[Mesa-dev] [PATCH] nir: fix typo in idiv lowering, causing large-udiv-udiv failures

[Ilia Mirkin]

On Tue, Nov 10, 2015 at 7:24 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Tue, Nov 10, 2015 at 7:02 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> On Tue, Nov 10, 2015 at 6:44 PM, Eric Anholt <eric at anholt.net> wrote:
>>> Ilia Mirkin <imirkin at alum.mit.edu> writes:
>>>
>>>> In nv50, and in the python script that Rob circulated, we do:
>>>>
>>>>    bld.mkCmp(OP_SET, CC_GE, TYPE_U32, (s = bld.getSSA()), TYPE_U32, m, b);
>>>>
>>>> Do the same in the nir div lowering pass. This fixes the large-udiv-udiv
>>>> piglit tests on freedreno.
>>>
>>> I assume you meant *-op-div-large-uint-uint.shader_test.
>>
>> Yes.
>>
>>>
>>> vc4 doesn't have uge yet, but I've got a patch to add it and it does
>>> fix one subtest.  What this lowering pass is actually doing has never
>>> really made sense to me, but it works, so:
>>>
>>> Acked-by: Eric Anholt <eric at anholt.net>
>>
>> It's a magical sequence of non-sensical operations which appear to
>> produce the proper result with high probability... what's so confusing
>> about that? :)
>>
>> More seriously, I think there are Newton-Raphson overtones in what
>> it's doing, but I never fully traced it down. It kind of loses me
>> after subtracting 2 from the integer representation of the float bits.
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>
> Where did you get it from? Is there a paper somewhere explaining it? I
> sort of have a morbid curiosity, perhaps because I got sucked into
> implementing division/sqrt/rsq for doubles.

I made the old freedreno impl based on nv50's div lowering code.
There's similarly weird code for r600, but I don't know if the r600 or
nv50 code came first, or where they came from. FWIW the nvc0 div logic
is quite different (implemented as a function call, which does rather
different things... I think). I also don't know where that came from.

Sorry!