On Tue, Sep 8, 2015 at 12:04 PM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote: > On 8 September 2015 at 10:57, Mathieu Malaterre <malat@xxxxxxxxxx> wrote: >> On Tue, Sep 8, 2015 at 11:45 AM, Andrew Haley <aph@xxxxxxxxxx> wrote: >>> On 09/08/2015 10:15 AM, Mathieu Malaterre wrote: >>>> What is even more surprising is that I can no longer reproduce the >>>> behavior using `valgrind` from my 32bits chroot. >>>> >>>> I understand that my bug description is relatively small, but I am >>>> eager to report a more specific gcc issue. If anyone could help me >>>> narrow down this issue, I'd appreciate your comments. >>>> >>>> Please note that that I disabled any kind of optimizations by using >>>> (explicitly!) -O0. >>> >>> I'm guessing that it's some silliness with the FPU, but that's a wild >>> guess. > > That was my first thought too. To rule it out you could compile with > -mfpmath=sse > > >> Technically this code path is *not* using floating point at all (by >> JPEG 2000 reversible kernel design). integer based shift&additions >> operations only. > > Then I suggest compiling with -fsanitize=undefined to see if there are > any undefined shifts. > > You could try that with both GCC and Clang, they both support UBsan > with slightly different feature sets. > >>> You first need to find out what part of the file is different, and >>> narrow it down from there. One question: how well do you understand >>> the OpenJPEG code base? >> >> Let me answer it this way: this is a huge task -for me- to narrow down >> this issue to a minimal C code. > > 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. Ah ! Thanks much for the info. I was only starring at: https://gcc.gnu.org/wiki/A_guide_to_testcase_reduction I'll give it a shot ASAP. Thx again.
