wgsl: support extended integer arithmetic for add, subtract, multiply

[dneto0]

SPIR-V has:

• add-with-carry OpIAddCarry
• sub-with-borrow OpISubBorrow
• unsigned multiply (1 word * 1 word produces 2 words)
• signed multiply (1 word * 1 word produces 2 words)

MSL has

• hadd(x,y) Returns (x + y) >> 1. The intermediate sum does not modulo overflow.
• rhadd(x,y) Returns (x + y + 1) >> 1. The intermediate sum does not modulo overflow.
• mulhi(x,y) Returns the top word of x y. (Presumably you do xy to get the bottom bits)
• madhi(x,y,c) return mulhi(x,y)+c.

I haven't looked at HLSL yet.

---

update: Fixed typo in expansion of madhi.

[alan-baker]

The SPIR-V opcodes OpIAddCarry and OpISubBorrow both interpret the inputs as unsigned integers only. So support is only partially available. For example, 2 + (-1) would yield a carry.

[kdashg]

WGSL meeting minutes 2021-05-11

* AB: should be post-MVP * MM: not clear what HLSL offers for this * TR: mul_high is exposed in DXIL, but not in HLSL * MM: what is our rule for this: will all implementations that rely on HLSL go straight to DXIL, or do we want compatibility with HLSL the language. * DN: DXIL path is at least one year from now for us, so we want support in HLSL itself * GR: also not clear that DXIL-only operations are as widely tested. * MM: so we should only implement things which are supported by HLSL.

[Kangz]

ANGLE polyfills these methods when translating HLSL to GLSL. The translations can be found here and here.

[Kangz]

This was in post-MVP, should it be post-V1 or V1?

[Kangz]

This was in post-MVP, should it be post-V1 or V1?

[litherum]

I think there's a typo in the OP:

madhi(x,y,c) return madhi(x,y)+c.

should be

madhi(x,y,c) return mulhi(x, y) + c.

[kdashg]

WGSL meeting minutes 2022-01-25

* DN: Google happy post-V1 * RM: ok either way * _Post V1_