Move monomorphize code to its own crate.

3 files changed, 1134 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 00000000000..429ed53d379
--- /dev/null
+++ b/compiler/rustc_monomorphize/src/partitioning/default.rs
@@ -0,0 +1,557 @@
+use std::collections::hash_map::Entry;
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
+use rustc_hir::definitions::DefPathDataName;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
+use rustc_middle::middle::exported_symbols::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, 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;
+                }
+                // 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.index == CRATE_DEF_INDEX {
+            if cgu_def_id.is_none() {
+                // If we have not found a module yet, take the crate root.
+                cgu_def_id = Some(DefId { krate: def_id.krate, index: CRATE_DEF_INDEX });
+            }
+            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).unwrap();
+    }
+
+    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: 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: 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 def_id = if let Some(def_id) = def_id.as_local() {
+        def_id
+    } 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 referenceable 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(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 00000000000..229468b47ff
--- /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, SymbolStr};
+
+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_cached_key(|cgu| cgu.name().as_str());
+
+    // This map keeps track of what got merged into what.
+    let mut cgu_contents: FxHashMap<Symbol, Vec<SymbolStr>> =
+        codegen_units.iter().map(|cgu| (cgu.name(), vec![cgu.name().as_str()])).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[..]).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.debugging_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 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))
+    };
+}