On Friday, November 20, 2015 05:36:35 PM Francisco Jerez wrote: > Kenneth Graunke <kenn...@whitecape.org> writes: > > > On Friday, November 20, 2015 02:38:10 PM Francisco Jerez wrote: > >> Kenneth Graunke <kenn...@whitecape.org> writes: > >> > >> > On Thursday, November 19, 2015 02:05:44 PM Kenneth Graunke wrote: > >> >> We've apparently always been botching JIP for sequences such as: > >> >> > >> >> do > >> >> cmp.f0.0 ... > >> >> (+f0.0) break > >> >> ... > >> >> if > >> >> ... > >> >> else > >> >> ... > >> >> endif > >> >> ... > >> >> while > >> >> > >> >> Normally, UIP is supposed to point to the final destination of the jump, > >> >> while in nested control flow, JIP is supposed to point to the end of the > >> >> current nesting level. It essentially bounces out of the current nested > >> >> control flow, to an instruction that has a JIP which bounces out another > >> >> level, and so on. > >> >> > >> >> In the above example, when setting JIP for the BREAK, we call > >> >> brw_find_next_block_end(), which begins a search after the BREAK for the > >> >> next ENDIF, ELSE, WHILE, or HALT. It ignores the IF and finds the ELSE, > >> >> setting JIP there. > >> >> > >> >> This makes no sense at all. The break is supposed to skip over the > >> >> whole if/else/endif block entirely. They have a sibling relationship, > >> >> not a nesting relationship. > >> >> > >> >> This patch fixes brw_find_next_block_end() to track depth as it does > >> >> its search, and ignore anything not at depth 0. So when it sees the > >> >> IF, it ignores everything until after the ENDIF. That way, it finds > >> >> the end of the right block. > >> >> > >> >> Caught while debugging a tessellation shader - no apparent effect on > >> >> Piglit. I did look for actual applications that were affected, and > >> >> found that GLBenchmark Manhattan had a BREAK with a bogus JIP. > >> >> > >> >> Cc: mesa-sta...@lists.freedesktop.org > >> >> Signed-off-by: Kenneth Graunke <kenn...@whitecape.org> > >> > > >> > I tried pretty hard to produce a Piglit test that showed an actual > >> > problem from doing this wrong - and I wasn't able to. > >> > > >> > It seems it just steps through some extra instructions which do > >> > nothing, and is pretty harmless. > >> > > >> From my understanding of how control flow is implemented, jumping to the > >> ENDIF instruction of an inactive IF-ENDIF construct (or similarly to the > >> WHILE instruction of an inactive loop) is fully equivalent to jumping to > >> the same point of an active (i.e. properly nested) but non-diverging > >> IF-ENDIF construct, and will behave the same: It will have no effect on > >> the current per-channel enables (because the IP of the ENDIF instruction > >> won't match the UIP value present at the top of the stack), and for that > >> reason will go on and jump to the instruction pointed to by the JIP > >> value of the ENDIF, which will be another ENDIF/WHILE/HALT instruction > >> closer to the right ENDIF/WHILE instruction that closes the current > >> block. > >> > >> > So I don't think this should actually go to stable after all. > >> > > >> Yeah, seems pretty harmless -- If it weren't harmeless you'd also need > >> to apply a similar fix to WHILE loops, but I don't think you do. > > > > Thanks, Curro! I appreciate you confirming the theory :) > > > > I haven't pushed the patch yet, so would you like to add a Reviewed-by? > > > > What WHILE fix are you thinking of? We may as well get it right, even > > if it is harmless. At a cursory glance, I didn't see anything wrong, > > as it uses the loop stack. But I might've missed something... > > Hmm, does it? AFAICT brw_find_next_block_end() will just give you the > first WHILE instruction it finds even if the corresponding loop starts > after the given offset. Not that it matters much anyway.
Ahh. I thought you meant the code for patching WHILE's JIP was
similarly broken. But you're right, I think as is,
DO
CMP.f0.0 ...
(+f0.0) BREAK
...
DO
...
WHILE
...
WHILE
will make the BREAK point to the first WHILE instead of the second.
The obvious fix would be to handle DO just like I handle IF.
Unfortunately, there is no DO instruction...and at this point, the
loop stack is gone.
I suppose one fix would be to make brw_find_next_block_end() only
accept a WHILE as the end of the block if the WHILE's jump target
is before our starting offset. In other words, if the WHILE's
top-of-the-loop is after our BREAK/CONTINUE, it is obviously the
wrong loop. (This works because WHILE's jumps are set as soon as
we emit the WHILE, so they're already in place by brw_set_uip_jip.)
I think that would work, but it would at least need a comment.
signature.asc
Description: This is a digitally signed message part.
_______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
