637 lines
27 KiB
Rust
637 lines
27 KiB
Rust
// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Partitioning Codegen Units for Incremental Compilation
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//! ======================================================
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//!
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//! The task of this module is to take the complete set of translation items of
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//! a crate and produce a set of codegen units from it, where a codegen unit
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//! is a named set of (translation-item, linkage) pairs. That is, this module
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//! decides which translation item appears in which codegen units with which
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//! linkage. The following paragraphs describe some of the background on the
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//! partitioning scheme.
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//!
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//! The most important opportunity for saving on compilation time with
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//! incremental compilation is to avoid re-translating and re-optimizing code.
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//! Since the unit of translation and optimization for LLVM is "modules" or, how
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//! we call them "codegen units", the particulars of how much time can be saved
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//! by incremental compilation are tightly linked to how the output program is
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//! partitioned into these codegen units prior to passing it to LLVM --
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//! especially because we have to treat codegen units as opaque entities once
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//! they are created: There is no way for us to incrementally update an existing
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//! LLVM module and so we have to build any such module from scratch if it was
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//! affected by some change in the source code.
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//!
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//! From that point of view it would make sense to maximize the number of
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//! codegen units by, for example, putting each function into its own module.
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//! That way only those modules would have to be re-compiled that were actually
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//! affected by some change, minimizing the number of functions that could have
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//! been re-used but just happened to be located in a module that is
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//! re-compiled.
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//!
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//! However, since LLVM optimization does not work across module boundaries,
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//! using such a highly granular partitioning would lead to very slow runtime
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//! code since it would effectively prohibit inlining and other inter-procedure
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//! optimizations. We want to avoid that as much as possible.
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//!
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//! Thus we end up with a trade-off: The bigger the codegen units, the better
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//! LLVM's optimizer can do its work, but also the smaller the compilation time
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//! reduction we get from incremental compilation.
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//!
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//! Ideally, we would create a partitioning such that there are few big codegen
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//! units with few interdependencies between them. For now though, we use the
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//! following heuristic to determine the partitioning:
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//!
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//! - There are two codegen units for every source-level module:
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//! - One for "stable", that is non-generic, code
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//! - One for more "volatile" code, i.e. monomorphized instances of functions
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//! defined in that module
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//! - Code for monomorphized instances of functions from external crates gets
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//! placed into every codegen unit that uses that instance.
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//!
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//! In order to see why this heuristic makes sense, let's take a look at when a
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//! codegen unit can get invalidated:
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//!
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//! 1. The most straightforward case is when the BODY of a function or global
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//! changes. Then any codegen unit containing the code for that item has to be
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//! re-compiled. Note that this includes all codegen units where the function
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//! has been inlined.
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//!
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//! 2. The next case is when the SIGNATURE of a function or global changes. In
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//! this case, all codegen units containing a REFERENCE to that item have to be
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//! re-compiled. This is a superset of case 1.
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//!
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//! 3. The final and most subtle case is when a REFERENCE to a generic function
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//! is added or removed somewhere. Even though the definition of the function
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//! might be unchanged, a new REFERENCE might introduce a new monomorphized
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//! instance of this function which has to be placed and compiled somewhere.
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//! Conversely, when removing a REFERENCE, it might have been the last one with
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//! that particular set of generic arguments and thus we have to remove it.
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//!
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//! From the above we see that just using one codegen unit per source-level
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//! module is not such a good idea, since just adding a REFERENCE to some
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//! generic item somewhere else would invalidate everything within the module
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//! containing the generic item. The heuristic above reduces this detrimental
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//! side-effect of references a little by at least not touching the non-generic
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//! code of the module.
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//!
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//! As another optimization, monomorphized functions from external crates get
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//! some special handling. Since we assume that the definition of such a
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//! function changes rather infrequently compared to local items, we can just
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//! instantiate external functions in every codegen unit where it is referenced
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//! -- without having to fear that doing this will cause a lot of unnecessary
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//! re-compilations. If such a reference is added or removed, the codegen unit
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//! has to be re-translated anyway.
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//! (Note that this only makes sense if external crates actually don't change
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//! frequently. For certain multi-crate projects this might not be a valid
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//! assumption).
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//!
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//! A Note on Inlining
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//! ------------------
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//! As briefly mentioned above, in order for LLVM to be able to inline a
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//! function call, the body of the function has to be available in the LLVM
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//! module where the call is made. This has a few consequences for partitioning:
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//!
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//! - The partitioning algorithm has to take care of placing functions into all
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//! codegen units where they should be available for inlining. It also has to
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//! decide on the correct linkage for these functions.
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//!
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//! - The partitioning algorithm has to know which functions are likely to get
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//! inlined, so it can distribute function instantiations accordingly. Since
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//! there is no way of knowing for sure which functions LLVM will decide to
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//! inline in the end, we apply a heuristic here: Only functions marked with
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//! #[inline] and (as stated above) functions from external crates are
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//! considered for inlining by the partitioner. The current implementation
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//! will not try to determine if a function is likely to be inlined by looking
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//! at the functions definition.
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//!
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//! Note though that as a side-effect of creating a codegen units per
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//! source-level module, functions from the same module will be available for
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//! inlining, even when they are not marked #[inline].
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use collector::InliningMap;
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use llvm;
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use monomorphize;
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use rustc::dep_graph::{DepNode, WorkProductId};
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use rustc::hir::def_id::DefId;
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use rustc::hir::map::DefPathData;
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use rustc::session::config::NUMBERED_CODEGEN_UNIT_MARKER;
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use rustc::ty::TyCtxt;
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use rustc::ty::item_path::characteristic_def_id_of_type;
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use rustc::ty::subst;
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use std::cmp::Ordering;
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use std::hash::{Hash, Hasher, SipHasher};
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use std::sync::Arc;
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use symbol_map::SymbolMap;
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use syntax::ast::NodeId;
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use syntax::parse::token::{self, InternedString};
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use trans_item::TransItem;
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use util::nodemap::{FnvHashMap, FnvHashSet, NodeSet};
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pub enum PartitioningStrategy {
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/// Generate one codegen unit per source-level module.
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PerModule,
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/// Partition the whole crate into a fixed number of codegen units.
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FixedUnitCount(usize)
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}
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pub struct CodegenUnit<'tcx> {
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/// A name for this CGU. Incremental compilation requires that
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/// name be unique amongst **all** crates. Therefore, it should
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/// contain something unique to this crate (e.g., a module path)
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/// as well as the crate name and disambiguator.
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name: InternedString,
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items: FnvHashMap<TransItem<'tcx>, llvm::Linkage>,
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}
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impl<'tcx> CodegenUnit<'tcx> {
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pub fn new(name: InternedString,
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items: FnvHashMap<TransItem<'tcx>, llvm::Linkage>)
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-> Self {
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CodegenUnit {
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name: name,
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items: items,
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}
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}
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pub fn empty(name: InternedString) -> Self {
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Self::new(name, FnvHashMap())
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}
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pub fn contains_item(&self, item: &TransItem<'tcx>) -> bool {
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self.items.contains_key(item)
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}
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pub fn name(&self) -> &str {
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&self.name
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}
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pub fn items(&self) -> &FnvHashMap<TransItem<'tcx>, llvm::Linkage> {
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&self.items
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}
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pub fn work_product_id(&self) -> Arc<WorkProductId> {
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Arc::new(WorkProductId(self.name().to_string()))
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}
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pub fn work_product_dep_node(&self) -> DepNode<DefId> {
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DepNode::WorkProduct(self.work_product_id())
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}
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pub fn compute_symbol_name_hash(&self, tcx: TyCtxt, symbol_map: &SymbolMap) -> u64 {
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let mut state = SipHasher::new();
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let all_items = self.items_in_deterministic_order(tcx, symbol_map);
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for (item, _) in all_items {
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let symbol_name = symbol_map.get(item).unwrap();
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symbol_name.hash(&mut state);
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}
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state.finish()
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}
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pub fn items_in_deterministic_order(&self,
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tcx: TyCtxt,
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symbol_map: &SymbolMap)
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-> Vec<(TransItem<'tcx>, llvm::Linkage)> {
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let mut items: Vec<(TransItem<'tcx>, llvm::Linkage)> =
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self.items.iter().map(|(item, linkage)| (*item, *linkage)).collect();
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// The codegen tests rely on items being process in the same order as
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// they appear in the file, so for local items, we sort by node_id first
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items.sort_by(|&(trans_item1, _), &(trans_item2, _)| {
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let node_id1 = local_node_id(tcx, trans_item1);
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let node_id2 = local_node_id(tcx, trans_item2);
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match (node_id1, node_id2) {
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(None, None) => {
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let symbol_name1 = symbol_map.get(trans_item1).unwrap();
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let symbol_name2 = symbol_map.get(trans_item2).unwrap();
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symbol_name1.cmp(symbol_name2)
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}
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// In the following two cases we can avoid looking up the symbol
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(None, Some(_)) => Ordering::Less,
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(Some(_), None) => Ordering::Greater,
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(Some(node_id1), Some(node_id2)) => {
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let ordering = node_id1.cmp(&node_id2);
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if ordering != Ordering::Equal {
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return ordering;
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}
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let symbol_name1 = symbol_map.get(trans_item1).unwrap();
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let symbol_name2 = symbol_map.get(trans_item2).unwrap();
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symbol_name1.cmp(symbol_name2)
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}
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}
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});
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return items;
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fn local_node_id(tcx: TyCtxt, trans_item: TransItem) -> Option<NodeId> {
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match trans_item {
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TransItem::Fn(instance) => {
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tcx.map.as_local_node_id(instance.def)
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}
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TransItem::Static(node_id) => Some(node_id),
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TransItem::DropGlue(_) => None,
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}
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}
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}
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}
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// Anything we can't find a proper codegen unit for goes into this.
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const FALLBACK_CODEGEN_UNIT: &'static str = "__rustc_fallback_codegen_unit";
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pub fn partition<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
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trans_items: I,
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strategy: PartitioningStrategy,
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inlining_map: &InliningMap<'tcx>,
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reachable: &NodeSet)
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-> Vec<CodegenUnit<'tcx>>
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where I: Iterator<Item = TransItem<'tcx>>
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{
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if let PartitioningStrategy::FixedUnitCount(1) = strategy {
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// If there is only a single codegen-unit, we can use a very simple
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// scheme and don't have to bother with doing much analysis.
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return vec![single_codegen_unit(tcx, trans_items, reachable)];
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}
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// In the first step, we place all regular translation items into their
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// respective 'home' codegen unit. Regular translation items are all
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// functions and statics defined in the local crate.
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let mut initial_partitioning = place_root_translation_items(tcx,
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trans_items,
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reachable);
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debug_dump(tcx, "INITIAL PARTITONING:", initial_partitioning.codegen_units.iter());
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// If the partitioning should produce a fixed count of codegen units, merge
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// until that count is reached.
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if let PartitioningStrategy::FixedUnitCount(count) = strategy {
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merge_codegen_units(&mut initial_partitioning, count, &tcx.crate_name[..]);
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debug_dump(tcx, "POST MERGING:", initial_partitioning.codegen_units.iter());
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}
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// In the next step, we use the inlining map to determine which addtional
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// translation items have to go into each codegen unit. These additional
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// translation items can be drop-glue, functions from external crates, and
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// local functions the definition of which is marked with #[inline].
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let post_inlining = place_inlined_translation_items(initial_partitioning,
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inlining_map);
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debug_dump(tcx, "POST INLINING:", post_inlining.0.iter());
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// Finally, sort by codegen unit name, so that we get deterministic results
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let mut result = post_inlining.0;
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result.sort_by(|cgu1, cgu2| {
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(&cgu1.name[..]).cmp(&cgu2.name[..])
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});
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result
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}
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struct PreInliningPartitioning<'tcx> {
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codegen_units: Vec<CodegenUnit<'tcx>>,
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roots: FnvHashSet<TransItem<'tcx>>,
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}
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struct PostInliningPartitioning<'tcx>(Vec<CodegenUnit<'tcx>>);
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fn place_root_translation_items<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
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trans_items: I,
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_reachable: &NodeSet)
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-> PreInliningPartitioning<'tcx>
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where I: Iterator<Item = TransItem<'tcx>>
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{
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let mut roots = FnvHashSet();
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let mut codegen_units = FnvHashMap();
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for trans_item in trans_items {
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let is_root = !trans_item.is_instantiated_only_on_demand();
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if is_root {
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let characteristic_def_id = characteristic_def_id_of_trans_item(tcx, trans_item);
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let is_volatile = trans_item.is_generic_fn();
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let codegen_unit_name = match characteristic_def_id {
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Some(def_id) => compute_codegen_unit_name(tcx, def_id, is_volatile),
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None => InternedString::new(FALLBACK_CODEGEN_UNIT),
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};
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let make_codegen_unit = || {
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CodegenUnit::empty(codegen_unit_name.clone())
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};
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let mut codegen_unit = codegen_units.entry(codegen_unit_name.clone())
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.or_insert_with(make_codegen_unit);
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let linkage = match trans_item.explicit_linkage(tcx) {
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Some(explicit_linkage) => explicit_linkage,
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None => {
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match trans_item {
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TransItem::Static(..) => llvm::ExternalLinkage,
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TransItem::DropGlue(..) => unreachable!(),
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// Is there any benefit to using ExternalLinkage?:
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TransItem::Fn(ref instance) => {
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if instance.substs.types.is_empty() {
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// This is a non-generic functions, we always
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// make it visible externally on the chance that
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// it might be used in another codegen unit.
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llvm::ExternalLinkage
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} else {
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// In the current setup, generic functions cannot
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// be roots.
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unreachable!()
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}
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}
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}
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}
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};
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codegen_unit.items.insert(trans_item, linkage);
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roots.insert(trans_item);
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}
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}
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// always ensure we have at least one CGU; otherwise, if we have a
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// crate with just types (for example), we could wind up with no CGU
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if codegen_units.is_empty() {
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let codegen_unit_name = InternedString::new(FALLBACK_CODEGEN_UNIT);
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codegen_units.entry(codegen_unit_name.clone())
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.or_insert_with(|| CodegenUnit::empty(codegen_unit_name.clone()));
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}
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PreInliningPartitioning {
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codegen_units: codegen_units.into_iter()
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.map(|(_, codegen_unit)| codegen_unit)
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.collect(),
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roots: roots,
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}
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}
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fn merge_codegen_units<'tcx>(initial_partitioning: &mut PreInliningPartitioning<'tcx>,
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target_cgu_count: usize,
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crate_name: &str) {
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assert!(target_cgu_count >= 1);
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let codegen_units = &mut initial_partitioning.codegen_units;
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// Merge the two smallest codegen units until the target size is reached.
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// Note that "size" is estimated here rather inaccurately as the number of
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// translation items in a given unit. This could be improved on.
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while codegen_units.len() > target_cgu_count {
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// Sort small cgus to the back
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codegen_units.sort_by_key(|cgu| -(cgu.items.len() as i64));
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let smallest = codegen_units.pop().unwrap();
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let second_smallest = codegen_units.last_mut().unwrap();
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for (k, v) in smallest.items.into_iter() {
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second_smallest.items.insert(k, v);
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}
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}
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for (index, cgu) in codegen_units.iter_mut().enumerate() {
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cgu.name = numbered_codegen_unit_name(crate_name, index);
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}
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// If the initial partitioning contained less than target_cgu_count to begin
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// with, we won't have enough codegen units here, so add a empty units until
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// we reach the target count
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while codegen_units.len() < target_cgu_count {
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let index = codegen_units.len();
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codegen_units.push(
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CodegenUnit::empty(numbered_codegen_unit_name(crate_name, index)));
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}
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}
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fn place_inlined_translation_items<'tcx>(initial_partitioning: PreInliningPartitioning<'tcx>,
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inlining_map: &InliningMap<'tcx>)
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-> PostInliningPartitioning<'tcx> {
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let mut new_partitioning = Vec::new();
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for codegen_unit in &initial_partitioning.codegen_units[..] {
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// Collect all items that need to be available in this codegen unit
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let mut reachable = FnvHashSet();
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for root in codegen_unit.items.keys() {
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follow_inlining(*root, inlining_map, &mut reachable);
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}
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let mut new_codegen_unit =
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CodegenUnit::empty(codegen_unit.name.clone());
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// Add all translation items that are not already there
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for trans_item in reachable {
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if let Some(linkage) = codegen_unit.items.get(&trans_item) {
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// This is a root, just copy it over
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new_codegen_unit.items.insert(trans_item, *linkage);
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} else if initial_partitioning.roots.contains(&trans_item) {
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// This item will be instantiated in some other codegen unit,
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// so we just add it here with AvailableExternallyLinkage
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// FIXME(mw): I have not seen it happening yet but having
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// available_externally here could potentially lead
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// to the same problem with exception handling tables
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// as in the case below.
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new_codegen_unit.items.insert(trans_item,
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llvm::AvailableExternallyLinkage);
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} else if trans_item.is_from_extern_crate() && !trans_item.is_generic_fn() {
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// FIXME(mw): It would be nice if we could mark these as
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// `AvailableExternallyLinkage`, since they should have
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// been instantiated in the extern crate. But this
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// sometimes leads to crashes on Windows because LLVM
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// does not handle exception handling table instantiation
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// reliably in that case.
|
|
new_codegen_unit.items.insert(trans_item, llvm::InternalLinkage);
|
|
} else {
|
|
assert!(trans_item.is_instantiated_only_on_demand());
|
|
// We can't be sure if this will also be instantiated
|
|
// somewhere else, so we add an instance here with
|
|
// InternalLinkage so we don't get any conflicts.
|
|
new_codegen_unit.items.insert(trans_item,
|
|
llvm::InternalLinkage);
|
|
}
|
|
}
|
|
|
|
new_partitioning.push(new_codegen_unit);
|
|
}
|
|
|
|
return PostInliningPartitioning(new_partitioning);
|
|
|
|
fn follow_inlining<'tcx>(trans_item: TransItem<'tcx>,
|
|
inlining_map: &InliningMap<'tcx>,
|
|
visited: &mut FnvHashSet<TransItem<'tcx>>) {
|
|
if !visited.insert(trans_item) {
|
|
return;
|
|
}
|
|
|
|
inlining_map.with_inlining_candidates(trans_item, |target| {
|
|
follow_inlining(target, inlining_map, visited);
|
|
});
|
|
}
|
|
}
|
|
|
|
fn characteristic_def_id_of_trans_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
|
|
trans_item: TransItem<'tcx>)
|
|
-> Option<DefId> {
|
|
match trans_item {
|
|
TransItem::Fn(instance) => {
|
|
// If this is a method, we want to put it into the same module as
|
|
// its self-type. If the self-type does not provide a characteristic
|
|
// DefId, we use the location of the impl after all.
|
|
|
|
if tcx.trait_of_item(instance.def).is_some() {
|
|
let self_ty = *instance.substs.types.get(subst::TypeSpace, 0);
|
|
// This is an implementation of a trait method.
|
|
return characteristic_def_id_of_type(self_ty).or(Some(instance.def));
|
|
}
|
|
|
|
if let Some(impl_def_id) = tcx.impl_of_method(instance.def) {
|
|
// This is a method within an inherent impl, find out what the
|
|
// self-type is:
|
|
let impl_self_ty = tcx.lookup_item_type(impl_def_id).ty;
|
|
let impl_self_ty = tcx.erase_regions(&impl_self_ty);
|
|
let impl_self_ty = monomorphize::apply_param_substs(tcx,
|
|
instance.substs,
|
|
&impl_self_ty);
|
|
|
|
if let Some(def_id) = characteristic_def_id_of_type(impl_self_ty) {
|
|
return Some(def_id);
|
|
}
|
|
}
|
|
|
|
Some(instance.def)
|
|
}
|
|
TransItem::DropGlue(dg) => characteristic_def_id_of_type(dg.ty()),
|
|
TransItem::Static(node_id) => Some(tcx.map.local_def_id(node_id)),
|
|
}
|
|
}
|
|
|
|
fn compute_codegen_unit_name<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
|
|
def_id: DefId,
|
|
volatile: bool)
|
|
-> InternedString {
|
|
// Unfortunately we cannot just use the `ty::item_path` infrastructure here
|
|
// because we need paths to modules and the DefIds of those are not
|
|
// available anymore for external items.
|
|
let mut mod_path = String::with_capacity(64);
|
|
|
|
let def_path = tcx.def_path(def_id);
|
|
mod_path.push_str(&tcx.crate_name(def_path.krate));
|
|
|
|
for part in tcx.def_path(def_id)
|
|
.data
|
|
.iter()
|
|
.take_while(|part| {
|
|
match part.data {
|
|
DefPathData::Module(..) => true,
|
|
_ => false,
|
|
}
|
|
}) {
|
|
mod_path.push_str("-");
|
|
mod_path.push_str(&part.data.as_interned_str());
|
|
}
|
|
|
|
if volatile {
|
|
mod_path.push_str(".volatile");
|
|
}
|
|
|
|
return token::intern_and_get_ident(&mod_path[..]);
|
|
}
|
|
|
|
fn single_codegen_unit<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
|
|
trans_items: I,
|
|
reachable: &NodeSet)
|
|
-> CodegenUnit<'tcx>
|
|
where I: Iterator<Item = TransItem<'tcx>>
|
|
{
|
|
let mut items = FnvHashMap();
|
|
|
|
for trans_item in trans_items {
|
|
let linkage = trans_item.explicit_linkage(tcx).unwrap_or_else(|| {
|
|
match trans_item {
|
|
TransItem::Static(node_id) => {
|
|
if reachable.contains(&node_id) {
|
|
llvm::ExternalLinkage
|
|
} else {
|
|
llvm::PrivateLinkage
|
|
}
|
|
}
|
|
TransItem::DropGlue(_) => {
|
|
llvm::InternalLinkage
|
|
}
|
|
TransItem::Fn(instance) => {
|
|
if trans_item.is_generic_fn() {
|
|
// FIXME(mw): Assigning internal linkage to all
|
|
// monomorphizations is potentially a waste of space
|
|
// since monomorphizations could be shared between
|
|
// crates. The main reason for making them internal is
|
|
// a limitation in MingW's binutils that cannot deal
|
|
// with COFF object that have more than 2^15 sections,
|
|
// which is something that can happen for large programs
|
|
// when every function gets put into its own COMDAT
|
|
// section.
|
|
llvm::InternalLinkage
|
|
} else if trans_item.is_from_extern_crate() {
|
|
// FIXME(mw): It would be nice if we could mark these as
|
|
// `AvailableExternallyLinkage`, since they should have
|
|
// been instantiated in the extern crate. But this
|
|
// sometimes leads to crashes on Windows because LLVM
|
|
// does not handle exception handling table instantiation
|
|
// reliably in that case.
|
|
llvm::InternalLinkage
|
|
} else if reachable.contains(&tcx.map
|
|
.as_local_node_id(instance.def)
|
|
.unwrap()) {
|
|
llvm::ExternalLinkage
|
|
} else {
|
|
// Functions that are not visible outside this crate can
|
|
// be marked as internal.
|
|
llvm::InternalLinkage
|
|
}
|
|
}
|
|
}
|
|
});
|
|
|
|
items.insert(trans_item, linkage);
|
|
}
|
|
|
|
CodegenUnit::new(
|
|
numbered_codegen_unit_name(&tcx.crate_name[..], 0),
|
|
items)
|
|
}
|
|
|
|
fn numbered_codegen_unit_name(crate_name: &str, index: usize) -> InternedString {
|
|
token::intern_and_get_ident(&format!("{}{}{}",
|
|
crate_name,
|
|
NUMBERED_CODEGEN_UNIT_MARKER,
|
|
index)[..])
|
|
}
|
|
|
|
fn debug_dump<'a, 'b, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
|
|
label: &str,
|
|
cgus: I)
|
|
where I: Iterator<Item=&'b CodegenUnit<'tcx>>,
|
|
'tcx: 'a + 'b
|
|
{
|
|
if cfg!(debug_assertions) {
|
|
debug!("{}", label);
|
|
for cgu in cgus {
|
|
debug!("CodegenUnit {}:", cgu.name);
|
|
|
|
for (trans_item, linkage) in &cgu.items {
|
|
debug!(" - {} [{:?}]", trans_item.to_string(tcx), linkage);
|
|
}
|
|
|
|
debug!("");
|
|
}
|
|
}
|
|
}
|