On Tue, Sep 8, 2015 at 10:00 PM, Mathieu Malaterre <malat@xxxxxxxxxx> wrote: > Andrew, > > On Tue, Sep 8, 2015 at 3:22 PM, Andrew Haley <aph@xxxxxxxxxx> wrote: >> On 09/08/2015 01:40 PM, Mathieu Malaterre wrote: >>> On Tue, Sep 8, 2015 at 2:00 PM, Mathieu Malaterre <malat@xxxxxxxxxx> wrote: >>>> FYI, >>>> >>>> On Tue, Sep 8, 2015 at 12:04 PM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> 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 ? Ok, I think I understand now. -O0 did produce code that is compatible with -ffloat-store. However I am still required to use -ffloat-store (explicitly) for any other optimization (at least required with -O2 in my case). Sorry for the noise,
