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. > _______________________________________________ > mesa-stable mailing list > mesa-sta...@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-stable
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