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

[Connor Abbott]

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.