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

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Message-ID: <buowrnakze2.fsf@dhlpc061.dev.necel.com>
Date:	Thu, 16 Dec 2010 15:25:25 +0900
From:	Miles Bader <miles@....org>
To:	"H. Peter Anvin" <hpa@...or.com>
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

"H. Peter Anvin" <hpa@...or.com> writes:
>> In this case it this_cpu_*_test_bit() return an int, but they act as a
>> bool and are used in if()s; where is the catch?
>
> 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.

It doesn't actually have to "booleanize" the value if it's used in a
boolean context though (and, AFAICT, usually won't).

My vague impression is that when used in a boolean context, gcc will
often generate the same or "equivalent" code for both variants -- but
sometimes a||b seems to generate better code; e.g.:

   static inline int test1a (int a, int b) { return a ? 1 : b; }
   int test1b (int a, int b) { if (test1a (a,b)) return a+b; else return 37; }

   static inline int test2a (int a, int b) { return a || b; }
   int test2b (int a, int b) { if (test2a (a,b)) return a+b; else return 37; }

=>

test1b:
	testl	%edi, %edi
	jne	.L2
	movl	$37, %eax
	testl	%esi, %esi
	jne	.L2
	rep
	ret
.L2:
	leal	(%rsi,%rdi), %eax
	ret

test2b:
	leal	(%rsi,%rdi), %edx
	movl	$37, %eax
	orl	%edi, %esi
	cmovne	%edx, %eax
	ret

	.ident	"GCC: (Debian 4.5.1-8) 4.5.1"


-Miles

-- 
Is it true that nothing can be known?  If so how do we know this?  -Woody Allen
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