Compilation performance regression on 3.4.0

[OlegYch]

Compiler version

3.4.0

Expectation

using sbt 1.9.8 in a project of about 500 files split equally between 4 modules compilation time was 70 seconds on scala 3.3.3
scalacOptions is -feature, -unchecked, -deprecation, -explaintypes, -encoding, UTF8, -language:_, -Yretain-trees, -Xmax-inlines:100, -no-indent

Actual

with scala version 3.4.0 same sources with same libs compile in about 120 seconds

[WojciechMazur]

Tracking compiler performance regression might be hard without getting into the codebase. You can start with checking if it maybe relates to #19907 or #19892 if so we'll try to fix it asap, hopefully in 3.4.1 or 3.4.2

[OlegYch]

those don't look related, it's a regression specifically moving from 3.3.3 to 3.4.0

[OlegYch]

for a single module with ~150 files it regresses from 17 seconds to 28

[OlegYch]

i think i have a lead - one file using reflection heavily via https://github.com/softwaremill/magnolia is compiled in 2 secs on 3.3 and in 3 secs on 3.4
without reflection usage it compiles in 1.3 secs on both 3.3 and 3.4
any idea what might have changed in reflection to cause that?

[Gedochao]

i think i have a lead - one file using reflection heavily via https://github.com/softwaremill/magnolia is compiled in 2 secs on 3.3 and in 3 secs on 3.4
without reflection usage it compiles in 1.3 secs on both 3.3 and 3.4
any idea what might have changed in reflection to cause that?

@jchyb @nicolasstucki any ideas, where we might look?

@OlegYch as was said earlier in the thread, it may be very hard to track this in any way without diving into the codebase.
I figure minimising it on your side might be quite difficult as well, but maybe you could help us bisect it instead? By bisecting I mean trying to compile with compiler nightly versions to try to pinpoint the exact change in the compiler which caused the performance drop.