src/dawn/native/ImmediateConstantsLayout.h

// Copyright 2025 The Dawn & Tint Authors
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#ifndef SRC_DAWN_NATIVE_IMMEDIATECONSTANTSLAYOUT_H_
#define SRC_DAWN_NATIVE_IMMEDIATECONSTANTSLAYOUT_H_

#include "dawn/common/Compiler.h"
#include "dawn/common/ityp_bitset.h"
#include "dawn/native/EnumClassBitmasks.h"
#include "dawn/native/IntegerTypes.h"

namespace dawn::native {

// Define common immediate data layout. Append members to expand layouts.
// NOTE: 'offsetof' doesn't support non-standard-layout structs. So use
// aggregate instead of inheritance for RenderImmediateConstants and
// ComputeImmediateConstants.
DAWN_ENABLE_STRUCT_PADDING_WARNINGS
struct UserImmediateConstants {
    uint32_t userImmediateData[kMaxExternalImmediateConstantsPerPipeline];
};

// 8 bytes of immediate data data to be used by the ClampFragDepth Tint transform.
struct ClampFragDepthArgs {
    float minClampFragDepth;
    float maxClampFragDepth;
};

// Define render pipeline immediate data layout. Append members to
// expand the layout.
struct RenderImmediateConstants {
    UserImmediateConstants userConstants;

    ClampFragDepthArgs clampFragDepth;

    // first index offset
    uint32_t firstVertex;
    uint32_t firstInstance;
};

struct NumWorkgroupsDimensions {
    uint32_t numWorkgroupsX;
    uint32_t numWorkgroupsY;
    uint32_t numWorkgroupsZ;
};

// Define compute pipeline immediate data layout. Append members to
// expand the layout.
struct ComputeImmediateConstants {
    UserImmediateConstants userConstants;

    NumWorkgroupsDimensions numWorkgroups;
};
DAWN_DISABLE_STRUCT_PADDING_WARNINGS

// Convert byte sizes and offsets into immediate constant indices and offsets
// (dividing everything by kImmediateConstantElementByteSize)
constexpr ImmediateConstantMask GetImmediateConstantBlockBits(size_t byteOffset, size_t byteSize) {
    // This bit math can be done in uint64_t because there are <= 64 bits in the mask.
    static_assert(ImmediateConstantMask{}.size() <= 64);
    uint64_t firstIndex = byteOffset / kImmediateConstantElementByteSize;
    uint64_t constantCount = byteSize / kImmediateConstantElementByteSize;
    return ((1u << constantCount) - 1u) << firstIndex;
}

// Returns the offset of the member in the packed immediates of the pipeline.
// The pointer-to-member is a pointer into the structure containing all the potential immediates.
// However pipelines don't need all of them and use a compacted layout with immediates
// in the same order, just some of them skipped. For example the pipeline mask 11001111,
// representing "userConstants: 4 | trivial_constants: 0 (2 at most)|clamp_frag:2",
// maps to pipeline immediate constant layout: "userConstants:4 | clamp_frag:2
template <typename Object, typename Member>
uint32_t GetImmediateByteOffsetInPipeline(Member Object::*ptr,
                                          const ImmediateConstantMask& pipelineImmediateMask) {
    Object obj = {};
    ptrdiff_t offset = reinterpret_cast<char*>(&(obj.*ptr)) - reinterpret_cast<char*>(&obj);

    const ImmediateConstantMask prefixBits =
        (1u << (offset / kImmediateConstantElementByteSize)) - 1u;

    return (prefixBits & pipelineImmediateMask).count() * kImmediateConstantElementByteSize;
}

template <typename Object, typename Member>
bool HasImmediateConstants(Member Object::*ptr,
                           const ImmediateConstantMask& pipelineImmediateMask) {
    Object obj = {};
    ptrdiff_t offset = reinterpret_cast<char*>(&(obj.*ptr)) - reinterpret_cast<char*>(&obj);
    size_t size = sizeof(Member);

    return pipelineImmediateMask.to_ulong() &
           GetImmediateConstantBlockBits(offset, size).to_ulong();
}

template <typename Object, typename Member>
std::optional<uint32_t> GetImmediateByteOffsetInPipelineIfAny(
    Member Object::* ptr,
    const ImmediateConstantMask& pipelineImmediateMask) {
    if (!HasImmediateConstants(ptr, pipelineImmediateMask)) {
        return std::nullopt;
    }

    return GetImmediateByteOffsetInPipeline(ptr, pipelineImmediateMask);
}

uint32_t GetImmediateIndexInPipeline(const uint32_t layoutOffset,
                                     const ImmediateConstantMask& pipelineImmediateMask);

}  // namespace dawn::native

#endif  // SRC_DAWN_NATIVE_IMMEDIATECONSTANTSLAYOUT_H_