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Re: [Qemu-devel] [PATCH] bitops.h: Add field32() and field64() functions
From: |
Blue Swirl |
Subject: |
Re: [Qemu-devel] [PATCH] bitops.h: Add field32() and field64() functions to extract bitfields |
Date: |
Sun, 8 Jul 2012 11:45:33 +0000 |
On Sun, Jul 8, 2012 at 11:37 AM, Avi Kivity <address@hidden> wrote:
> On 06/28/2012 08:58 AM, Markus Armbruster wrote:
>>>>
>>>> Better code is an argument only if the effect can be demonstrated.
>>>
>>> I don't know even for which compilers or CPUs this is true so it's
>>> unlikely I could demonstrate it. However, googling finds a few
>>> articles in defense of this.
>>
>> Hearsay. Your honor, I rest my case :)
>
> On x86_64, conversion from unsigned to unsigned long takes zero
> instructions, but conversion from int to long takes one instruction. So
> expressions like a[i] are one instruction shorter if the index is unsigned.
>
> unsigned is also slightly safer from a security perspective, since you
> only need to consider overflow, not underflow.
Q.E.D. Expect patches to convert bitops to unsigned shortly.
>
> I used to be an int fan but I have been converted. My fingers still
> prefer int though.
>
> --
> error compiling committee.c: too many arguments to function
>
>