20161117 - Mechanics of Vulkan SPIR-V Specialization

References
SPIR-V | Vulkan

SPIR-V Background

• SPIR-V is a single linear stream of variable length instructions.
• Each instruction starts with a 32-bit packed word {MSB: 16-bit WordCount, LSB: 16-bit Opcode}.
• The WordCount enables unknown Opcode to be ignored.
• OpSpecConstant* instructions used to define a specialization constant.
• OpSpecConstantOp builds a new specializaton constant from doing an operation.
• OpSpecConstantOp does not support complex math like sin() and cos().
• OpSpecConstant* instructions have a Result ID and sometimes a default Value.
• OpDecorate with SpecId associates VkSpecializationMapEntry.constantID with a Result ID.
• OpSpecConstant* gets reduced to OpConstant* instructions at PSO creation time.

Human readable SPIR-V example.

%1 = OpTypeInt 32 1       ; declare a signed 32-bit type
OpDecorate %2 SpecId 3    ; set 'constantID = 3' 
%2 = OpSpecConstant %1 -8 ; some signed 32-bit integer constant

Vulkan Background
Showing below the related structures which show how to set specialization constants.

typedef struct VkSpecializationMapEntry {
  uint32_t constantID; // SpecId in SPIR-V source.
  uint32_t offset;     // Byte offset in VkSpecializationInfo.pData for value.
  size_t   size;
} VkSpecializationMapEntry;

// Passed into  VkPipelineShaderStageCreateInfo
typedef struct VkSpecializationInfo {
  uint32_t                        mapEntryCount;
  const VkSpecializationMapEntry* pMapEntries;
  size_t                          dataSize;
  const void*                     pData;
} VkSpecializationInfo;