I routinely use a virtual gentoo ~amd64 machine in VirtualBox for debugging software problems and yesterday I found one after switching from gcc-4.9.3 to gcc-5.3.0 (in the gentoo guest).
After emerging mesa-11.1.2-r1 I noticed that a game wouldn't start, and after several hours of debugging I tracked it down to gcc5. To demonstrate the bug, in an xterm window type "glxinfo": # glxinfo name of display: :0.0 libGL error: unable to load driver: swrast_dri.so libGL error: failed to load driver: swrast X Error of failed request: GLXBadContext Major opcode of failed request: 153 (GLX) Minor opcode of failed request: 6 (X_GLXIsDirect) Serial number of failed request: 48 Current serial number in output stream: 47 I could 'fix' the bug by replacing just that one file (swrast_dri.so) with a copy compiled with gcc-4.9.3. I'm surprised it worked, but it did. I tried building mesa using gcc5 on a real ~amd64 machine and did *not* see the same problem, FWIW, but though I'd mention it here. BTW I tried building the vbox guest additions with gcc5 as a test but that didn't fix the problem. I don't have a real machine that's built entirely with gcc5 so I can't test that idea yet, and that won't be any time soon. I'm sticking to gcc4 for the foreseeable future while you guys do more testing :)
