1 files changed, 111 insertions, 0 deletions
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))
+}