[Flac-dev] Fixed: ERROR: mismatch in decoded data, verify FAILED!

[Josh Coalson]

> > After some intense debugging, I found the problem.  One block in
> > the file
> > triggered a very rare bug in the LPC coefficient quantizer caused
> > by
> > insufficient floating point precision.  There is a snippet to
> > compute the
> > log(base 2) of a number:
> > 
> > 	floor(log(cmax) / M_LN2)
> > 
> > It turns out on at least some x86's compiling with gcc-2.91 (at
> > least) the
> > result of floor(log(8.0) / M_LN2) is 2.0 not 3.0.  Anyway, I made a
> > fix and
> > also added code to detect and handle similar problems (related to
> > FP
> > accuracy).
> 
> This appears to be triggered by -ffast-math.
> 
> mizar:[/tmp] cat log.c
> #include <math.h>
> 
> int main() {
>   double x = 8.0;
>   printf("log(x) / M_LN2 = %.40f\n", log(x) / M_LN2);
>   printf("floor(log(x) / M_LN2) = %.40f\n", floor(log(x) / M_LN2));
>   return 0;
> }
> mizar:[/tmp] gcc -o log log.c -lm
> mizar:[/tmp] ./log
> log(x) / M_LN2 = 3.0000000000000000000000000000000000000000
> floor(log(x) / M_LN2) = 3.0000000000000000000000000000000000000000
> mizar:[/tmp] gcc -ffast-math -o log log.c -lm
> mizar:[/tmp] ./log
> log(x) / M_LN2 = 2.9999999999999995559107901499373838305473
> floor(log(x) / M_LN2) = 2.0000000000000000000000000000000000000000
> mizar:[/tmp] 
> 
> This happens for me with both gcc 2.95.4 and 3.0.  According to the
> documentation, this may not even be a bug.  -ffast-math allows gcc to
> violate
> ANSI/IEEE floating point rules as an optimization, which means that
> sometimes
> mathematically incorrect results may be generated.  If flac is
> expecting exact
> results here, maybe it shouldn't be using -ffast-math.

Wow, you're thorough!  I did some more testing with and
without this option.  It appears that, at least for
flac, -ffast-math is actually very slightly slower in
addition to being dangerous, so I'll take it out of the
gcc CFLAGS.

Josh


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