[pulseaudio-discuss] [PATCH] tests: add tolerant variation for comparing the rewind result

[David Henningsson]

On 2015-05-25 14:57, Arun Raghavan wrote:
> On 25 May 2015 at 12:49, hwang4 <hui.wang at canonical.com> wrote:
>>
>>
>> On 2015年05月25日 14:40, David Henningsson wrote:
>>>
>>>
>>>
>>> On 2015-05-25 06:49, Hui Wang wrote:
>>>>
>>>> On 32bits OS, this test case fails. The reason is when rewinding to
>>>> the middle of a block, some of float parameters in the saved_state
>>>> are stored in the memory from FPU registers, and those parameters will
>>>> be used for next time to process data with lfe. Here if FPU register
>>>> is over 32bits, the storing from FPU register to memory will introduce
>>>> some variation, and this small variation will introduce small
>>>> variation to the rewinding result.
>>>
>>>
>>> Very interesting finding. I didn't know that storing things back and forth
>>> to memory could change the computation result.
>>>
>>> And the fact that it only happens on 32-bit platforms and only with
>>> optimisations makes it even stranger. Makes me wonder if this is actually an
>>> gcc optimisation bug.
>>>
>> Probably.
>
> Did I misunderstand something here? Fixing a problem with the compiler
> in our test seems to be incorrect.

My assumption is that as long as we have -ffast-math on, then these are 
things we have to live with (and thus calling it a "gcc optimisation 
bug" is inaccurate, sorry about that).

Being pragmatic: since the difference is one bit only, no one will hear 
any difference. But if you feel like digging into it further, I'm 
curious to know the result. :-)

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic