From: Albert Cahalan <albert@users.sf.net>
To: linux-kernel mailing list <linux-kernel@vger.kernel.org>
Cc: willy@w.ods.org, davem@redhat.com, jamie@shareable.org,
chip@pobox.com, jamie@shareable.org, albert@users.sf.net
Subject: Re: [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers
Date: 10 Aug 2003 00:03:54 -0400 [thread overview]
Message-ID: <1060488233.780.65.camel@cube> (raw)
Willy Tarreau writes:
>On Sat, Aug 09, 2003 at 01:21:17AM +0100, Jamie Lokier wrote:
>> Albert Cahalan wrote:
>>> // tell gcc what to expect: if(unlikely(err)) die(err);
>>> #define likely(x) __builtin_expect(!!(x),1)
>>> #define unlikely(x) __builtin_expect(!!(x),0)
>>> #define expected(x,y) __builtin_expect((x),(y))
>>
>> You may want to check that GCC generates the same code as for
>>
>> #define likely(x) __builtin_expect((x),1)
>> #define unlikely(x) __builtin_expect((x),0)
>>
>> I tried this once, and I don't recall the precise result
>> but I do recall it generating different code (i.e. not
>> optimal for one of the definitions).
I looked at the assembly (ppc, gcc 3.2.3) and didn't
see any overhead.
Note that the kernel code is broken for the likely()
macro, since 42 is a perfectly good truth value in C.
> anyway, in __builtin_expect(!!(x),0) there is a useless
> conversion, because it's totally equivalent to
> __builtin_expect((x),0) (how could !!x be 0 if x isn't ?),
> but I'm pretty sure that the compiler will add the test.
The !!x gives you a 0 or 1 while converting the type.
So a (char*)0xfda9c80300000000ull will become a 1 of
an acceptable type, allowing the macro to work as desired.
next reply other threads:[~2003-08-10 4:14 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-08-10 4:03 Albert Cahalan [this message]
2003-08-10 7:29 ` [PATCH] 2.4.22pre10: {,un}likely_p() macros for pointers Willy Tarreau
2003-08-10 8:02 ` Willy Tarreau
2003-08-11 1:23 ` Chip Salzenberg
2003-08-11 2:09 ` Jamie Lokier
2003-08-11 2:39 ` Chip Salzenberg
2003-08-11 4:02 ` Albert Cahalan
2003-08-11 4:30 ` Jamie Lokier
2003-08-11 5:30 ` Willy Tarreau
2003-08-11 5:42 ` Jamie Lokier
2003-08-11 13:09 ` Albert Cahalan
2003-08-11 18:55 ` Andrew Morton
2003-08-11 23:13 ` Albert Cahalan
2003-08-13 19:42 ` Jamie Lokier
2003-08-11 4:55 ` Jamie Lokier
2003-08-11 5:26 ` Willy Tarreau
2003-08-11 5:38 ` Jamie Lokier
-- strict thread matches above, loose matches on Subject: below --
2003-08-05 12:44 Albert Cahalan
2003-08-09 0:21 ` Jamie Lokier
2003-08-09 8:13 ` Willy Tarreau
2003-08-09 8:51 ` David S. Miller
2003-08-09 9:36 ` Jamie Lokier
2003-08-09 10:10 ` Herbert Xu
2003-08-09 10:42 ` Alan Cox
2003-08-09 16:23 ` Jamie Lokier
2003-08-04 17:06 Chip Salzenberg
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