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

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Message-ID: <buooc8mko3x.fsf@dhlpc061.dev.necel.com>
Date:	Thu, 16 Dec 2010 19:29:06 +0900
From:	Miles Bader <miles@....org>
To:	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.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

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.

-Miles

-- 
Suburbia: where they tear out the trees and then name streets after them.
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