src/tint/lang/wgsl/ast/transform/offset_first_index.h

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#ifndef SRC_TINT_LANG_WGSL_AST_TRANSFORM_OFFSET_FIRST_INDEX_H_
#define SRC_TINT_LANG_WGSL_AST_TRANSFORM_OFFSET_FIRST_INDEX_H_

#include "src/tint/lang/wgsl/ast/transform/transform.h"

namespace tint::ast::transform {

/// Adds firstVertex/Instance (injected via push constants) to
/// vertex/instance index builtins.
///
/// This transform assumes that Name transform has been run before.
///
/// Some shading languages start vertex and instance numbering at 0,
/// regardless of the firstVertex/firstInstance value specified. This transform
/// adds the value of firstVertex/firstInstance to each builtin. This action is
/// performed by adding a new push constant equal to original builtin +
/// firstVertex/firstInstance to each function that references one of
/// these builtins.
///
/// For D3D, this affects both firstVertex and firstInstance. For OpenGL,
/// it applies to only firstInstance. For this reason, the first_vertex_offset
/// and first_instance_offset are optional, where std::nullopt indicates that
/// no subsitution is to be performed.
///
/// Before:
/// ```
///   @builtin(vertex_index) var<in> vert_idx : u32;
///   @builtin(instance_index) var<in> inst_idx : u32;
///   fn func() -> u32 {
///     return vert_idx * inst_idx;
///   }
/// ```
///
/// After:
/// ```
/// struct PushConstants {
///   first_vertex : u32,
///   first_instance : u32,
/// }
///
/// var<push_constant> push_constants : PushConstants;
///
///   @builtin(vertex_index) var<in> vert_idx : u32;
///   @builtin(instance_index) var<in> inst_idx : u32;
///   fn func() -> u32 {
///     return (vert_idx + push_constants.first_vertex) * (inst_idx +
///     push_constants.first_instance);
///   }
/// ```
///
class OffsetFirstIndex final : public Castable<OffsetFirstIndex, Transform> {
  public:
    /// Transform configuration options
    struct Config final : public Castable<Config, ast::transform::Data> {
        /// Constructor
        /// @param first_vertex_off Offset of the firstVertex push constant
        /// @param first_instance_off Location of the firstInstance push constant
        Config(std::optional<int32_t> first_vertex_off, std::optional<int32_t> first_instance_off);

        /// Destructor
        ~Config() override;

        /// Offset of the firstVertex push constant
        const std::optional<uint32_t> first_vertex_offset;

        /// Offset of the firstInstance push constant
        const std::optional<uint32_t> first_instance_offset;
    };

    /// Constructor
    OffsetFirstIndex();

    /// Destructor
    ~OffsetFirstIndex() override;

    /// @copydoc Transform::Apply
    ApplyResult Apply(const Program& program,
                      const DataMap& inputs,
                      DataMap& outputs) const override;
};

}  // namespace tint::ast::transform

#endif  // SRC_TINT_LANG_WGSL_AST_TRANSFORM_OFFSET_FIRST_INDEX_H_