On 2/13/19 7:53 AM, Juan A. Suarez Romero wrote:
> On Tue, 2019-02-12 at 16:22 -0800, Ian Romanick wrote:
>> On 2/12/19 12:58 AM, Juan A. Suarez Romero wrote:
>>> opt_split_alu_of_phi moves ALU instruction to the end of continue block.
>>>
>>> But if the continue block ends with a jump instruction (an explicit
>>> "continue" instruction) then the ALU must be inserted before the jump,
>>> as it is illegal to add instructions after the jump.
>>
>> I'm assuming you found this by inspection? Since this pass only
>> operates when the first block of the loop only has two predecessors (the
>> block before the loop and the implicit continue at the end of the loop),
>> this shouldn't be a a problem in practice... or were you able to trigger
>> it somehow?
>
> Found when dealing with the SPIR-V code that I've sent in
> https://lists.freedesktop.org/archives/mesa-dev/2019-February/214906.html
>
>
> The obtained NIR code has an explicit continue at the end of the loop (see
> http://paste.debian.net/1067619/, in particular the loop with header block_2).
That loop is a mess. Wow... I'm impressed. :) I see a continue inside
an else-clause at line 107 and a break at line 112. I now understand
the fundamental problem. Am I correct that this problem would also
occur before 8fb8ebfbb05? And that's what "nir: allow stitching of
non-empty block" fixes?
Did nir_validate trip on this? If not, it seems like it should...
though that might cause problems when nir_validate is run immediately
following translation into NIR.
It seems like we could craft a couple *simple* piglit tests that
exercise this. I don't want to rely on a giant, ugly test case as our
only coverage for this issue. I'm sure debugging this was not fun, and
I don't want someone to have to go through that again should a similar
issue creep back in. I can take a stab at that if you don't already
have something ready.
Either way, I think this change is obviously correct. This patch is
Reviewed-by: Ian Romanick <ian.d.roman...@intel.com>
> J.A.
>
>
>
>>> CC: Ian Romanick <ian.d.roman...@intel.com>
>>> Fixes: 0881e90c099 ("nir: Split ALU instructions in loops that read phis")
>>> ---
>>> src/compiler/nir/nir_opt_if.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
>>> index 9afb901be14..932af9e37ab 100644
>>> --- a/src/compiler/nir/nir_opt_if.c
>>> +++ b/src/compiler/nir/nir_opt_if.c
>>> @@ -488,7 +488,7 @@ opt_split_alu_of_phi(nir_builder *b, nir_loop *loop)
>>> *
>>> * Insert the new instruction at the end of the continue block.
>>> */
>>> - b->cursor = nir_after_block(continue_block);
>>> + b->cursor = nir_after_block_before_jump(continue_block);
>>>
>>> nir_ssa_def *const alu_copy =
>>> clone_alu_and_replace_src_defs(b, alu, continue_srcs);
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