-ffloat-store behavior (Re: Susprising behavior of gcc on x86 (-m32))

[Mathieu Malaterre <malat@debian.org>]

Andrew,

On Tue, Sep 8, 2015 at 3:22 PM, Andrew Haley <aph@redhat.com> wrote:
> On 09/08/2015 01:40 PM, Mathieu Malaterre wrote:
>> On Tue, Sep 8, 2015 at 2:00 PM, Mathieu Malaterre <malat@debian.org> wrote:
>>> FYI,
>>>
>>> On Tue, Sep 8, 2015 at 12:04 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>> [...]
>>>> That's not the only option. You could compile one file with GCC and
>>>> all others with Clang and see if you can reproduce it. Repeat for each
>>>> file, which will narrow down the file where the problem occurs. Then
>>>> you can try splitting that file into smaller pieces, with one function
>>>> per file, and repeat the process. That would tell you which function
>>>> or functions get miscompiled by GCC.
>>>
>>> Ok so if I compile eveything with gcc and then only `tcd.c` using
>>> clang, then everything works as expected (no symptoms).
>>> ref: https://github.com/uclouvain/openjpeg/blob/master/src/lib/openjp2/tcd.c
>>>
>>> I'll repeat your approach to find the culprit function.
>>
>> And the culprit function is `opj_tcd_makelayer`:
>>
>> https://github.com/uclouvain/openjpeg/blob/master/src/lib/openjp2/tcd.c#L218
>>
>> Other than the `if (dd / dr >= thresh)` I do not see anything
>> obviously suspicious.
>
> I see floating point, despite your earlier denial.  :-)

lol. Sorry about that :(

> Libopenjpeg has a bad reputation for messing with the floating-
> point state.  Please make sure the library is not linked with
> -ffast-math.
>
> Beyond that, a few printf()s and "diff" should find the problem.

So here what seems to be working for me, replace:

if (dd / dr >= thresh)

with:

double div; /* OPJ_FLOAT64 */
div = dd / dr;
if (div >= thresh)

However reading the documentation of gcc:

https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Optimize-Options.html#index-ffloat-store-1074

It appears that -ffloat-store is not activated by default (I did check
that using also the output of `gcc -Q -v`).

So could someone please let me know why `gcc -m32` (no other option!)
produce different behavior (=removes excess precision if my
understanding is correct) in the two above cases ?

Thanks much again for help,