linux-kernel - Re: x86: A fast way to check capabilities of the current cpu

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Message-ID: <4D0A3267.40300@zytor.com>
Date:	Thu, 16 Dec 2010 07:38:15 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Miles Bader <miles@....org>
CC:	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	Pekka Enbeerg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: x86: A fast way to check capabilities of the current cpu

On 12/16/2010 02:29 AM, Miles Bader wrote:
> Miguel Ojeda <miguel.ojeda.sandonis@...il.com> writes:
>>>> If they aren't, and are stored in a variable for whatever reason, then
>>>> the || form will generate additional instructions to booleanize the
>>>> value for no good reason.
>>
>> I think hpa was talking about some code where gcc can not optimize out
>> the assignment (e.g. volatile, complex code, using the int outside
>> conditional expressions, etc.).
> 
> Sure, but that seems to assume that the alternatives are otherwise
> equivalent in the common case, when used in a boolean context.
> 
> If that's not true then one risks pessimizing the common case to make an
> uncommon case more efficient.
> 

The alternatives are equivalent when used in the common context.  Your
examples are bogus, because they don't account for the
__builtin_constant_p().

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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