[Python-Dev] Accessing globals without dict lookup

Tim Peters tim.one@comcast.net
Tue, 12 Feb 2002 18:04:44 -0500

[Neil Schemenauer]
> I seem to remember someone saying that GCC generated better code for:
>         if (exceptional) {
>             do exceptional things
>             break / return / goto
>         }
>         do normal things
> Is GCC in the dumb category?

Yes, any compiler that doesn't do branch prediction based on *semantic*
analysis is dirt dumb.  A simple example of semantic prediction is
"comparing a pointer to NULL is probably going to yield false".  Ditto
comparing a number for equality with 0.

I'd like to see a reference for the pattern above; it goes against the very
common "forward branches usually aren't taken" heuristic.  Note that
Vladimir applied that gimmick to an extreme in obmalloc.c's malloc()

> Also, the Linux is starting to use this set of macros more often:
["likely" and "unlikely"]

They're late to the party.  Cray had a "percent true" directive 20 years
ago, allowing for 48 bits of precision in specifying how (un)likely <wink>.

> ...
> I don't have GCC >= 2.96 otherwise I would have tried sprinkling some of
> those macros in ceval and testing the effect.

Maybe more interesting:  One of the folks at Zope Corp reported getting
significant speedups by using some gcc option that can feed real-life branch
histories back into the compiler.  Less work and less error-prone than
guessing annotations.