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Don't merge CGUs in incremental builds. #112441

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Don't merge CGUs in incremental builds. #112441

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3 changes: 2 additions & 1 deletion compiler/rustc_monomorphize/src/partitioning.rs
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Expand Up @@ -161,10 +161,11 @@ where

debug_dump(tcx, "INITIAL PARTITIONING", &codegen_units);

// njn: update comment
// Merge until we have at most `max_cgu_count` codegen units.
// `merge_codegen_units` is responsible for updating the CGU size
// estimates.
{
if cx.tcx.sess.opts.incremental.is_none() {
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Currently -Ccodegen-units overrides the 256 default for incr comp. With this change that no longer happens. I think user's intent should be preserved even with incr comp. Maybe they care about runtime perf too and many codegen units is a huge regression (eg in a game) or maybe duplication of inlined functions costs more than not merging saves in reusable cgus. Either way the user should have the option to do what is best for them.

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Sure, as written I've done this too crudely. If this were to land various other changes would be needed, e.g. documentation. I have changed the PR to draft status to better reflect this.

let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus");
merge_codegen_units(cx, &mut codegen_units);
debug_dump(tcx, "POST MERGING", &codegen_units);
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