Move monomorphize code to its own crate.

1 files changed, 466 insertions, 0 deletions

diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs
new file mode 100644
index 00000000000..7a7a56a034e
--- /dev/null
+++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs
@@ -0,0 +1,466 @@
+//! 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::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.debugging_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);
+    }
+
+    // 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_cached_key(|cgu| cgu.name().as_str());
+
+    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.debugging_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()),
+        )
+    });
+
+    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.debugging_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;
+                }
+                for scope in &tcx.instance_mir(instance.def).source_scopes {
+                    if let Some((ref inlined, _)) = scope.inlined {
+                        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))
+    };
+}