[Pkg-octave-devel] Unit test bug in octave-signal

[Oliver Heimlich]

On 04.12.2017 18:00, Oliver Heimlich wrote:
> On 13.11.2017 16:26, Sébastien Villemot wrote:
>> On Mon, Nov 13, 2017 at 04:04:38PM +0100, Sébastien Villemot wrote:
>>
>>> This has already been said several times on this list, but it is important to
>>> remember that it is normal to get slightly different numerical results on i386.
>>> The reason is that in i386, numerical computations are done with the x87 FPU,
>>> which internally stores double-floats with extra precision (80-bits), which
>>> means that when several consecutive mathematical operations are done within FPU
>>> registers, they do not conform to the IEEE 754 standard for double floats.
>>> On the contrary, amd64 uses SSE2 registers, which are 64-bit wide, and
>>> therefore conformant to the IEEE standard.
>>
>> And I forgot to add that the easy workaround is to add the -ffloat-store option
>> to gcc. With this option, the result of every floating point operation is
>> copied back into memory (and read again) before moving to the next operation.
>> Therefore, every intermediate result gets truncated to 64-bit double float. As
>> a consequence, one gets exactly the same results as on amd64. But this has an
>> obvious performance cost (many additional memory read/writes, as many as there
>> are intermediate results). So one has to ponder carefully before using this option.
> 
> 
> I could simply reproduce the numeric errors in a VirtualBox VM with
> Debian 32-bit.  However, using “-ffloat-store” didn't fix it.  The only
> way to make it work was to use either “-mfpmath=sse -msse” or
> “-mpfmath=sse -msse2”.
> 
> Also, I noticed that the function crlibm_init, which is called, sets FPU
> flags to use double, not double exteded.  However, it doesn't help
> either.

I found the solution: One has to enable GCC's compiler option
“-frounding-math”.  Then, the crlibm library can set the FPU's
arithmetic as needed.

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