bpo-37774: use Py_LIKELY/Py_UNLIKELY for vectorcall

[jdemeyer]

CC @markshannon @methane

https://bugs.python.org/issue37774

[methane]

I think __GNUC__ > 2 is redundant.

[jdemeyer]

Redundant because Python won't compile with GCC 2.x anyway?

[methane]

Because:

• I don't know GCC 2.x supports all of C99 features we use in PEP 7.
• I don't know how to use GCC 2 to compile Python in recent days.
• I don't know who try to compile Python with GCC 2.

[methane]

I found this page: https://gcc.gnu.org/c99status.html
We use some GCC>=3.0 features already.

• designated initializers
• new block scopes for selection and iteration statements
• stdbool.h (GCC 2.95 had <stdbool.h>, but based on an early draft with major differences from C99 semantics.)

[pablogsal]

I am not super fan of exposing these macros, by I agree with @methane in that they are battle tested. Do we have some benchmarks numbers for the effect of this (with PGO, as PGO also activates branch-prediction annotations)?

[methane]

I don't think these macros are useful for PGO build. In case of obmalloc.c, it only affects non-PGO build.

[jdemeyer]

Rebased to latest master, fixed conflicts in obmalloc.c

@markshannon: what's your opinion on this? It achieves the same speedup as #14735 but in a more structural way.

[matrixise]

@markshannon do you have time for a review of this PR? Thank you

[csabella]

This has three approvals - should it be merged once the conflicts are resolved?

[markshannon]

As @methane has said, given PGO does this for us, what's the point?

Having said that. I'd be OK with https://github.com/python/cpython/pull/15144/files#diff-7f17c8d8448b7b6f90549035d2147a9fR4978 being added, as performance is deliberately being sacrificed when sys.settrace is used.

As for the others, probably not.

[markshannon]

I see no benefit to this.
I'd rather leave this sort of tweaking to tools as they can adapt as the surrounding code changes.

[markshannon]

@jdemeyer do you mind if I close this?

[markshannon]

This is rather out of date now, so I'm closing it.

[kumaraditya303]

Closing as there are conflicts and the benefit is unclear.