[Python-Dev] Accessing globals without dict lookup (original) (raw)

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() function.

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 .

... 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.