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

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Date:	Thu, 16 Dec 2010 11:17:25 +0100
From:	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:	Miles Bader <miles@....org>
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

On Thu, Dec 16, 2010 at 7:25 AM, Miles Bader <miles@....org> wrote:
> "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; }
>

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.).

>=>
>
> 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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