From: Mark Mitchell <mark@codesourcery.com>
To: trevor_smigiel@playstation.sony.com
Cc: Hans-Peter Nilsson <hp@bitrange.com>, gcc <gcc@gcc.gnu.org>,
Russell_Olsen@playstation.sony.com,
Andrew_Pinski@PlayStation.Sony.Com,
Mark Mendell <mendell@ca.ibm.com>
Subject: Re: __builtin_expect for indirect function calls
Date: Sat, 05 Jan 2008 05:44:00 -0000 [thread overview]
Message-ID: <477F190B.2000807@codesourcery.com> (raw)
In-Reply-To: <20080103233308.GD5853@playstation.sony.com>
trevor_smigiel@playstation.sony.com wrote:
> Currently, the prototype for __builtin_expect is
>
> long __builtin_expect (long expression, long constant);
>
> Extending it to functions would change it to
>
> T __builtin_expect (T expression, T expected);
Yes, it really makes more sense for __builtin_expect to be polymorphic
in this way anyhow. I consider it a design wart that it's defined in
terms of "long". (For example, this means you can't use it for "long
long"!)
> Rather than the above definition, we could choose not to inspect the
> context and just say:
> * T is the type of 'expression'
> * 'expected' is allowed to be a non-constant
>
> In this case I think there would only be compatibility issues with
> unusual uses of __builtin_expect, for example, if it was being used in a
> function argument and its type effected overload resolution.
I think this is the abstractly right approach. However, you're right
that there are backwards-compatibility issues. Another issue is that on
platforms where "long" and "int" do not have the same width, something like:
sizeof(__builtin_expect(1, 1))
will change its value. And, the current prototype is documented in the
manual.
What do people think? Do we have the leeway to change this? Or should
we add __builtin_expect2? Or add an -fno-polymorphic-builtin-expect?
Or...?
Thanks,
--
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713
next prev parent reply other threads:[~2008-01-05 5:44 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
2007-12-26 19:10 ` Mark Mitchell
2008-01-03 23:36 ` trevor_smigiel
2008-01-05 5:44 ` Mark Mitchell [this message]
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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