Re: libstdc++/8230: Buggy allocator behaviour

[Gabriel Dos Reis <gdr@integrable-solutions.net>]

The following reply was made to PR libstdc++/8230; it has been noted by GNATS.

From: Gabriel Dos Reis <gdr@integrable-solutions.net>
To: Matt Austern <austern@apple.com>
Cc: Benjamin Kosnik <bkoz@redhat.com>, bkoz@gcc.gnu.org, gcc-bugs@gcc.gnu.org,
   jkanze@caicheuvreux.com, gcc-gnats@gcc.gnu.org, libstdc++@gcc.gnu.org,
   rth@redhat.com
Subject: Re: libstdc++/8230: Buggy allocator behaviour
Date: 14 Nov 2002 23:27:17 +0100

 Matt Austern <austern@apple.com> writes:
 
 [...]
 
 | > In general, I would like to see us starting using use 
 | > __builtin_expect()
 | > at places where we do checks for limits cases.  Thoughts?
 | 
 | Two questions.
 | 
 | First, is it known how much of a speedup this gives on typical 
 | platforms?
 
 I didn't make measurements by myself but I'm trusting its implementor
 (RTH) :-)
 
 Maybe RTH could provide us with some datapoint here?
 
 | Second, if the answer to question 1 is "a lot": is there some sensible
 | way to use this in the compiler itself?  I suppose it'd have to involve
 | some ghastly macro hackery because of the bootstrap problem, but
 | it we're talking big performance wins, it might be worth it.
 
 If the answer to the first question is "a lot", then I agree with you
 that it makes perfect sense to use it in the compiler itself.  I
 could imagine something along the lines
 
 #if !defined(__GNUC__) || __GNUC__ < 3  /* correct the version number */
 #  define __builtin_expect(EXP, PROB) EXP
 #endif
 
 in system.h or wherever appropriate.
 
 -- Gaby