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Re: [Qemu-devel] [PATCH v3 4/5] qtest: precompute hex nibs

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 4/5] qtest: precompute hex nibs
Date: Thu, 07 May 2015 14:27:28 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 05/06/2015 10:18 AM, John Snow wrote:

>> To find out, add just buffering.  Something like this in your patch
>> instead of byte2hex():
>>          for (i = 0; i < len; i++) {
>> -            qtest_sendf(chr, "%02x", data[i]);
>> +            snprintf(&enc[i * 2], 2, "%02x", data[i]);
>>          }
>> If the speedup is pretty much entirely due to buffering (which I
>> suspect), then your commit message could use a bit of love :)
> When you're right, you're right. The difference may not be statistically
> meaningful, but with today's current planetary alignment, using
> sprintf() to batch the sends instead of my home-rolled nib computation
> function, I can eke out a few more tenths of a second.

I'm a bit surprised - making a function call per byte generally executes
more instructions than open-coding the conversion (albeit the branch
prediction in the hardware probably does fairly well over long strings,
since it is a tight and predictable loop).  Remember, sprintf() has to
decode the format string on every call (unless the compiler is smart
enough to open-code what sprintf would do).

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

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