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[experiment] Make incremental compilation respect the -Ccodegen-units flag. #67834

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[experiment] Make incremental compilation respect the -Ccodegen-units flag. #67834

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25 changes: 5 additions & 20 deletions src/librustc_mir/monomorphize/partitioning.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -112,14 +112,6 @@ use rustc_span::symbol::Symbol;
use crate::monomorphize::collector::InliningMap;
use crate::monomorphize::collector::{self, MonoItemCollectionMode};

pub enum PartitioningStrategy {
/// Generates one codegen unit per source-level module.
PerModule,

/// Partition the whole crate into a fixed number of codegen units.
FixedUnitCount(usize),
}

// 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"))
Expand All @@ -128,7 +120,7 @@ fn fallback_cgu_name(name_builder: &mut CodegenUnitNameBuilder<'_>) -> Symbol {
pub fn partition<'tcx, I>(
tcx: TyCtxt<'tcx>,
mono_items: I,
strategy: PartitioningStrategy,
max_cgu_count: usize,
inlining_map: &InliningMap<'tcx>,
) -> Vec<CodegenUnit<'tcx>>
where
Expand All @@ -148,11 +140,10 @@ where

debug_dump(tcx, "INITIAL PARTITIONING:", initial_partitioning.codegen_units.iter());

// If the partitioning should produce a fixed count of codegen units, merge
// until that count is reached.
if let PartitioningStrategy::FixedUnitCount(count) = strategy {
// Merge until we have at most `max_cgu_count` codegen units.
{
let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus");
merge_codegen_units(tcx, &mut initial_partitioning, count);
merge_codegen_units(tcx, &mut initial_partitioning, max_cgu_count);
debug_dump(tcx, "POST MERGING:", initial_partitioning.codegen_units.iter());
}

Expand Down Expand Up @@ -875,13 +866,7 @@ fn collect_and_partition_mono_items(
let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {
sync::join(
|| {
let strategy = if tcx.sess.opts.incremental.is_some() {
PartitioningStrategy::PerModule
} else {
PartitioningStrategy::FixedUnitCount(tcx.sess.codegen_units())
};

partition(tcx, items.iter().cloned(), strategy, &inlining_map)
partition(tcx, items.iter().cloned(), tcx.sess.codegen_units(), &inlining_map)
.into_iter()
.map(Arc::new)
.collect::<Vec<_>>()
Expand Down
7 changes: 7 additions & 0 deletions src/librustc_session/session.rs
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Expand Up @@ -741,6 +741,13 @@ impl Session {
return n as usize;
}

// If incremental compilation is turned on, we default to a high number
// codegen units in order to reduce the "collatoral damage" small
// changes cause.
if self.opts.incremental.is_some() {
return 256;
}

// Why is 16 codegen units the default all the time?
//
// The main reason for enabling multiple codegen units by default is to
Expand Down