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

[H. Peter Anvin]

On 12/16/2010 02:29 AM, Miles Bader wrote:
> Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx> 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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