[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH v2] utils: Add pow2ceil()
|
From: |
Eric Blake |
|
Subject: |
Re: [Qemu-devel] [PATCH v2] utils: Add pow2ceil() |
|
Date: |
Thu, 12 Mar 2015 10:45:58 -0600 |
|
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 |
On 03/12/2015 09:29 AM, Richard Henderson wrote:
> On 02/25/2015 02:45 AM, Markus Armbruster wrote:
>> return 0x8000000000000000u >> (clz64(value - 1) - 1);
>
> I realize this was weeks ago, but it would certainly be preferable to shift a
> small constant left than a large constant right.
>
> Most RISC machines can't form 0x8000000000000000ull without loading 1 and then
> left shifting to start with. So end the end you're better off with
>
> return 1ull << (63 - clz64(value));
Since the value being shifted is a constant either way, can't gcc figure
out the equivalence and generate the optimal code to begin with? If
not, should it be opened as a gcc bug for potential optimization?
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature