Is Vulkan's VkMemoryHeapFlagBits missing value

问题:

At Vulkan specs 1.0.9 (pg. 180), we have the following:

typedef struct VkMemoryHeap {
  VkDeviceSize size;
  VkMemoryHeapFlags flags;
} VkMemoryHeap;

and this description:

• size is the total memory size in bytes in the heap.

• flags is a bitmask of attribute flags for the heap. The bits specified in flags are:

typedef enum VkMemoryHeapFlagBits {
VK_MEMORY_HEAP_DEVICE_LOCAL_BIT = 0x00000001,
} VkMemoryHeapFlagBits;

But when I query VkPhysicalDeviceMemoryProperties I've got flags with zero values. My code matches the output from Vulkan SDK vkjson_info.exe tool, which output a JSON file with properties from my GPU.

Is something missing here ?

回答1:

No. It is perfectly valid for VkMemoryHeap::flags to be 0. A particular flag is either present or absent; it's not an enumeration where the value can attain one of a small number of different possibilities. Usually, flags are independent of one another.

That's how bitflags work; they represent boolean conditions. A particular flag is either present or absent. If a flag is present, then the object attains the meaning that the flag defines. If it is absent, then the object doesn't have that meaning.

A heap with VK_MEMORY_HEAP_DEVICE_LOCAL_BIT means exactly what the specification says it means: "the heap corresponds to device local memory". A heap without that flag set means that heap does not "correspond to device local memory".

The specification requires that at least one heap have this flag set. But that's all.