src/transform/first_index_offset.h

// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef SRC_TRANSFORM_FIRST_INDEX_OFFSET_H_
#define SRC_TRANSFORM_FIRST_INDEX_OFFSET_H_

#include <string>

#include "src/ast/variable_decl_statement.h"
#include "src/transform/transform.h"

namespace tint {
namespace transform {

/// Adds firstVertex/Instance (injected via root constants) to
/// vertex/instance index builtins.
///
/// This transform assumes that Name transform has been run before.
///
/// Unlike other APIs, D3D always starts vertex and instance numbering at 0,
/// regardless of the firstVertex/Instance value specified. This transformer
/// adds the value of firstVertex/Instance to each builtin. This action is
/// performed by adding a new constant equal to original builtin +
/// firstVertex/Instance to each function that references one of these builtins.
///
/// Note that D3D does not have any semantics for firstVertex/Instance.
/// Therefore, these values must by passed to the shader.
///
/// Before:
///   [[builtin(vertex_index)]] var<in> vert_idx : u32;
///   fn func() -> u32 {
///     return vert_idx;
///   }
///
/// After:
///   [[block]]
///   struct TintFirstIndexOffsetData {
///     tint_first_vertex_index : u32;
///     tint_first_instance_index : u32;
///   };
///   [[builtin(vertex_index)]] var<in> tint_first_index_offset_vert_idx : u32;
///   [[binding(N), group(M)]] var<uniform> tint_first_index_data :
///                                                    TintFirstIndexOffsetData;
///   fn func() -> u32 {
///     const vert_idx = (tint_first_index_offset_vert_idx +
///                       tint_first_index_data.tint_first_vertex_index);
///     return vert_idx;
///   }
///
class FirstIndexOffset : public Transform {
 public:
  /// Data is outputted by the FirstIndexOffset transform.
  /// Data holds information about shader usage and constant buffer offsets.
  struct Data : public Castable<Data, transform::Data> {
    /// Constructor
    /// @param has_vtx_index True if the shader uses vertex_index
    /// @param has_inst_index True if the shader uses instance_index
    /// @param first_vtx_offset Offset of first vertex into constant buffer
    /// @param first_inst_offset Offset of first instance into constant buffer
    Data(bool has_vtx_index,
         bool has_inst_index,
         uint32_t first_vtx_offset,
         uint32_t first_inst_offset);

    /// Copy constructor
    Data(const Data&);

    /// Destructor
    ~Data() override;

    /// True if the shader uses vertex_index
    bool const has_vertex_index;
    /// True if the shader uses instance_index
    bool const has_instance_index;
    /// Offset of first vertex into constant buffer
    uint32_t const first_vertex_offset;
    /// Offset of first instance into constant buffer
    uint32_t const first_instance_offset;
  };

  /// Constructor
  /// @param binding the binding() for firstVertex/Instance uniform
  /// @param group the group() for firstVertex/Instance uniform
  FirstIndexOffset(uint32_t binding, uint32_t group);
  ~FirstIndexOffset() override;

  /// Runs the transform on `program`, returning the transformation result.
  /// @param program the source program to transform
  /// @param data optional extra transform-specific input data
  /// @returns the transformation result
  Output Run(const Program* program, const DataMap& data = {}) override;

 private:
  struct State {
    /// Adds uniform buffer with firstVertex/Instance to the program builder
    /// @returns variable of new uniform buffer
    ast::Variable* AddUniformBuffer();
    /// Adds constant with modified original_name builtin to func
    /// @param original_name the name of the original builtin used in function
    /// @param field_name name of field in firstVertex/Instance buffer
    /// @param buffer_var variable of firstVertex/Instance buffer
    ast::VariableDeclStatement* CreateFirstIndexOffset(
        const std::string& original_name,
        const std::string& field_name,
        ast::Variable* buffer_var);

    ProgramBuilder* const dst;
    uint32_t const binding;
    uint32_t const group;

    bool has_vertex_index = false;
    bool has_instance_index = false;
    uint32_t vertex_index_offset = 0;
    uint32_t instance_index_offset = 0;
  };

  uint32_t binding_;
  uint32_t group_;
};
}  // namespace transform
}  // namespace tint

#endif  // SRC_TRANSFORM_FIRST_INDEX_OFFSET_H_