diff options
Diffstat (limited to 'compiler/rustc_monomorphize/src/partitioning')
| -rw-r--r-- | compiler/rustc_monomorphize/src/partitioning/default.rs | 560 | ||||
| -rw-r--r-- | compiler/rustc_monomorphize/src/partitioning/merging.rs | 111 | ||||
| -rw-r--r-- | compiler/rustc_monomorphize/src/partitioning/mod.rs | 515 |
3 files changed, 1186 insertions, 0 deletions
diff --git a/compiler/rustc_monomorphize/src/partitioning/default.rs b/compiler/rustc_monomorphize/src/partitioning/default.rs new file mode 100644 index 000000000..15276569c --- /dev/null +++ b/compiler/rustc_monomorphize/src/partitioning/default.rs @@ -0,0 +1,560 @@ +use std::collections::hash_map::Entry; + +use rustc_data_structures::fx::{FxHashMap, FxHashSet}; +use rustc_hir::def::DefKind; +use rustc_hir::def_id::{DefId, LOCAL_CRATE}; +use rustc_hir::definitions::DefPathDataName; +use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags; +use rustc_middle::middle::exported_symbols::{SymbolExportInfo, SymbolExportLevel}; +use rustc_middle::mir::mono::{CodegenUnit, CodegenUnitNameBuilder, Linkage, Visibility}; +use rustc_middle::mir::mono::{InstantiationMode, MonoItem}; +use rustc_middle::ty::print::characteristic_def_id_of_type; +use rustc_middle::ty::{self, visit::TypeVisitable, DefIdTree, InstanceDef, TyCtxt}; +use rustc_span::symbol::Symbol; + +use super::PartitioningCx; +use crate::collector::InliningMap; +use crate::partitioning::merging; +use crate::partitioning::{ + MonoItemPlacement, Partitioner, PostInliningPartitioning, PreInliningPartitioning, +}; + +pub struct DefaultPartitioning; + +impl<'tcx> Partitioner<'tcx> for DefaultPartitioning { + fn place_root_mono_items( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>, + ) -> PreInliningPartitioning<'tcx> { + let mut roots = FxHashSet::default(); + let mut codegen_units = FxHashMap::default(); + let is_incremental_build = cx.tcx.sess.opts.incremental.is_some(); + let mut internalization_candidates = FxHashSet::default(); + + // Determine if monomorphizations instantiated in this crate will be made + // available to downstream crates. This depends on whether we are in + // share-generics mode and whether the current crate can even have + // downstream crates. + let export_generics = + cx.tcx.sess.opts.share_generics() && cx.tcx.local_crate_exports_generics(); + + let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx); + let cgu_name_cache = &mut FxHashMap::default(); + + for mono_item in mono_items { + match mono_item.instantiation_mode(cx.tcx) { + InstantiationMode::GloballyShared { .. } => {} + InstantiationMode::LocalCopy => continue, + } + + let characteristic_def_id = characteristic_def_id_of_mono_item(cx.tcx, mono_item); + let is_volatile = is_incremental_build && mono_item.is_generic_fn(); + + let codegen_unit_name = match characteristic_def_id { + Some(def_id) => compute_codegen_unit_name( + cx.tcx, + cgu_name_builder, + def_id, + is_volatile, + cgu_name_cache, + ), + None => fallback_cgu_name(cgu_name_builder), + }; + + let codegen_unit = codegen_units + .entry(codegen_unit_name) + .or_insert_with(|| CodegenUnit::new(codegen_unit_name)); + + let mut can_be_internalized = true; + let (linkage, visibility) = mono_item_linkage_and_visibility( + cx.tcx, + &mono_item, + &mut can_be_internalized, + export_generics, + ); + if visibility == Visibility::Hidden && can_be_internalized { + internalization_candidates.insert(mono_item); + } + + codegen_unit.items_mut().insert(mono_item, (linkage, visibility)); + roots.insert(mono_item); + } + + // Always ensure we have at least one CGU; otherwise, if we have a + // crate with just types (for example), we could wind up with no CGU. + if codegen_units.is_empty() { + let codegen_unit_name = fallback_cgu_name(cgu_name_builder); + codegen_units.insert(codegen_unit_name, CodegenUnit::new(codegen_unit_name)); + } + + PreInliningPartitioning { + codegen_units: codegen_units + .into_iter() + .map(|(_, codegen_unit)| codegen_unit) + .collect(), + roots, + internalization_candidates, + } + } + + fn merge_codegen_units( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + initial_partitioning: &mut PreInliningPartitioning<'tcx>, + ) { + merging::merge_codegen_units(cx, initial_partitioning); + } + + fn place_inlined_mono_items( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + initial_partitioning: PreInliningPartitioning<'tcx>, + ) -> PostInliningPartitioning<'tcx> { + let mut new_partitioning = Vec::new(); + let mut mono_item_placements = FxHashMap::default(); + + let PreInliningPartitioning { + codegen_units: initial_cgus, + roots, + internalization_candidates, + } = initial_partitioning; + + let single_codegen_unit = initial_cgus.len() == 1; + + for old_codegen_unit in initial_cgus { + // Collect all items that need to be available in this codegen unit. + let mut reachable = FxHashSet::default(); + for root in old_codegen_unit.items().keys() { + follow_inlining(*root, cx.inlining_map, &mut reachable); + } + + let mut new_codegen_unit = CodegenUnit::new(old_codegen_unit.name()); + + // Add all monomorphizations that are not already there. + for mono_item in reachable { + if let Some(linkage) = old_codegen_unit.items().get(&mono_item) { + // This is a root, just copy it over. + new_codegen_unit.items_mut().insert(mono_item, *linkage); + } else { + if roots.contains(&mono_item) { + bug!( + "GloballyShared mono-item inlined into other CGU: \ + {:?}", + mono_item + ); + } + + // This is a CGU-private copy. + new_codegen_unit + .items_mut() + .insert(mono_item, (Linkage::Internal, Visibility::Default)); + } + + if !single_codegen_unit { + // If there is more than one codegen unit, we need to keep track + // in which codegen units each monomorphization is placed. + match mono_item_placements.entry(mono_item) { + Entry::Occupied(e) => { + let placement = e.into_mut(); + debug_assert!(match *placement { + MonoItemPlacement::SingleCgu { cgu_name } => { + cgu_name != new_codegen_unit.name() + } + MonoItemPlacement::MultipleCgus => true, + }); + *placement = MonoItemPlacement::MultipleCgus; + } + Entry::Vacant(e) => { + e.insert(MonoItemPlacement::SingleCgu { + cgu_name: new_codegen_unit.name(), + }); + } + } + } + } + + new_partitioning.push(new_codegen_unit); + } + + return PostInliningPartitioning { + codegen_units: new_partitioning, + mono_item_placements, + internalization_candidates, + }; + + fn follow_inlining<'tcx>( + mono_item: MonoItem<'tcx>, + inlining_map: &InliningMap<'tcx>, + visited: &mut FxHashSet<MonoItem<'tcx>>, + ) { + if !visited.insert(mono_item) { + return; + } + + inlining_map.with_inlining_candidates(mono_item, |target| { + follow_inlining(target, inlining_map, visited); + }); + } + } + + fn internalize_symbols( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + partitioning: &mut PostInliningPartitioning<'tcx>, + ) { + if partitioning.codegen_units.len() == 1 { + // Fast path for when there is only one codegen unit. In this case we + // can internalize all candidates, since there is nowhere else they + // could be accessed from. + for cgu in &mut partitioning.codegen_units { + for candidate in &partitioning.internalization_candidates { + cgu.items_mut().insert(*candidate, (Linkage::Internal, Visibility::Default)); + } + } + + return; + } + + // Build a map from every monomorphization to all the monomorphizations that + // reference it. + let mut accessor_map: FxHashMap<MonoItem<'tcx>, Vec<MonoItem<'tcx>>> = Default::default(); + cx.inlining_map.iter_accesses(|accessor, accessees| { + for accessee in accessees { + accessor_map.entry(*accessee).or_default().push(accessor); + } + }); + + let mono_item_placements = &partitioning.mono_item_placements; + + // For each internalization candidates in each codegen unit, check if it is + // accessed from outside its defining codegen unit. + for cgu in &mut partitioning.codegen_units { + let home_cgu = MonoItemPlacement::SingleCgu { cgu_name: cgu.name() }; + + for (accessee, linkage_and_visibility) in cgu.items_mut() { + if !partitioning.internalization_candidates.contains(accessee) { + // This item is no candidate for internalizing, so skip it. + continue; + } + debug_assert_eq!(mono_item_placements[accessee], home_cgu); + + if let Some(accessors) = accessor_map.get(accessee) { + if accessors + .iter() + .filter_map(|accessor| { + // Some accessors might not have been + // instantiated. We can safely ignore those. + mono_item_placements.get(accessor) + }) + .any(|placement| *placement != home_cgu) + { + // Found an accessor from another CGU, so skip to the next + // item without marking this one as internal. + continue; + } + } + + // If we got here, we did not find any accesses from other CGUs, + // so it's fine to make this monomorphization internal. + *linkage_and_visibility = (Linkage::Internal, Visibility::Default); + } + } + } +} + +fn characteristic_def_id_of_mono_item<'tcx>( + tcx: TyCtxt<'tcx>, + mono_item: MonoItem<'tcx>, +) -> Option<DefId> { + match mono_item { + MonoItem::Fn(instance) => { + let def_id = match instance.def { + ty::InstanceDef::Item(def) => def.did, + ty::InstanceDef::VTableShim(..) + | ty::InstanceDef::ReifyShim(..) + | ty::InstanceDef::FnPtrShim(..) + | ty::InstanceDef::ClosureOnceShim { .. } + | ty::InstanceDef::Intrinsic(..) + | ty::InstanceDef::DropGlue(..) + | ty::InstanceDef::Virtual(..) + | ty::InstanceDef::CloneShim(..) => return None, + }; + + // 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(def_id).is_some() { + let self_ty = instance.substs.type_at(0); + // This is a default implementation of a trait method. + return characteristic_def_id_of_type(self_ty).or(Some(def_id)); + } + + if let Some(impl_def_id) = tcx.impl_of_method(def_id) { + if tcx.sess.opts.incremental.is_some() + && tcx.trait_id_of_impl(impl_def_id) == tcx.lang_items().drop_trait() + { + // Put `Drop::drop` into the same cgu as `drop_in_place` + // since `drop_in_place` is the only thing that can + // call it. + return None; + } + + // When polymorphization is enabled, methods which do not depend on their generic + // parameters, but the self-type of their impl block do will fail to normalize. + if !tcx.sess.opts.unstable_opts.polymorphize || !instance.needs_subst() { + // This is a method within an impl, find out what the self-type is: + let impl_self_ty = tcx.subst_and_normalize_erasing_regions( + instance.substs, + ty::ParamEnv::reveal_all(), + tcx.type_of(impl_def_id), + ); + if let Some(def_id) = characteristic_def_id_of_type(impl_self_ty) { + return Some(def_id); + } + } + } + + Some(def_id) + } + MonoItem::Static(def_id) => Some(def_id), + MonoItem::GlobalAsm(item_id) => Some(item_id.def_id.to_def_id()), + } +} + +fn compute_codegen_unit_name( + tcx: TyCtxt<'_>, + name_builder: &mut CodegenUnitNameBuilder<'_>, + def_id: DefId, + volatile: bool, + cache: &mut CguNameCache, +) -> Symbol { + // Find the innermost module that is not nested within a function. + let mut current_def_id = def_id; + let mut cgu_def_id = None; + // Walk backwards from the item we want to find the module for. + loop { + if current_def_id.is_crate_root() { + if cgu_def_id.is_none() { + // If we have not found a module yet, take the crate root. + cgu_def_id = Some(def_id.krate.as_def_id()); + } + break; + } else if tcx.def_kind(current_def_id) == DefKind::Mod { + if cgu_def_id.is_none() { + cgu_def_id = Some(current_def_id); + } + } else { + // If we encounter something that is not a module, throw away + // any module that we've found so far because we now know that + // it is nested within something else. + cgu_def_id = None; + } + + current_def_id = tcx.parent(current_def_id); + } + + let cgu_def_id = cgu_def_id.unwrap(); + + *cache.entry((cgu_def_id, volatile)).or_insert_with(|| { + let def_path = tcx.def_path(cgu_def_id); + + let components = def_path.data.iter().map(|part| match part.data.name() { + DefPathDataName::Named(name) => name, + DefPathDataName::Anon { .. } => unreachable!(), + }); + + let volatile_suffix = volatile.then_some("volatile"); + + name_builder.build_cgu_name(def_path.krate, components, volatile_suffix) + }) +} + +// Anything we can't find a proper codegen unit for goes into this. +fn fallback_cgu_name(name_builder: &mut CodegenUnitNameBuilder<'_>) -> Symbol { + name_builder.build_cgu_name(LOCAL_CRATE, &["fallback"], Some("cgu")) +} + +fn mono_item_linkage_and_visibility<'tcx>( + tcx: TyCtxt<'tcx>, + mono_item: &MonoItem<'tcx>, + can_be_internalized: &mut bool, + export_generics: bool, +) -> (Linkage, Visibility) { + if let Some(explicit_linkage) = mono_item.explicit_linkage(tcx) { + return (explicit_linkage, Visibility::Default); + } + let vis = mono_item_visibility(tcx, mono_item, can_be_internalized, export_generics); + (Linkage::External, vis) +} + +type CguNameCache = FxHashMap<(DefId, bool), Symbol>; + +fn mono_item_visibility<'tcx>( + tcx: TyCtxt<'tcx>, + mono_item: &MonoItem<'tcx>, + can_be_internalized: &mut bool, + export_generics: bool, +) -> Visibility { + let instance = match mono_item { + // This is pretty complicated; see below. + MonoItem::Fn(instance) => instance, + + // Misc handling for generics and such, but otherwise: + MonoItem::Static(def_id) => { + return if tcx.is_reachable_non_generic(*def_id) { + *can_be_internalized = false; + default_visibility(tcx, *def_id, false) + } else { + Visibility::Hidden + }; + } + MonoItem::GlobalAsm(item_id) => { + return if tcx.is_reachable_non_generic(item_id.def_id) { + *can_be_internalized = false; + default_visibility(tcx, item_id.def_id.to_def_id(), false) + } else { + Visibility::Hidden + }; + } + }; + + let def_id = match instance.def { + InstanceDef::Item(def) => def.did, + InstanceDef::DropGlue(def_id, Some(_)) => def_id, + + // These are all compiler glue and such, never exported, always hidden. + InstanceDef::VTableShim(..) + | InstanceDef::ReifyShim(..) + | InstanceDef::FnPtrShim(..) + | InstanceDef::Virtual(..) + | InstanceDef::Intrinsic(..) + | InstanceDef::ClosureOnceShim { .. } + | InstanceDef::DropGlue(..) + | InstanceDef::CloneShim(..) => return Visibility::Hidden, + }; + + // The `start_fn` lang item is actually a monomorphized instance of a + // function in the standard library, used for the `main` function. We don't + // want to export it so we tag it with `Hidden` visibility but this symbol + // is only referenced from the actual `main` symbol which we unfortunately + // don't know anything about during partitioning/collection. As a result we + // forcibly keep this symbol out of the `internalization_candidates` set. + // + // FIXME: eventually we don't want to always force this symbol to have + // hidden visibility, it should indeed be a candidate for + // internalization, but we have to understand that it's referenced + // from the `main` symbol we'll generate later. + // + // This may be fixable with a new `InstanceDef` perhaps? Unsure! + if tcx.lang_items().start_fn() == Some(def_id) { + *can_be_internalized = false; + return Visibility::Hidden; + } + + let is_generic = instance.substs.non_erasable_generics().next().is_some(); + + // Upstream `DefId` instances get different handling than local ones. + let Some(def_id) = def_id.as_local() else { + return if export_generics && is_generic { + // If it is an upstream monomorphization and we export generics, we must make + // it available to downstream crates. + *can_be_internalized = false; + default_visibility(tcx, def_id, true) + } else { + Visibility::Hidden + }; + }; + + if is_generic { + if export_generics { + if tcx.is_unreachable_local_definition(def_id) { + // This instance cannot be used from another crate. + Visibility::Hidden + } else { + // This instance might be useful in a downstream crate. + *can_be_internalized = false; + default_visibility(tcx, def_id.to_def_id(), true) + } + } else { + // We are not exporting generics or the definition is not reachable + // for downstream crates, we can internalize its instantiations. + Visibility::Hidden + } + } else { + // If this isn't a generic function then we mark this a `Default` if + // this is a reachable item, meaning that it's a symbol other crates may + // access when they link to us. + if tcx.is_reachable_non_generic(def_id.to_def_id()) { + *can_be_internalized = false; + debug_assert!(!is_generic); + return default_visibility(tcx, def_id.to_def_id(), false); + } + + // If this isn't reachable then we're gonna tag this with `Hidden` + // visibility. In some situations though we'll want to prevent this + // symbol from being internalized. + // + // There's two categories of items here: + // + // * First is weak lang items. These are basically mechanisms for + // libcore to forward-reference symbols defined later in crates like + // the standard library or `#[panic_handler]` definitions. The + // definition of these weak lang items needs to be referencable by + // libcore, so we're no longer a candidate for internalization. + // Removal of these functions can't be done by LLVM but rather must be + // done by the linker as it's a non-local decision. + // + // * Second is "std internal symbols". Currently this is primarily used + // for allocator symbols. Allocators are a little weird in their + // implementation, but the idea is that the compiler, at the last + // minute, defines an allocator with an injected object file. The + // `alloc` crate references these symbols (`__rust_alloc`) and the + // definition doesn't get hooked up until a linked crate artifact is + // generated. + // + // The symbols synthesized by the compiler (`__rust_alloc`) are thin + // veneers around the actual implementation, some other symbol which + // implements the same ABI. These symbols (things like `__rg_alloc`, + // `__rdl_alloc`, `__rde_alloc`, etc), are all tagged with "std + // internal symbols". + // + // The std-internal symbols here **should not show up in a dll as an + // exported interface**, so they return `false` from + // `is_reachable_non_generic` above and we'll give them `Hidden` + // visibility below. Like the weak lang items, though, we can't let + // LLVM internalize them as this decision is left up to the linker to + // omit them, so prevent them from being internalized. + let attrs = tcx.codegen_fn_attrs(def_id); + if attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) { + *can_be_internalized = false; + } + + Visibility::Hidden + } +} + +fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibility { + if !tcx.sess.target.default_hidden_visibility { + return Visibility::Default; + } + + // Generic functions never have export-level C. + if is_generic { + return Visibility::Hidden; + } + + // Things with export level C don't get instantiated in + // downstream crates. + if !id.is_local() { + return Visibility::Hidden; + } + + // C-export level items remain at `Default`, all other internal + // items become `Hidden`. + match tcx.reachable_non_generics(id.krate).get(&id) { + Some(SymbolExportInfo { level: SymbolExportLevel::C, .. }) => Visibility::Default, + _ => Visibility::Hidden, + } +} diff --git a/compiler/rustc_monomorphize/src/partitioning/merging.rs b/compiler/rustc_monomorphize/src/partitioning/merging.rs new file mode 100644 index 000000000..02bb8dea0 --- /dev/null +++ b/compiler/rustc_monomorphize/src/partitioning/merging.rs @@ -0,0 +1,111 @@ +use std::cmp; + +use rustc_data_structures::fx::FxHashMap; +use rustc_hir::def_id::LOCAL_CRATE; +use rustc_middle::mir::mono::{CodegenUnit, CodegenUnitNameBuilder}; +use rustc_span::symbol::Symbol; + +use super::PartitioningCx; +use crate::partitioning::PreInliningPartitioning; + +pub fn merge_codegen_units<'tcx>( + cx: &PartitioningCx<'_, 'tcx>, + initial_partitioning: &mut PreInliningPartitioning<'tcx>, +) { + assert!(cx.target_cgu_count >= 1); + let codegen_units = &mut initial_partitioning.codegen_units; + + // Note that at this point in time the `codegen_units` here may not be in a + // deterministic order (but we know they're deterministically the same set). + // We want this merging to produce a deterministic ordering of codegen units + // from the input. + // + // Due to basically how we've implemented the merging below (merge the two + // smallest into each other) we're sure to start off with a deterministic + // order (sorted by name). This'll mean that if two cgus have the same size + // the stable sort below will keep everything nice and deterministic. + codegen_units.sort_by(|a, b| a.name().as_str().partial_cmp(b.name().as_str()).unwrap()); + + // This map keeps track of what got merged into what. + let mut cgu_contents: FxHashMap<Symbol, Vec<Symbol>> = + codegen_units.iter().map(|cgu| (cgu.name(), vec![cgu.name()])).collect(); + + // Merge the two smallest codegen units until the target size is reached. + while codegen_units.len() > cx.target_cgu_count { + // Sort small cgus to the back + codegen_units.sort_by_cached_key(|cgu| cmp::Reverse(cgu.size_estimate())); + let mut smallest = codegen_units.pop().unwrap(); + let second_smallest = codegen_units.last_mut().unwrap(); + + // Move the mono-items from `smallest` to `second_smallest` + second_smallest.modify_size_estimate(smallest.size_estimate()); + for (k, v) in smallest.items_mut().drain() { + second_smallest.items_mut().insert(k, v); + } + + // Record that `second_smallest` now contains all the stuff that was in + // `smallest` before. + let mut consumed_cgu_names = cgu_contents.remove(&smallest.name()).unwrap(); + cgu_contents.get_mut(&second_smallest.name()).unwrap().append(&mut consumed_cgu_names); + + debug!( + "CodegenUnit {} merged into CodegenUnit {}", + smallest.name(), + second_smallest.name() + ); + } + + let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx); + + if cx.tcx.sess.opts.incremental.is_some() { + // If we are doing incremental compilation, we want CGU names to + // reflect the path of the source level module they correspond to. + // For CGUs that contain the code of multiple modules because of the + // merging done above, we use a concatenation of the names of + // all contained CGUs. + let new_cgu_names: FxHashMap<Symbol, String> = cgu_contents + .into_iter() + // This `filter` makes sure we only update the name of CGUs that + // were actually modified by merging. + .filter(|(_, cgu_contents)| cgu_contents.len() > 1) + .map(|(current_cgu_name, cgu_contents)| { + let mut cgu_contents: Vec<&str> = cgu_contents.iter().map(|s| s.as_str()).collect(); + + // Sort the names, so things are deterministic and easy to + // predict. + + // We are sorting primitive &strs here so we can use unstable sort + cgu_contents.sort_unstable(); + + (current_cgu_name, cgu_contents.join("--")) + }) + .collect(); + + for cgu in codegen_units.iter_mut() { + if let Some(new_cgu_name) = new_cgu_names.get(&cgu.name()) { + if cx.tcx.sess.opts.unstable_opts.human_readable_cgu_names { + cgu.set_name(Symbol::intern(&new_cgu_name)); + } else { + // If we don't require CGU names to be human-readable, we + // use a fixed length hash of the composite CGU name + // instead. + let new_cgu_name = CodegenUnit::mangle_name(&new_cgu_name); + cgu.set_name(Symbol::intern(&new_cgu_name)); + } + } + } + } else { + // If we are compiling non-incrementally we just generate simple CGU + // names containing an index. + for (index, cgu) in codegen_units.iter_mut().enumerate() { + cgu.set_name(numbered_codegen_unit_name(cgu_name_builder, index)); + } + } +} + +fn numbered_codegen_unit_name( + name_builder: &mut CodegenUnitNameBuilder<'_>, + index: usize, +) -> Symbol { + name_builder.build_cgu_name_no_mangle(LOCAL_CRATE, &["cgu"], Some(index)) +} diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs new file mode 100644 index 000000000..ff2d38693 --- /dev/null +++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs @@ -0,0 +1,515 @@ +//! Partitioning Codegen Units for Incremental Compilation +//! ====================================================== +//! +//! The task of this module is to take the complete set of monomorphizations of +//! a crate and produce a set of codegen units from it, where a codegen unit +//! is a named set of (mono-item, linkage) pairs. That is, this module +//! decides which monomorphization appears in which codegen units with which +//! linkage. The following paragraphs describe some of the background on the +//! partitioning scheme. +//! +//! The most important opportunity for saving on compilation time with +//! incremental compilation is to avoid re-codegenning and re-optimizing code. +//! Since the unit of codegen and optimization for LLVM is "modules" or, how +//! we call them "codegen units", the particulars of how much time can be saved +//! by incremental compilation are tightly linked to how the output program is +//! partitioned into these codegen units prior to passing it to LLVM -- +//! especially because we have to treat codegen units as opaque entities once +//! they are created: There is no way for us to incrementally update an existing +//! LLVM module and so we have to build any such module from scratch if it was +//! affected by some change in the source code. +//! +//! From that point of view it would make sense to maximize the number of +//! codegen units by, for example, putting each function into its own module. +//! That way only those modules would have to be re-compiled that were actually +//! affected by some change, minimizing the number of functions that could have +//! been re-used but just happened to be located in a module that is +//! re-compiled. +//! +//! However, since LLVM optimization does not work across module boundaries, +//! using such a highly granular partitioning would lead to very slow runtime +//! code since it would effectively prohibit inlining and other inter-procedure +//! optimizations. We want to avoid that as much as possible. +//! +//! Thus we end up with a trade-off: The bigger the codegen units, the better +//! LLVM's optimizer can do its work, but also the smaller the compilation time +//! reduction we get from incremental compilation. +//! +//! Ideally, we would create a partitioning such that there are few big codegen +//! units with few interdependencies between them. For now though, we use the +//! following heuristic to determine the partitioning: +//! +//! - There are two codegen units for every source-level module: +//! - One for "stable", that is non-generic, code +//! - One for more "volatile" code, i.e., monomorphized instances of functions +//! defined in that module +//! +//! In order to see why this heuristic makes sense, let's take a look at when a +//! codegen unit can get invalidated: +//! +//! 1. The most straightforward case is when the BODY of a function or global +//! changes. Then any codegen unit containing the code for that item has to be +//! re-compiled. Note that this includes all codegen units where the function +//! has been inlined. +//! +//! 2. The next case is when the SIGNATURE of a function or global changes. In +//! this case, all codegen units containing a REFERENCE to that item have to be +//! re-compiled. This is a superset of case 1. +//! +//! 3. The final and most subtle case is when a REFERENCE to a generic function +//! is added or removed somewhere. Even though the definition of the function +//! might be unchanged, a new REFERENCE might introduce a new monomorphized +//! instance of this function which has to be placed and compiled somewhere. +//! Conversely, when removing a REFERENCE, it might have been the last one with +//! that particular set of generic arguments and thus we have to remove it. +//! +//! From the above we see that just using one codegen unit per source-level +//! module is not such a good idea, since just adding a REFERENCE to some +//! generic item somewhere else would invalidate everything within the module +//! containing the generic item. The heuristic above reduces this detrimental +//! side-effect of references a little by at least not touching the non-generic +//! code of the module. +//! +//! A Note on Inlining +//! ------------------ +//! As briefly mentioned above, in order for LLVM to be able to inline a +//! function call, the body of the function has to be available in the LLVM +//! module where the call is made. This has a few consequences for partitioning: +//! +//! - The partitioning algorithm has to take care of placing functions into all +//! codegen units where they should be available for inlining. It also has to +//! decide on the correct linkage for these functions. +//! +//! - The partitioning algorithm has to know which functions are likely to get +//! inlined, so it can distribute function instantiations accordingly. Since +//! there is no way of knowing for sure which functions LLVM will decide to +//! inline in the end, we apply a heuristic here: Only functions marked with +//! `#[inline]` are considered for inlining by the partitioner. The current +//! implementation will not try to determine if a function is likely to be +//! inlined by looking at the functions definition. +//! +//! Note though that as a side-effect of creating a codegen units per +//! source-level module, functions from the same module will be available for +//! inlining, even when they are not marked `#[inline]`. + +mod default; +mod merging; + +use rustc_data_structures::fx::{FxHashMap, FxHashSet}; +use rustc_data_structures::sync; +use rustc_hir::def_id::DefIdSet; +use rustc_middle::mir; +use rustc_middle::mir::mono::MonoItem; +use rustc_middle::mir::mono::{CodegenUnit, Linkage}; +use rustc_middle::ty::print::with_no_trimmed_paths; +use rustc_middle::ty::query::Providers; +use rustc_middle::ty::TyCtxt; +use rustc_span::symbol::Symbol; + +use crate::collector::InliningMap; +use crate::collector::{self, MonoItemCollectionMode}; + +pub struct PartitioningCx<'a, 'tcx> { + tcx: TyCtxt<'tcx>, + target_cgu_count: usize, + inlining_map: &'a InliningMap<'tcx>, +} + +trait Partitioner<'tcx> { + fn place_root_mono_items( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>, + ) -> PreInliningPartitioning<'tcx>; + + fn merge_codegen_units( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + initial_partitioning: &mut PreInliningPartitioning<'tcx>, + ); + + fn place_inlined_mono_items( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + initial_partitioning: PreInliningPartitioning<'tcx>, + ) -> PostInliningPartitioning<'tcx>; + + fn internalize_symbols( + &mut self, + cx: &PartitioningCx<'_, 'tcx>, + partitioning: &mut PostInliningPartitioning<'tcx>, + ); +} + +fn get_partitioner<'tcx>(tcx: TyCtxt<'tcx>) -> Box<dyn Partitioner<'tcx>> { + let strategy = match &tcx.sess.opts.unstable_opts.cgu_partitioning_strategy { + None => "default", + Some(s) => &s[..], + }; + + match strategy { + "default" => Box::new(default::DefaultPartitioning), + _ => tcx.sess.fatal("unknown partitioning strategy"), + } +} + +pub fn partition<'tcx>( + tcx: TyCtxt<'tcx>, + mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>, + max_cgu_count: usize, + inlining_map: &InliningMap<'tcx>, +) -> Vec<CodegenUnit<'tcx>> { + let _prof_timer = tcx.prof.generic_activity("cgu_partitioning"); + + let mut partitioner = get_partitioner(tcx); + let cx = &PartitioningCx { tcx, target_cgu_count: max_cgu_count, inlining_map }; + // In the first step, we place all regular monomorphizations into their + // respective 'home' codegen unit. Regular monomorphizations are all + // functions and statics defined in the local crate. + let mut initial_partitioning = { + let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_roots"); + partitioner.place_root_mono_items(cx, mono_items) + }; + + initial_partitioning.codegen_units.iter_mut().for_each(|cgu| cgu.estimate_size(tcx)); + + debug_dump(tcx, "INITIAL PARTITIONING:", initial_partitioning.codegen_units.iter()); + + // Merge until we have at most `max_cgu_count` codegen units. + { + let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus"); + partitioner.merge_codegen_units(cx, &mut initial_partitioning); + debug_dump(tcx, "POST MERGING:", initial_partitioning.codegen_units.iter()); + } + + // In the next step, we use the inlining map to determine which additional + // monomorphizations have to go into each codegen unit. These additional + // monomorphizations can be drop-glue, functions from external crates, and + // local functions the definition of which is marked with `#[inline]`. + let mut post_inlining = { + let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_inline_items"); + partitioner.place_inlined_mono_items(cx, initial_partitioning) + }; + + post_inlining.codegen_units.iter_mut().for_each(|cgu| cgu.estimate_size(tcx)); + + debug_dump(tcx, "POST INLINING:", post_inlining.codegen_units.iter()); + + // Next we try to make as many symbols "internal" as possible, so LLVM has + // more freedom to optimize. + if !tcx.sess.link_dead_code() { + let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_internalize_symbols"); + partitioner.internalize_symbols(cx, &mut post_inlining); + } + + let instrument_dead_code = + tcx.sess.instrument_coverage() && !tcx.sess.instrument_coverage_except_unused_functions(); + + if instrument_dead_code { + assert!( + post_inlining.codegen_units.len() > 0, + "There must be at least one CGU that code coverage data can be generated in." + ); + + // Find the smallest CGU that has exported symbols and put the dead + // function stubs in that CGU. We look for exported symbols to increase + // the likelihood the linker won't throw away the dead functions. + // FIXME(#92165): In order to truly resolve this, we need to make sure + // the object file (CGU) containing the dead function stubs is included + // in the final binary. This will probably require forcing these + // function symbols to be included via `-u` or `/include` linker args. + let mut cgus: Vec<_> = post_inlining.codegen_units.iter_mut().collect(); + cgus.sort_by_key(|cgu| cgu.size_estimate()); + + let dead_code_cgu = + if let Some(cgu) = cgus.into_iter().rev().find(|cgu| { + cgu.items().iter().any(|(_, (linkage, _))| *linkage == Linkage::External) + }) { + cgu + } else { + // If there are no CGUs that have externally linked items, + // then we just pick the first CGU as a fallback. + &mut post_inlining.codegen_units[0] + }; + dead_code_cgu.make_code_coverage_dead_code_cgu(); + } + + // Finally, sort by codegen unit name, so that we get deterministic results. + let PostInliningPartitioning { + codegen_units: mut result, + mono_item_placements: _, + internalization_candidates: _, + } = post_inlining; + + result.sort_by(|a, b| a.name().as_str().partial_cmp(b.name().as_str()).unwrap()); + + result +} + +pub struct PreInliningPartitioning<'tcx> { + codegen_units: Vec<CodegenUnit<'tcx>>, + roots: FxHashSet<MonoItem<'tcx>>, + internalization_candidates: FxHashSet<MonoItem<'tcx>>, +} + +/// For symbol internalization, we need to know whether a symbol/mono-item is +/// accessed from outside the codegen unit it is defined in. This type is used +/// to keep track of that. +#[derive(Clone, PartialEq, Eq, Debug)] +enum MonoItemPlacement { + SingleCgu { cgu_name: Symbol }, + MultipleCgus, +} + +struct PostInliningPartitioning<'tcx> { + codegen_units: Vec<CodegenUnit<'tcx>>, + mono_item_placements: FxHashMap<MonoItem<'tcx>, MonoItemPlacement>, + internalization_candidates: FxHashSet<MonoItem<'tcx>>, +} + +fn debug_dump<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, label: &str, cgus: I) +where + I: Iterator<Item = &'a CodegenUnit<'tcx>>, + 'tcx: 'a, +{ + let dump = move || { + use std::fmt::Write; + + let s = &mut String::new(); + let _ = writeln!(s, "{}", label); + for cgu in cgus { + let _ = + writeln!(s, "CodegenUnit {} estimated size {} :", cgu.name(), cgu.size_estimate()); + + for (mono_item, linkage) in cgu.items() { + let symbol_name = mono_item.symbol_name(tcx).name; + let symbol_hash_start = symbol_name.rfind('h'); + let symbol_hash = symbol_hash_start.map_or("<no hash>", |i| &symbol_name[i..]); + + let _ = writeln!( + s, + " - {} [{:?}] [{}] estimated size {}", + mono_item, + linkage, + symbol_hash, + mono_item.size_estimate(tcx) + ); + } + + let _ = writeln!(s, ""); + } + + std::mem::take(s) + }; + + debug!("{}", dump()); +} + +#[inline(never)] // give this a place in the profiler +fn assert_symbols_are_distinct<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, mono_items: I) +where + I: Iterator<Item = &'a MonoItem<'tcx>>, + 'tcx: 'a, +{ + let _prof_timer = tcx.prof.generic_activity("assert_symbols_are_distinct"); + + let mut symbols: Vec<_> = + mono_items.map(|mono_item| (mono_item, mono_item.symbol_name(tcx))).collect(); + + symbols.sort_by_key(|sym| sym.1); + + for &[(mono_item1, ref sym1), (mono_item2, ref sym2)] in symbols.array_windows() { + if sym1 == sym2 { + let span1 = mono_item1.local_span(tcx); + let span2 = mono_item2.local_span(tcx); + + // Deterministically select one of the spans for error reporting + let span = match (span1, span2) { + (Some(span1), Some(span2)) => { + Some(if span1.lo().0 > span2.lo().0 { span1 } else { span2 }) + } + (span1, span2) => span1.or(span2), + }; + + let error_message = format!("symbol `{}` is already defined", sym1); + + if let Some(span) = span { + tcx.sess.span_fatal(span, &error_message) + } else { + tcx.sess.fatal(&error_message) + } + } + } +} + +fn collect_and_partition_mono_items<'tcx>( + tcx: TyCtxt<'tcx>, + (): (), +) -> (&'tcx DefIdSet, &'tcx [CodegenUnit<'tcx>]) { + let collection_mode = match tcx.sess.opts.unstable_opts.print_mono_items { + Some(ref s) => { + let mode_string = s.to_lowercase(); + let mode_string = mode_string.trim(); + if mode_string == "eager" { + MonoItemCollectionMode::Eager + } else { + if mode_string != "lazy" { + let message = format!( + "Unknown codegen-item collection mode '{}'. \ + Falling back to 'lazy' mode.", + mode_string + ); + tcx.sess.warn(&message); + } + + MonoItemCollectionMode::Lazy + } + } + None => { + if tcx.sess.link_dead_code() { + MonoItemCollectionMode::Eager + } else { + MonoItemCollectionMode::Lazy + } + } + }; + + let (items, inlining_map) = collector::collect_crate_mono_items(tcx, collection_mode); + + tcx.sess.abort_if_errors(); + + let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || { + sync::join( + || { + let mut codegen_units = partition( + tcx, + &mut items.iter().cloned(), + tcx.sess.codegen_units(), + &inlining_map, + ); + codegen_units[0].make_primary(); + &*tcx.arena.alloc_from_iter(codegen_units) + }, + || assert_symbols_are_distinct(tcx, items.iter()), + ) + }); + + if tcx.prof.enabled() { + // Record CGU size estimates for self-profiling. + for cgu in codegen_units { + tcx.prof.artifact_size( + "codegen_unit_size_estimate", + cgu.name().as_str(), + cgu.size_estimate() as u64, + ); + } + } + + let mono_items: DefIdSet = items + .iter() + .filter_map(|mono_item| match *mono_item { + MonoItem::Fn(ref instance) => Some(instance.def_id()), + MonoItem::Static(def_id) => Some(def_id), + _ => None, + }) + .collect(); + + if tcx.sess.opts.unstable_opts.print_mono_items.is_some() { + let mut item_to_cgus: FxHashMap<_, Vec<_>> = Default::default(); + + for cgu in codegen_units { + for (&mono_item, &linkage) in cgu.items() { + item_to_cgus.entry(mono_item).or_default().push((cgu.name(), linkage)); + } + } + + let mut item_keys: Vec<_> = items + .iter() + .map(|i| { + let mut output = with_no_trimmed_paths!(i.to_string()); + output.push_str(" @@"); + let mut empty = Vec::new(); + let cgus = item_to_cgus.get_mut(i).unwrap_or(&mut empty); + cgus.sort_by_key(|(name, _)| *name); + cgus.dedup(); + for &(ref cgu_name, (linkage, _)) in cgus.iter() { + output.push(' '); + output.push_str(cgu_name.as_str()); + + let linkage_abbrev = match linkage { + Linkage::External => "External", + Linkage::AvailableExternally => "Available", + Linkage::LinkOnceAny => "OnceAny", + Linkage::LinkOnceODR => "OnceODR", + Linkage::WeakAny => "WeakAny", + Linkage::WeakODR => "WeakODR", + Linkage::Appending => "Appending", + Linkage::Internal => "Internal", + Linkage::Private => "Private", + Linkage::ExternalWeak => "ExternalWeak", + Linkage::Common => "Common", + }; + + output.push('['); + output.push_str(linkage_abbrev); + output.push(']'); + } + output + }) + .collect(); + + item_keys.sort(); + + for item in item_keys { + println!("MONO_ITEM {}", item); + } + } + + (tcx.arena.alloc(mono_items), codegen_units) +} + +fn codegened_and_inlined_items<'tcx>(tcx: TyCtxt<'tcx>, (): ()) -> &'tcx DefIdSet { + let (items, cgus) = tcx.collect_and_partition_mono_items(()); + let mut visited = DefIdSet::default(); + let mut result = items.clone(); + + for cgu in cgus { + for (item, _) in cgu.items() { + if let MonoItem::Fn(ref instance) = item { + let did = instance.def_id(); + if !visited.insert(did) { + continue; + } + let body = tcx.instance_mir(instance.def); + for block in body.basic_blocks() { + for statement in &block.statements { + let mir::StatementKind::Coverage(_) = statement.kind else { continue }; + let scope = statement.source_info.scope; + if let Some(inlined) = scope.inlined_instance(&body.source_scopes) { + result.insert(inlined.def_id()); + } + } + } + } + } + } + + tcx.arena.alloc(result) +} + +pub fn provide(providers: &mut Providers) { + providers.collect_and_partition_mono_items = collect_and_partition_mono_items; + providers.codegened_and_inlined_items = codegened_and_inlined_items; + + providers.is_codegened_item = |tcx, def_id| { + let (all_mono_items, _) = tcx.collect_and_partition_mono_items(()); + all_mono_items.contains(&def_id) + }; + + providers.codegen_unit = |tcx, name| { + let (_, all) = tcx.collect_and_partition_mono_items(()); + all.iter() + .find(|cgu| cgu.name() == name) + .unwrap_or_else(|| panic!("failed to find cgu with name {:?}", name)) + }; +} |