[FFmpeg-devel] [PATCH] all: silence clang -Wabsolute-value for unsigned subtractions

[Michael Niedermayer]

On Sat, Aug 22, 2015 at 04:26:53PM +0200, Nicolas George wrote:
> Le quintidi 5 fructidor, an CCXXIII, Ganesh Ajjanagadde a écrit :
> > +    /* cast for -Wabsolute-value in clang 3.5+ */
> > +    return abs((int) (t1 - t2)) + abs((int) ((c1 & 0x000000ff) - (c2 & 0x000000ff))) +
> > +        abs((int) (((c1 & 0x0000ff00) >> 8) - ((c2 & 0x0000ff00) >> 8))) +
> > +        abs((int) (((c1 & 0x00ff0000) >> 16) - ((c2 & 0x00ff0000) >> 16)));
> 
> I believe this is not correct, with or without the cast (the cast is
> implicit due to the prototype of abs(), of course): if the result is morally
> negative, converting the morally-negative unsigned to signed is an undefined
> behaviour.

i dont know what "morally negative" means exactly but convertion
of out of range values to signed integers is not undefined behavior
in C. It is implementation defined or raises a implementation defined
signal
Please see 6.3.1.3 Signed and unsigned integers in ISO/IEC 9899:TC3

I doubt that FFmpeg will work on platforms that
define this significantly differently than how it is defined for the
currently supported platforms,

[...]

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