Add countTrailingZeros

[gents83]

Adds countTrailingZeros from gfx-rs/wgpu#4402

Use lsb or ctz.

It uses:

• HLSL: firstbitlow
• GLSL: findLSB
• MSL: ctz
• SPV: spirv::GLOp::FindILsb

PS: It's my first integration in Naga, so please double check it carefully :)

[evahop]

@gents83
Please disregard my review, I did not mean to start one if i did.
The behavior of countTrailingZeros is different than that of findLSB, firstbitlow etc. So the backends need to produce something equivalent. The majority of which produce -1 for zero where countTrailingZeros should return 32. I think you where much closer in your initial attempts.

I've updated a mistake from a previous comment to avoid confusion,
but something along the lines of x == 0 ? 32 : findLSB(x) - 1 seems right to me.

In the case of findLSB it's not supported bellow 400 & es 310 so there's a version check to ensure it's supported.

[evahop]

Actually that's not quite right either, since findLSB index starts at zero. Subtracting 1 puts us back at -1. I think we can just do away with the subtraction.

[gents83]

It should be lsb except when zero, right?

0...0001 = 0
0...0010 = 1
0...0100 = 2
....
1...0000 = 31
0...0000 = 32

Sounds correct?

[teoxoy]

Sounds correct, so GLSL, SPV and HLSL need the following transform:

min(uint(findLSB(n)), 32)

min should be faster than branching here.

[codecov-commenter]

Codecov Report

Merging #2243 (25921d0) into master (60c0fc0) will increase coverage by 0.02%.
The diff coverage is 94.11%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##           master    gfx-rs/naga#2243      +/-   ##
==========================================
+ Coverage   82.02%   82.04%   +0.02%     
==========================================
  Files          80       80              
  Lines       43161    43246      +85     
==========================================
+ Hits        35401    35481      +80     
- Misses       7760     7765       +5     

Impacted Files	Coverage Δ
src/lib.rs	33.88% <ø> (ø)
src/proc/typifier.rs	76.24% <ø> (ø)
src/valid/expression.rs	65.99% <ø> (ø)
src/back/hlsl/writer.rs	91.14% <87.50%> (-0.05%)	⬇️
src/back/glsl/mod.rs	86.20% <95.00%> (+0.07%)	⬆️
src/back/spv/block.rs	91.36% <97.29%> (+0.13%)	⬆️
src/back/msl/writer.rs	90.92% <100.00%> (+<0.01%)	⬆️
src/back/wgsl/writer.rs	91.70% <100.00%> (+<0.01%)	⬆️
src/front/wgsl/parse/conv.rs	98.97% <100.00%> (+<0.01%)	⬆️
src/proc/mod.rs	75.57% <100.00%> (+0.08%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[gents83]

@evahop & @teoxoy it should be fine now
PS: sorry for the multiple commits, for some reasons I wasn't able to have makefile running correctly locally

[teoxoy]

Thanks for the PR, the logic looks good!
Left a few comments regarding a few bitcasts.

[teoxoy]

Also, @evahop, thanks for helping out with this PR!

[gents83]

Tnx for precious suggestions!

[teoxoy]

HLSL backend looks good now.
I noticed that the spv backend is not generating what it should.
I also opened #2258 to fix the countLeadingZeros impl which is not working properly for the same reasons.

[teoxoy]

@gents83 - just a heads up that we landed #2257, you can now use get_constant_composite

[gents83]

@gents83 - just a heads up that we landed #2257, you can now use get_constant_composite

Ok - integrating this one too :)

[teoxoy]

Looks good!