From: Hans-Peter Nilsson <hp@bitrange.com>
To: trevor_smigiel@playstation.sony.com
Cc: gcc <gcc@gcc.gnu.org>, Russell_Olsen@playstation.sony.com
Subject: Re: __builtin_expect for indirect function calls
Date: Sat, 22 Dec 2007 03:42:00 -0000 [thread overview]
Message-ID: <20071221220630.Y67443@dair.pair.com> (raw)
In-Reply-To: <20071218000552.GV3656@playstation.sony.com>
On Mon, 17 Dec 2007, trevor_smigiel@playstation.sony.com wrote:
> When we can't hint the real target, we want to hint the most common
> target. There are potentially clever ways for the compiler to do this
> automatically, but I'm most interested in giving the user some way to do
> it explicitly. One possiblity is to have something similar to
> __builtin_expect, but for functions. For example, I propose:
>
> __builtin_expect_call (FP, PFP)
Is there a hidden benefit? I mean, isn't this really
expressable using builtin_expect as-is, at least when it comes
to the syntax? Like:
>
> which returns the value of FP with the same type as FP, and tells the
> compiler that PFP is the expected target of FP. Trival examples:
>
> typedef void (*fptr_t)(void);
>
> extern void foo(void);
>
> void
> call_fp (fptr_t fp)
> {
> /* Call the function pointed to by fp, but predict it as if it is
> calling foo() */
> __builtin_expect_call (fp, foo)();
__builtin_expect (fp, foo); /* alt __builtin_expect (fp == foo, 1); */
fp ();
> }
>
> void
> call_fp_predicted (fptr_t fp, fptr_t predicted)
> {
> /* same as above but the function we are calling doesn't have to be
> known at compile time */
> __builtin_expect_call (fp, predicted)();
__builtin_expect (fp, predicted);
fp();
I guess the information just isn't readily available in the
preferred form when needed and *that* part could more or less
simply be fixed?
brgds, H-P
next prev parent reply other threads:[~2007-12-22 3:16 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-18 0:52 trevor_smigiel
2007-12-18 2:27 ` Jonathan Adamczewski
2007-12-22 3:42 ` Hans-Peter Nilsson [this message]
2007-12-26 19:10 ` Mark Mitchell
2008-01-03 23:36 ` trevor_smigiel
2008-01-05 5:44 ` Mark Mitchell
2008-01-05 10:40 ` Richard Guenther
2008-01-06 19:44 ` Mark Mitchell
2008-01-07 21:15 ` Mark Mendell
2008-01-08 15:36 ` Dave Korn
2008-01-08 15:51 ` Dave Korn
2008-01-03 23:46 ` trevor_smigiel
2008-01-06 20:42 Ross Ridge
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