[Lang] ti.sqr(x) is now deprecated: please use x ** 2 instead

[yuanming-hu]

Related issue = #833

People tend to think sqr means square root instead of square (https://forum.taichi.graphics/t/what-the-result-of-ti-sqr-x-x-is-a-vector-if-any-element-of-x-is-negative/125/3).

Fortunately, we have ** 2 now thanks to @archibate...

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[archibate]

sgtm, thx, also consider:
[x] remove ti.sqr from doc.
[ ] import deprecated, deprecate ti.sqr.
[ ] remove ti.sqr from lang/ops.py.

[yuanming-hu]

Thanks. It's actually already deprecated:
https://github.com/taichi-dev/taichi/pull/937/files#diff-d13b207c22d2668af6db2c8d635648e9R81-R83

Given there are quite a few other repos using ti.sqr, I think we should keep it there for a while and then remove it.

[archibate]

Thank for the hard work! I believe you've grep -wr 'ti.sqr' examples'ed, if so, LGTM now.

[archibate]

Feel free to merge this now.
Btw, what does ti.inv and mean? Can we remove that too?
Also found that UnaryOpType::rcp and UnaryOpType::rsqrt are not used at all, can we remove them? (I guess rcp is the same meaning as inv: 1/x..)

[yuanming-hu]

Thanks. inv is for inverse. Yes, maybe we should remove it since it's confusing. We should consider exposing rcp and rsqrt. Let's move the discussions to #833

[k-ye]

I have no idea why, but this PR made sdf_renderer.py on Metal ~8x times slower... In literally the previous commit 8277acf, I still got 24 samples/s, now it's around 3.5 samples/s... @yuanming-hu @archibate @xumingkuan could you help test on the CUDA/OpenGL backends as well?

[archibate]

8x slower...

me too. I tweaked norm_sqr and found:
CUDA: 4.5sps vs 1.2sps, ~4x slower.
OpenGL: 6.8sps vs error: 0(68) : error C1503: undefined variable "nan".

Seems it's boardcast_if_scalar harms the perf? Could you also checkout the constant fold #839 and see if that helps?

[archibate]

I realized why! Because boardcast_if_scalar cast the 2 into 2.0, which disables unrolling (i.e. use pow_f32 instead of pow_i32)
I think self**2 -> self * self will solve this.
Also refactor boardcast_if_scalar can solve this.

[k-ye]

I realized why! Because boardcast_if_scalar cast the 2 into 2.0, which disables unrolling (i.e. use pow_f32 instead of pow_i32)

Ah i see, thx! Yep I changed to self * self and it's fast again!

Could you also checkout the constant fold #839 and see if that helps?

Unfortunately no, but i guess it's not supposed to help in this case...