The scalar layouts section makes multiple untrue claims that will mislead readers and lead to difficult-to-debug data corruption.
|
In Vulkan 1.4, this functionality is now a core requirement. By using the `scalar` layout, you can ensure that your data structures on the GPU match your C{pp} structures perfectly, byte-for-byte. |
Scalar layout (as it is implemented in GLSL and Slang) in fact does not match C++ structure layout exactly, there is a notable corner case with nested structs:
struct Inner
{
uint a;
uint16_t b;
};
struct Outer
{
Inner ab;
uint16_t c;
};With scalar layout (in Slang and GLSL), sizeof(Outer) == 8, as Outer::c gets packed into the trailing padding of Outer::ab. In C++, sizeof(Outer) == 12. Slang does have -fvk-use-c-layout (slangc) / OptionKind::ForceCLayout (API) / CDataLayout (in-language) that does actually match the C++ layout (with the only known caveat being empty structs, which are 0 bytes in Slang and 1 in C/C++). As for HLSL/DXC, they have changed their scalar layout to match C/C++ somewhat recently: microsoft/DirectXShaderCompiler#7996.
|
If you are using Slang, you don't even need to worry about manual layout qualifiers for most cases. Slang's layout engine handles the `scalar` rules for you when targeting Vulkan 1.4: |
This is just untrue. StructuredBuffer<T> in Slang does not default to scalar layout. You also don't target Vulkan versions with Slang, you target SPIR-V versions instead, but none of those change the default layout to be scalar layout. You have to either specify it globally with -fvk-use-scalar-layout (slangc), OptionKind::GLSLForceScalarLayout (Slang API) or specify it per-buffer with RWStructuredBuffer<T, ScalarDataLayout> buf;.
|
=== GLSL: The Manual Struggle |
Not really a factual error, but swapping std430 -> scalar is not exactly what I would call a "manual struggle". It is just as manual as it is in Slang.
|
If you are using modern languages like Slang, this becomes even easier. Slang defaults to a more natural, C{pp}-like layout, and its Vulkan backend handles the scalar layout details for you automatically when targeting Vulkan 1.4. |
Again, Slang does not default to a "C++-like" layout. It doesn't even default to scalar layout. And I don't know why this would be in plural, "modern languages". None of the big three shading languages default to scalar layout.
|
By understanding these low-level architectural details, you've moved beyond "writing shaders" and started "programming the hardware." |
This is a completely meaningless, tautological statement.
The scalar layouts section makes multiple untrue claims that will mislead readers and lead to difficult-to-debug data corruption.
Vulkan-Tutorial/en/Advanced_Vulkan_Compute/02_Compute_Architecture/04_vulkan_1_4_scalar_layouts.adoc
Line 33 in eda64e1
Scalar layout (as it is implemented in GLSL and Slang) in fact does not match C++ structure layout exactly, there is a notable corner case with nested structs:
With scalar layout (in Slang and GLSL),
sizeof(Outer) == 8, asOuter::cgets packed into the trailing padding ofOuter::ab. In C++,sizeof(Outer) == 12. Slang does have-fvk-use-c-layout(slangc) /OptionKind::ForceCLayout(API) /CDataLayout(in-language) that does actually match the C++ layout (with the only known caveat being empty structs, which are 0 bytes in Slang and 1 in C/C++). As for HLSL/DXC, they have changed their scalar layout to match C/C++ somewhat recently: microsoft/DirectXShaderCompiler#7996.Vulkan-Tutorial/en/Advanced_Vulkan_Compute/02_Compute_Architecture/04_vulkan_1_4_scalar_layouts.adoc
Line 43 in eda64e1
This is just untrue.
StructuredBuffer<T>in Slang does not default to scalar layout. You also don't target Vulkan versions with Slang, you target SPIR-V versions instead, but none of those change the default layout to be scalar layout. You have to either specify it globally with-fvk-use-scalar-layout(slangc),OptionKind::GLSLForceScalarLayout(Slang API) or specify it per-buffer withRWStructuredBuffer<T, ScalarDataLayout> buf;.Vulkan-Tutorial/en/Advanced_Vulkan_Compute/02_Compute_Architecture/04_vulkan_1_4_scalar_layouts.adoc
Line 58 in eda64e1
Not really a factual error, but swapping
std430->scalaris not exactly what I would call a "manual struggle". It is just as manual as it is in Slang.Vulkan-Tutorial/en/Advanced_Vulkan_Compute/02_Compute_Architecture/04_vulkan_1_4_scalar_layouts.adoc
Line 110 in eda64e1
Again, Slang does not default to a "C++-like" layout. It doesn't even default to scalar layout. And I don't know why this would be in plural, "modern languages". None of the big three shading languages default to scalar layout.
Vulkan-Tutorial/en/Advanced_Vulkan_Compute/02_Compute_Architecture/04_vulkan_1_4_scalar_layouts.adoc
Line 116 in eda64e1
This is a completely meaningless, tautological statement.