime_extension.contains(index),
_ => true
}
});
let terminator = block.terminator_mut();
match &terminator.kind {
TerminatorKind::Drop {
location,
target,
..
} => {
if let Some(index) = location.as_local() {
if requires_lifetime_extension.contains(index) {
terminator.kind = TerminatorKind::Goto { target: *target };
}
}
}
_ => {}
}
}
}