[FFmpeg-devel] [PATCH 1/2] lavc/pcm_tablegen: slight speedup of table generation

Ganesh Ajjanagadde gajjanag at mit.edu
Sun Jan 3 22:14:27 CET 2016

On Sun, Jan 3, 2016 at 9:35 AM, Kieran Kunhya <kierank at obe.tv> wrote:
>> It is still "speed critical" enough for people to retain
>> CONFIG_HARDCODED_TABLES. My goal here is simple: I want to get cycle
>> count down enough so that hardcoded tables can be removed here.
> How are you going to guarantee this across all arches?

First of all, I have no idea what "guarantee" you are looking for
here. What I guarantee is this: on every architecture, this change
results in speedup (not to a mathematical level of guarantee, but to a
practical level of gurantee).

What I don't know is at what point one considers hardcoded tables,
associated ifdefry, etc useful or not. That is inherently subjective.
Same goes for code "readability" - also subjective. I also don't know
the relative gains across architectures.

What I do know is that there are clear inconsistencies in opinions
here regarding this (e.g not "speed critical" yet needs a configure
option), and I want to understand the heart of the matter.

The thread I created is a step towards a scientific understanding of
the actual impact of hardcoded tables, something that seems to not
have been done in the past, when it should have been done at the time
of introduction.

Furthermore, since when has FFmpeg "guaranteed" speedups across all
arches? Impacts of even a single line change can vary across arches.
Does that mean that all patches under review get benched on each and
every architecture that we support? Of course not.


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