Add floating-point pack/unpack proposal (#6191)

* add initial proposal

* update proposal

* update proposal

* update proposal

* update proposal

* fix typo

* improve wording

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Co-authored-by: Yong He <[email protected]>

docs/proposals/018-packed-data-intrinsics.md

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+SP #018: Data pack/unpack intrinsics
+====================================
+
+Adds intrinsics for converting numeric vector data to and from packed unsigned integer values.
+
+## Status
+
+Status: Design Review.
+
+Implementation: N/A
+
+Author: Darren Wihandi and Slang community.
+
+Reviewer: Yong He.
+
+## Background
+
+Data packing/unpacking intrinsics provide great utility. Slang's core module, which derives from HLSL, already defines integer pack/unpack intrinsics that were introduced in SM 6.6.
+Floating-point variants, however, are undefined. Floating-point pack/unpack intrinsics exist as built-in intrinsics in GLSL, SPIRV, Metal, and WGSL but not in HLSL and Slang, and
+there is no way to access the intrinsics provided by the other shader language targets.
+
+## Proposed Approach
+
+We propose to add new packed-data intrinsics to cover both floating-point and integer types. Although the HLSL integer intrinsics are already implemented, integer variants are also
+added to obtain independence from the HLSL specifications and syntax. For floating-point processing, different variants exist that handle conversion between unorm, snorm, and
+unnormalized/standard IEEE 754 floats and vectors of 16-bit and 32-bit floats.
+
+### Floating-point Unpack Functions
+
+A set of unpack intrinsics are added to decompose a 32-bit integer of packed 8-bit or 16-bit float chunks and reinterpret them as a vector of unorm, snorm, or standard floats and
+halfs. Each 8-bit or 16-bit chunk is converted to either a normalized float through a conversion rule or a standard IEEE-754 floating-point.
+```
+float4 unpackUnorm4x8ToFloat(uint packedVal);
+half4 unpackUnorm4x8ToHalf(uint packedVal);
+
+float4 unpackSnorm4x8ToFloat(uint packedVal);
+half4 unpackSnorm4x8ToHalf(uint packedVal);
+
+float2 unpackUnorm2x16ToFloat(uint packedVal);
+half2 unpackUnorm2x16ToHalf(uint packedVal);
+
+float2 unpackSnorm2x16ToFloat(uint packedVal);
+half2 unpackSnorm2x16ToHalf(uint packedVal);
+
+float2 unpackHalf2x16ToFloat(uint packedVal);
+half2 unpackHalf2x16ToHalf(uint packedVal);
+```
+### Floating-point Pack Functions
+
+A set of pack intrinsics are added to pack a vector of unorm, snorm, or standard floats and halves to a 32-bit integer of packed 8-bit or 16-bit float chunks. Each vector element is converted
+to an 8-bit or 16-bit integer chunk through conversion rules, then packed into one 32-bit integer value.
+
+```
+uint packUnorm4x8(float4 unpackedVal);
+uint packUnorm4x8(half4 unpackedVal);
+
+uint packSnorm4x8(float4 unpackedVal);
+uint packSnorm4x8(half4 unpackedVal);
+
+uint packUnorm2x16(float2 unpackedVal);
+uint packUnorm2x16(half2 unpackedVal);
+
+uint packSnorm2x16(float2 unpackedVal);
+uint packSnorm2x16(half2 unpackedVal);
+
+uint packHalf2x16(float2 unpackedVal);
+uint packHalf2x16(half2 unpackedVal);
+```
+
+### Integer Unpack Functions
+
+A set of unpack intrinsics are added to decompose a 32-bit integer containing four packed 8-bit signed or unsigned integer values and reinterpret them as vectors of 16-bit or 32-bit integers.
+These intrinsics support sign-extension for signed integers and zero-extension for unsigned integers.
+
+```
+uint32_t4 unpackUint4x8ToUint32(uint packedVal);
+uint16_t4 unpackUint4x8ToUint16(uint packedVal);
+
+int32_t4 unpackInt4x8ToInt32(uint packedVal);
+int16_t4 unpackInt4x8ToInt16(uint packedVal);
+```
+
+### Integer Pack Functions
+
+A set of pack intrinsics are added to convert a vector of 16-bit or 32-bit signed or unsigned integers into a 32-bit packed representation, storing only the lower 8 bits of each value.
+The clamped variants ensure each 8-bit value is clamped to [0, 255] for unsigned values and [-128, 127] for signed values.
+
+```
+uint packUint4x8(uint32_t4 unpackedVal);
+uint packUint4x8(uint16_t4 unpackedVal);
+
+uint packInt4x8(int32_t4 unpackedVal);
+uint packInt4x8(int16_t4 unpackedVal);
+
+uint packUint4x8Clamp(int32_t4 unpackedVal);
+uint packUint4x8Clamp(int16_t4 unpackedVal);
+
+uint packInt4x8Clamp(int32_t4 unpackedVal);
+uint packInt4x8Clamp(int16_t4 unpackedVal);
+```
+
+### Implementation Details
+
+#### Normalized float conversion rules
+Normalized float conversion rules are standard across GLSL/SPIRV, Metal, and WGSL. Slang follows these standards. The conversion rules for each target are detailed in:
+- Section 8.4, `Floating-Point Pack and Unpack Functions`, of the GLSL language specification, which is also used by the SPIR-V extended instrucsion for GLSL.
+- Section 7.7.1, `Conversion Rules for Normalized Integer Pixel Data Types`, of the Metal Shading language specification.
+- Sections [16.9  Data Packing Built-in Functions](https://www.w3.org/TR/WGSL/#pack-builtin-functions) and [16.10 Data Unpacking Built-in Functions](https://www.w3.org/TR/WGSL/#unpack-builtin-functions) of the WebGPU Shading language specification.
+
+#### Targets without built-in intrinsics
+For targets without built-in intrinsics, the implementation is done manually using a combination of arithmetic and bitwise operations.
+
+#### Built-in packed Datatypes
+Unlike HLSL's implementation, which introduces new packed datatypes (uint8_t4_packed and int8_t4_packed), unsigned 32-bit integers are used directly, and no new packed datatypes are introduced.