Douglas Wilson
Hi ghc-devs,
I've been working on a new mode of adding cost-centres to programs and I'd like to ask some questions and solicit some feedback. The code is here [fn:1], it works, provided one enables -fprof-core on all modules.
I've recently been trying to pick some low hanging fruit from ghc compilation performance. A common frustration was in the difference between profiled and non-profiled builds. Often I thought had I found a problem in the profiled build, only to find it was optimized away in the non-profiled build. Several times an issue was tail-calls not happening in profiled builds.
To solve this problem I've been working on a new way of inserting cost-centres: adding them to core after simplification (currently at the end of corePrepPgm) rather than adding them to HsSyn before simplification. This makes it harder to map cost-centres into source code (You have to -ddump-prep currently), but in exchange you are profiling the same core program as the non-profilied build.
Sounds quite reasonable to me.
I intend to investigate whether I can use SourceNotes to create SrcSpans for the generated cost-centres to somewhat alleviate the need to inspect dumped core.
Yes, I think this would be a great idea. I have also wondered in the past whether it would be worthwhile (optionally?) adding a CSS-like name SourceNote (e.g. in addition to a SrcSpan, include the name of the containing binding; for instance "insert.go"). This would make it a bit easier to understand a backtrace at-a-glance.
Do you have any examples of programs for which existing profiling tools are inadequate due to how cost-centres affect simplification? There is an example in #12893 but something self-contained would be great!
I'm afraid I don't know of any off-hand. Cheers, - Ben