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

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Message-ID: <AANLkTimhdmWGhxW9qnFWYxiVDManH4XG4RqTLJb_Fdbk@mail.gmail.com>
Date:	Wed, 15 Dec 2010 22:30:05 +0100
From:	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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 Wed, Dec 15, 2010 at 10:03 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 12/15/2010 12:56 PM, Andrew Morton wrote:
>> On Wed, 15 Dec 2010 14:07:39 -0600 (CST)
>> Christoph Lameter <cl@...ux.com> wrote:
>>
>>> +#define cpu_has(c, bit)                                                     \
>>> +    (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 :  \
>>>       test_cpu_cap(c, bit))
>>>
>>> +#define this_cpu_has(bit)                                           \
>>> +    (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 :  \
>>> +     this_cpu_test_bit(bit, (unsigned long *)&cpu_info.x86_capability))
>>> +
>>
>> Isn't
>>
>>       a ? 1 : b
>>
>> a complex way of writing
>>
>>       a || b
>>
>
> Not if b is not a bool.
>

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?

>        -hpa
>
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