[Ffmpeg-devel] ffmpeg on ppc with gcc 4.0

Derk-Jan Hartman d.hartman
Sun Apr 10 15:28:38 CEST 2005

OK, that is what i wanted to know, thank you Romain.


On 10 apr 2005, at 08:11, Romain Dolbeau wrote:
> Derk-Jan Hartman <d.hartman at student.utwente.nl> wrote:
>> I'm trying to compile ffmpeg under Mac OS X Tiger with gcc 4.0 and
>> i'm running into the following problem that did not occur with gcc
>> 3.0 under Panther.
>> I tried to google a bit, but I'm unclear about what the proper
>> solution is to this issue. Someone with more gcc OSX knowledge than
>> me better fix this I guess.
> Short version: do as it tells you, add the option
> "-force_cpusubtype_ALL" to the gcc-4.0 command-line.
> Long version: dcbz is a (mis)feature of the PowerPC, allowing to put
> zero in a cache block. This is good, because you can zero a block w/o
> having to bother loading the block first. This is very bad, because it
> exposes an implementation detail (the cache line size) to userland
> software. In the past, all PPC sold by Apple had 32 bytes cache lines,
> and a lot of software assumed it would always be true. Unfortunately,
> the PPC970 (G5) has 128 bytes cache lines... So on the 970, dcbz zero 
> 32
> bytes, while dcbzl zero a whole cache line (... unless I got them
> backward). dsputil_ppc.c knows about that, and use the proper
> instruction on all processors. Unfortunately, it seems gcc-4.0 doesn't
> allow dcbzl in non-G5 specific software... Using 
> "-force_cpusubtype_ALL"
> forces gcc-4.0 to accept the code anyway, which is no problem, as it
> won't be used on non-G5 cpu.
> I'll try to find a better solution... if I find the time :-(
> -- 
> Romain Dolbeau
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Universiteit Twente
Derk-Jan Hartman (d.hartman at student.utwente dot nl)

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