Bug#756777: Dirac FTBFS on armhf and blocks migration of arm64 fix.

[Sebastian Ramacher]

On 2014-10-13 03:29:19, peter green wrote:
> Found 756777 1.0.2-6
> Thanks
> >| Checking encode and decode of colourbars
> >|
> >|   2: colourbars                                      FAILED (colourbars.at:7)
> >|
> I've just reproduced this locally with both sid's version in sid and
> jessie's version in jessie. So this doesn't appear to be related to the
> change in the sid package but related to some change in the build
> environment. If I had to guess i'd say a computer

I think it's a toolchain bug. I did debug the issue some time after
filing the bug report, but forgot to report my findings to the bug. The
same version built fine in a wheezy chroot on harris, but fails in a
unstable chroot.

Here is the backtrace I got at the time:

#0  dirac::PictureCompressor::SelectQuantisers (this=this at entry=0x207ccc4, coeff_data=..., bands=..., lambda=lambda at entry=6.25, est_bits=..., 
    cb_mode=QUANT_SINGLE, pp=..., csort=csort at entry=dirac::Y_COMP) at picture_compress.cpp:632
#1  0x00036d94 in dirac::PictureCompressor::CodeResidue (this=this at entry=0x207ccc4, my_buffer=..., pnum=<optimized out>, 
    p_picture_byteio=p_picture_byteio at entry=0x20e4ae8) at picture_compress.cpp:449
#2  0x000232ee in dirac::SequenceCompressor::CompressNextPicture (this=0x207cba8) at seq_compress.cpp:447
#3  0x00018e72 in DiracEncoder::CompressNextPicture (this=this at entry=0x207c400) at dirac_encoder.cpp:607
#4  0x0001b122 in CompressNextPicture (this=0x207c400) at dirac_encoder.cpp:1141
#5  dirac_encoder_output (encoder=0x207c240) at dirac_encoder.cpp:1106
#6  0x00014b20 in main (argc=<optimized out>, argv=<optimized out>) at encmain.cpp:1234

The offending line is "est_bits[b] = SelectMultiQuants( coeff_data ,
bands , b, lambda, ...". Printing b (or another index somewhere up the
call trace - I don't remember exactly) immediately before this line,
makes the crash go away. This makes me think gcc fails at optimizing the
code and generates broken output.

I didn't debug it any further.

Cheers
-- 
Sebastian Ramacher
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