Re: [Qemu-devel] [PATCH v2] utils: Add pow2ceil()

[Eric Blake <eblake@redhat.com>]

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On 02/23/2015 06:59 AM, Markus Armbruster wrote:
> Alexey Kardashevskiy <aik@ozlabs.ru> writes:
> 
>> This adds a helper to get closest bigger power-of-two value.
>>

> 
> Here's how I'd do these functions:
> 
> int64_t pow2floor(int64_t value)
> {
>     assert(value > 0);
>     return 0x8000000000000000u >> clz64(value);
> }

Needs to be 0x8000000000000000ull for 32-bit machines to compile correctly.

Why is the parameter int64_t?  Wouldn't it be more useful to have:

uint64_t pow2floor(uint64_t value)

> 
> int64_t pow2ceil(int64_t value)
> {

Again, why allow signed inputs?

>     assert(value <= 0x4000000000000000)
>     if (value <= 1)
> 	return 1;

In particular, this slams all negative values to a result of 1, which
doesn't necessarily make sense.

>     return 0x8000000000000000u >> (clz64(value - 1) - 1);
> }
> 
> 
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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