Add extension for GLSL ES 3.1 bit functions

[kenrussell]

Per the WebGL working group conference call of 2025-02-06 and #3704 it was agreed to add an extension for the math operations added in GLSL ES 3.1:

• Integer Functions
  • bitfieldExtract and bitfieldInsert
  • bitfieldReverse
  • bitCount
  • findLSB and findMSB
  • uaddCarry and usubBorrow
  • umulExtended and imulExtended
• Floating-Point Pack and Unpack Functions
  • [un]packUnorm4x8
  • [un]packSnorm4x8
• Common Functions
  • mix with genIType/genUType/genBType
  • frexp and ldexp

[csmartdalton]

Could we shoehorn in textureGather functions as well?

• textureGather
• textureGatherOffset

They're loosely related to the other math operations and I see they are already implemented in ANGLE

[lexaknyazev]

@csmartdalton They have a few annoying portability issues and it's hard to call them math-related as they need texture samplers. What specific use case do you have in mind?

[csmartdalton]

@csmartdalton They have a few annoying portability issues and it's hard to call them math-related as they need texture samplers. What specific use case do you have in mind?

We use textureGather for feathering and I would like to see it in WebGL, whether as its own extension or in a group. If it never makes it, that's ok too since we can just fetch 4 texels manually. What are the annoying portability issues?

[TV4Fun]

Is there any update on this? This would be an extremely handy feature to have.

[castano]

Seconded. This would be very useful, in particular bitfieldReverse and bitCount. They are easily emulated, but when available provide significant performance benefits.