[Mesa-stable] [PATCH] i965: Fix JIP to properly skip over unrelated control flow.

[Francisco Jerez]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On Friday, November 20, 2015 02:38:10 PM Francisco Jerez wrote:
>> Kenneth Graunke <kenneth at 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-stable at lists.freedesktop.org
>> >> Signed-off-by: Kenneth Graunke <kenneth at 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.

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