On Sat, Jan 17, 2015 at 5:17 AM, Kenneth Graunke <kenn...@whitecape.org> wrote: > On Tuesday, January 13, 2015 03:40:32 PM Matt Turner wrote: >> On Tue, Jan 13, 2015 at 3:35 PM, Matt Turner <matts...@gmail.com> wrote: >> > Otherwise, we would have necessarily read the results or eliminated the >> > dead SEND. In either case, no work around is necessary. >> > >> > Noticed when debugging the problem the previous patch fixed that any >> > time we hit a math instruction, we'd walk through subsequent >> > instructions, and of course each time discover that its result was in >> > fact used. >> > --- >> >> I was thinking through the pre-send dependency work around: >> >> /** >> * Implements this workaround for the original 965: >> * >> * "[DevBW, DevCL] Implementation Restrictions: As the hardware does not >> * check for post destination dependencies on this instruction, software >> * must ensure that there is no destination hazard for the case of >> ‘write >> * followed by a posted write’ shown in the following example. >> * >> * 1. mov r3 0 >> * 2. send r3.xy <rest of send instruction> >> * 3. mov r2 r3 >> * >> * Due to no post-destination dependency check on the ‘send’, the above >> * code sequence could have two instructions (1 and 2) in flight at the >> * same time that both consider ‘r3’ as the target of their final >> writes. >> */ >> >> While this is a hardware problem or something, isn't it impossible for >> us to hit? If the first MOV's results weren't read, we would have dead >> code eliminated it. If they were read (necessarily between it and the >> SEND), we would never have both instructions in flight at once. > > It's definitely pretty rare, though I'm not certain I can say it never > happens. If you care to look into it further, I found the bug report > which spawned this code: > > https://bugs.freedesktop.org/show_bug.cgi?id=58960 > > The attachment contains a sample application which I managed to compile via: > $ for file in *.h; do moc-qt4 $file > moc-$(basename $file .h).cpp; done > $ g++ -Wall -g $(pkg-config --libs --cflags QtCore QtGui QtOpenGL gl) *.cpp > > It would be great if we could make a Piglit test.
No regressions from this patch as well, and no changes to the generated code in that program. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
