[Speex-dev] [patch] Mac Universal Binaries

Jean-Marc Valin jean-marc.valin at usherbrooke.ca
Thu May 3 20:36:21 PDT 2007

> One issue I found was the cpu behaviour when converting from 
> float/double to int32 when the source float is outside the range
>  of values that can be represented by the int32.
> For instance:
>       float           int32_ppc      in32_x86
>       2147483649.0    2147483647   -2147483648
>       2147483648.0    2147483647   -2147483648
>       2147483647.0    2147483647    2147483647  (0x7fffffff)
>         ....
>      -2147483648.0   -2147483648   -2147483648
>      -2147483649.0   -2147483648   -2147483648  (0x80000000)
>      -2147483650.0   -2147483648   -2147483648
> As you can see out of range floats are correctly clipped on PPC,
> but only correctly clipped for negative floats on x86.

Well, AFAIK C89 says the behaviour in undefined, so I wouldn't rely on
that too much. Even on PPC, it's quite possible someday the gcc folks
will be able to 1) determine at compile time that a conversion somewhere
will overflow and 2) optimise it out (since it's undefined anyway) or do
other funny things with it. That's why I wouldn't depend on the right
thing happening, even on PPC.

> libsndfile has code a bit like this:
>     if (CPU_CLIPS_POSITIVE == 0 && scaled_value >= 1.0 * 0x7FFFFFFF)
>         int_value = 0x7fffffff ;
>     if (CPU_CLIPS_NEGATIVE == 0 && scaled_value <= (-8.0 * 0x10000000))
>         int_value = 0x80000000 ;
> On PPC, the above two lines get optimised out. On Intel x86, only 
> the second gets optimised out.

Sounds a bit dangerous to be considering what I wrote above. I tend to
consider the following: "If your code depends on non-specified
behaviour, sooner or later a gcc developer will come up with a clever
way of breaking it".

>> Do they affect all code or just highly-optimized code?
> The float -> int issue affects all code where the source float
> is greater than 0x7fffffff. Yes it is a rather obscure corner
> case.

Well, how obscure it is really depends on the range of your samples. I'd
tend to say that if you expect something that fits in 16-bits than you
don't really care what happens if the input it 2^16 times too big,
because it's catastrophic anyway.

> AFAIAC, all code should have a test suite. For universal binary
> builds (and any cross compile builds), the test suite must be run 
> on both platforms.

I'm afraid I haven't been Doing It Your Way so far :-( But I'm quite
willing to integrate contributed test cases :-)


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