An unlikely() experiment

Many of you might know of GCC's __builtin_expect() and that it allows us to tell the compiler which code branches are more likely to occur. The documentation on this does warn that normally it's a bad idea to use this "as programmers are notoriously bad at predicting how their programs actually perform" [1].  So I choose to ignore that, and give it a try anyway... elog(ERROR) for example, surely that branch should always be in a cold path? ... ereport() too perhaps?

Anyway, I knocked up a patch which went an added an errcheck() macro which is defined the same as unlikely() is, and I stuck these around just the "if" conditions for tests before elog(ERROR) calls, such as:

gcc version 5.2.1 on i7-4712HQ

pgbench -M prepared -T 60 -S

Master: 12246 tps

Patched 13132 tsp (107%)

pgbench -M prepared -T 60 -S -c 8 -j 8

Master: 122982 tps

Patched: 129318 tps (105%)

I thought this was quite interesting.

Ok, so perhaps it's not that likely that we'd want to litter these errcheck()s around everywhere, so I added a bit more logging as I thought it's probably just a handful of calls that are making this improvement. I changed the macro to call a function to log the __FILE__ and __LINE__, then I loaded the results into Postgres, aggregated by file and line and sorted by count(*) desc. Here's a sample of them (against bbbd807):

Perhaps it would be worth doing something with the hottest ones maybe?

Alternatively, if there was some way to mark the path as cold from within the path, rather than from the if() condition before the path, then we could perhaps do something in the elog() macro instead. I just couldn't figure out a way to do this.

Does anyone have any thoughts on this?

[1] https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

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