## Appendix A: Vulkan Environment for SPIR-V

Shaders for Vulkan are defined by the Khronos SPIR-V Specification as well as the Khronos SPIR-V Extended Instructions for GLSL Specification. This appendix defines additional SPIR-V requirements applying to Vulkan shaders.

### Versions and Formats

A Vulkan 1.1 implementation must support the 1.0, 1.1, 1.2, and 1.3 versions of SPIR-V and the 1.0 version of the SPIR-V Extended Instructions for GLSL.

A SPIR-V module passed into vkCreateShaderModule is interpreted as a series of 32-bit words in host endianness, with literal strings packed as described in section 2.2 of the SPIR-V Specification. The first few words of the SPIR-V module must be a magic number and a SPIR-V version number, as described in section 2.3 of the SPIR-V Specification.

### Capabilities

The SPIR-V capabilities listed below must be supported if the corresponding feature or extension is enabled, or if no features or extensions are listed for that capability. Extensions are only listed when there is not also a feature bit associated with that capability.

Table 58. List of SPIR-V Capabilities and enabling features or extensions
SPIR-V OpCapability Vulkan feature or extension name

Matrix

Shader

InputAttachment

Sampled1D

Image1D

SampledBuffer

ImageBuffer

ImageQuery

DerivativeControl

Geometry

Tessellation

Float64

Int64

Int16

TessellationPointSize

GeometryPointSize

ImageGatherExtended

StorageImageMultisample

UniformBufferArrayDynamicIndexing

SampledImageArrayDynamicIndexing

StorageBufferArrayDynamicIndexing

StorageImageArrayDynamicIndexing

ClipDistance

CullDistance

ImageCubeArray

imageCubeArray

SampleRateShading

SparseResidency

MinLod

SampledCubeArray

imageCubeArray

ImageMSArray

StorageImageExtendedFormats

InterpolationFunction

StorageImageReadWithoutFormat

StorageImageWriteWithoutFormat

MultiViewport

multiViewport

DrawParameters

MultiView

DeviceGroup

VariablePointersStorageBuffer

VariablePointers

StorageBuffer16BitAccess

StorageBuffer16BitAccess

UniformAndStorageBuffer16BitAccess

UniformAndStorageBuffer16BitAccess

StoragePushConstant16

storagePushConstant16

StorageInputOutput16

GroupNonUniform

VK_SUBGROUP_FEATURE_BASIC_BIT

GroupNonUniformVote

VK_SUBGROUP_FEATURE_VOTE_BIT

GroupNonUniformArithmetic

VK_SUBGROUP_FEATURE_ARITHMETIC_BIT

GroupNonUniformBallot

VK_SUBGROUP_FEATURE_BALLOT_BIT

GroupNonUniformShuffle

VK_SUBGROUP_FEATURE_SHUFFLE_BIT

GroupNonUniformShuffleRelative

VK_SUBGROUP_FEATURE_SHUFFLE_RELATIVE_BIT

GroupNonUniformClustered

VK_SUBGROUP_FEATURE_CLUSTERED_BIT

GroupNonUniformQuad

The application can pass a SPIR-V module to vkCreateShaderModule that uses the SPV_KHR_variable_pointers SPIR-V extension.

The application can pass a SPIR-V module to vkCreateShaderModule that uses the SPV_KHR_shader_draw_parameters SPIR-V extension.

The application can pass a SPIR-V module to vkCreateShaderModule that uses the SPV_KHR_16bit_storage SPIR-V extension.

The application can pass a SPIR-V module to vkCreateShaderModule that uses the SPV_KHR_storage_buffer_storage_class SPIR-V extension.

The application must not pass a SPIR-V module containing any of the following to vkCreateShaderModule:

• any OpCapability not listed above,

• an unsupported capability, or

• a capability which corresponds to a Vulkan feature or extension which has not been enabled.

### Validation Rules within a Module

A SPIR-V module passed to vkCreateShaderModule must conform to the following rules:

• Every entry point must have no return value and accept no arguments.

• Recursion: The static function-call graph for an entry point must not contain cycles.

• The Logical addressing model must be selected.

• Scope for execution must be limited to:

• Workgroup

• Subgroup

• Scope for memory must be limited to:

• Device

• Device scope only extends to the queue family, not the whole device.

• Workgroup

• Subgroup

• Invocation

• Scope for Non Uniform Group Operations must be limited to:

• Subgroup

• Storage Class must be limited to:

• UniformConstant

• Input

• Uniform

• Output

• Workgroup

• Private

• Function

• PushConstant

• Image

• StorageBuffer

• Memory semantics must obey the following rules:

• Acquire must not be used with OpAtomicStore.

• Release must not be used with OpAtomicLoad.

• AcquireRelease must not be used with OpAtomicStore or OpAtomicLoad.

• Sequentially consistent atomics and barriers are not supported and SequentiallyConsistent is treated as AcquireRelease. SequentiallyConsistent should not be used.

• OpMemoryBarrier must use one of Acquire, Release, AcquireRelease, or SequentiallyConsistent and must include at least one storage class.

• If the semantics for OpControlBarrier includes one of Acquire, Release, AcquireRelease, or SequentiallyConsistent, then it must include at least one storage class.

• SubgroupMemory, CrossWorkgroupMemory, and AtomicCounterMemory are ignored.

• Any OpVariable with an Initializer operand must have one of the following as its Storage Class operand:

• Output

• Private

• Function

• The OriginLowerLeft execution mode must not be used; fragment entry points must declare OriginUpperLeft.

• The PixelCenterInteger execution mode must not be used. Pixels are always centered at half-integer coordinates.

• Any variable in the UniformConstant storage class must be typed as either:

• OpTypeImage

• OpTypeSampler

• OpTypeSampledImage

• An array of one of these types.

• Images and Samplers

• OpTypeImage must declare a scalar 32-bit float or 32-bit integer type for the “Sampled Type”. (RelaxedPrecision can be applied to a sampling instruction and to the variable holding the result of a sampling instruction.)

• If the Sampled Type of an OpTypeImage declaration does not match the numeric format of the corresponding resource in type, as shown in the SPIR-V Sampled Type column of the Interpretation of Numeric Format table, the values obtained by reading or sampling from the image are undefined.

• If the signedness of any read or sample operation does not match the signedness of the corresponding resource then the values obtained are undefined.

• OpTypeImage must have a “Sampled” operand of 1 (sampled image) or 2 (storage image).

• If shaderStorageImageReadWithoutFormat is not enabled and an OpTypeImage has “Image Format” operand of Unknown, any variables created with the given type must be decorated with NonReadable.

• If shaderStorageImageWriteWithoutFormat is not enabled and an OpTypeImage has “Image Format” operand of Unknown, any variables created with the given type must be decorated with NonWritable.

• OpImageQuerySizeLod, and OpImageQueryLevels must only consume an “Image” operand whose type has its “Sampled” operand set to 1.

• The (u,v) coordinates used for a SubpassData must be the <id> of a constant vector (0,0), or if a layer coordinate is used, must be a vector that was formed with constant 0 for the u and v components.

• The “Depth” operand of OpTypeImage is ignored.

• Objects of types OpTypeImage, OpTypeSampler, OpTypeSampledImage, and arrays of these types must not be stored to or modified.

• Any image operation must use at most one of the Offset, ConstOffset, and ConstOffsets image operands.

• Image operand Offset must only be used with OpImage*Gather instructions.

• The “Component” operand of OpImageGather, and OpImageSparseGather must be the <id> of a constant instruction.

• Structure types must not contain opaque types.

• Decorations

• Any BuiltIn decoration not listed in Built-In Variables must not be used.

• Any BuiltIn decoration that corresponds only to Vulkan features or extensions that have not been enabled must not be used.

• The GLSLShared and GLSLPacked decorations must not be used.

• The Flat, NoPerspective, Sample, and Centroid decorations must not be used on variables with storage class other than Input or on variables used in the interface of non-fragment shader entry points.

• The Patch decoration must not be used on variables in the interface of a vertex, geometry, or fragment shader stage’s entry point.

• Only the round-to-nearest-even and the round-towards-zero rounding modes can be used for the FPRoundingMode decoration.

• The FPRoundingMode decoration can only be used for the floating-point conversion instructions as described in the SPV_KHR_16bit_storage SPIR-V extension.

• DescriptorSet and Binding decorations must obey the constraints on storage class, type, and descriptor type described in DescriptorSet and Binding Assignment

• OpTypeRuntimeArray must only be used for:

• the last member of an OpTypeStruct that is in the StorageBuffer storage class decorated as Block, or that is in the Uniform storage class decorated as BufferBlock.

• If OpControlBarrier is used in fragment, vertex, tessellation evaluation, or geometry stages, the execution Scope must be Subgroup.

• For each compute shader entry point, either a LocalSize execution mode or an object decorated with the WorkgroupSize decoration must be specified.

• “Result Type” for Non Uniform Group Operations must be limited to 32-bit floating-point, 32-bit integer, boolean, or vectors of these types.

• If the Float64 capability is enabled, 64-bit floating-point and vector of 64-bit floating-point types are also permitted.

• The “Id” operand of OpGroupNonUniformBroadcast must be the <id> of a constant instruction.

• If OpGroupNonUniformBallotBitCount is used, the group operation must be one of:

• Reduce

• InclusiveScan

• ExclusiveScan

• Atomic instructions must declare a scalar 32-bit integer type, for the value pointed to by Pointer.

• The Pointer operand of all atomic instructions must have a Storage Class limited to:

• Uniform

• Workgroup

• Image

• StorageBuffer

• The Base operand of OpPtrAccessChain must point to one of the following storage classes:

• Workgroup, if VariablePointers is enabled.

• StorageBuffer, if VariablePointers or VariablePointersStorageBuffer is enabled.

• PhysicalStorageBufferEXT, if the PhysicalStorageBuffer64EXT addressing model is enabled.

### Precision and Operation of SPIR-V Instructions

The following rules apply to half, single, and double-precision floating point instructions:

• Positive and negative infinities and positive and negative zeros are generated as dictated by IEEE 754, but subject to the precisions allowed in the following table.

• Dividing a non-zero by a zero results in the appropriately signed IEEE 754 infinity.

• Signaling NaNs are not required to be generated and exceptions are never raised. Signaling NaN may be converted to quiet NaNs values by any floating point instruction.

• The following instructions must not flush denormalized values: OpConstant, OpConstantComposite, OpSpecConstant, OpSpecConstantComposite, OpLoad, OpStore, OpBitcast, OpPhi, OpSelect, OpFunctionCall, OpReturnValue, OpVectorExtractDynamic, OpVectorInsertDynamic, OpVectorShuffle, OpCompositeConstruct, OpCompositeExtract, OpCompositeInsert, OpCopyMemory, OpCopyObject.

• Any denormalized value input into a shader or potentially generated by any instruction in a shader (except those listed above) may be flushed to 0.

• The rounding mode cannot be set, and results will be correctly rounded, as described below.

• NaNs may not be generated. Instructions that operate on a NaN may not result in a NaN.

The precision of double-precision instructions is at least that of single precision.

The precision of operations is defined either in terms of rounding, as an error bound in ULP, or as inherited from a formula as follows.

Correctly Rounded

Operations described as “correctly rounded” will return the infinitely precise result, x, rounded so as to be representable in floating-point. The rounding mode used is not defined but must obey the following rules. If x is exactly representable then x will be returned. Otherwise, either the floating-point value closest to and no less than x or the value closest to and no greater than x will be returned.

ULP

Where an error bound of n ULP (units in the last place) is given, for an operation with infinitely precise result x the value returned must be in the range [x - n * ulp(x), x + n * ulp(x)]. The function ulp(x) is defined as follows:

If there exist non-equal floating-point numbers a and b such that a ≤ x ≤ b then ulp(x) is the minimum possible distance between such numbers, $$ulp(x) = \mathrm{min}_{a,b} | b - a |$$. If such numbers do not exist then ulp(x) is defined to be the difference between the two finite floating-point numbers nearest to x.

Where the range of allowed return values includes any value of magnitude larger than that of the largest representable finite floating-point number, operations may, additionally, return either an infinity of the appropriate sign or the finite number with the largest magnitude of the appropriate sign. If the infinitely precise result of the operation is not mathematically defined then the value returned is undefined.

Inherited From …​

Where an operation’s precision is described as being inherited from a formula, the result returned must be at least as accurate as the result of computing an approximation to x using a formula equivalent to the given formula applied to the supplied inputs. Specifically, the formula given may be transformed using the mathematical associativity, commutativity and distributivity of the operators involved to yield an equivalent formula. The SPIR-V precision rules, when applied to each such formula and the given input values, define a range of permitted values. If NaN is one of the permitted values then the operation may return any result, otherwise let the largest permitted value in any of the ranges be Fmax and the smallest be Fmin. The operation must return a value in the range [x - E, x + E] where $$E = \mathrm{max} \left( | x - F_{\mathrm{min}} |, | x - F_{\mathrm{max}} | \right)$$.

For single precision (32 bit) instructions, precisions are required to be at least as follows, unless decorated with RelaxedPrecision:

Table 59. Precision of core SPIR-V Instructions
Instruction Precision

OpFAdd

Correctly rounded.

OpFSub

Correctly rounded.

OpFMul, OpVectorTimesScalar, OpMatrixTimesScalar

Correctly rounded.

OpFOrdEqual, OpFUnordEqual

Correct result.

OpFOrdLessThan, OpFUnordLessThan

Correct result.

OpFOrdGreaterThan, OpFUnordGreaterThan

Correct result.

OpFOrdLessThanEqual, OpFUnordLessThanEqual

Correct result.

OpFOrdGreaterThanEqual, OpFUnordGreaterThanEqual

Correct result.

OpFDiv(x,y)

2.5 ULP for |y| in the range [2-126, 2126].

conversions between types

Correctly rounded.

Table 60. Precision of GLSL.std.450 Instructions
Instruction Precision

fma()

Inherited from OpFMul followed by OpFAdd.

exp(x), exp2(x)

3 + 2 × |x| ULP.

log(), log2()

3 ULP outside the range [0.5, 2.0]. Absolute error < 2-21 inside the range [0.5, 2.0].

pow(x, y)

Inherited from exp2(y × log2(x)).

sqrt()

Inherited from 1.0 / inversesqrt().

inversesqrt()

2 ULP.

GLSL.std.450 extended instructions specifically defined in terms of the above instructions inherit the above errors. GLSL.std.450 extended instructions not listed above and not defined in terms of the above have undefined precision.

For the OpSRem and OpSMod instructions, if either operand is negative the result is undefined.

 Note While the OpSRem and OpSMod instructions are supported by the Vulkan environment, they require non-negative values and thus do not enable additional functionality beyond what OpUMod provides.

### Compatibility Between SPIR-V Image Formats And Vulkan Formats

Images which are read from or written to by shaders must have SPIR-V image formats compatible with the Vulkan image formats backing the image under the circumstances described for texture image validation. The compatibile formats are:

Table 61. SPIR-V and Vulkan Image Format Compatibility
SPIR-V Image Format Compatible Vulkan Format

Rgba32f

VK_FORMAT_R32G32B32A32_SFLOAT

Rgba16f

VK_FORMAT_R16G16B16A16_SFLOAT

R32f

VK_FORMAT_R32_SFLOAT

Rgba8

VK_FORMAT_R8G8B8A8_UNORM

Rgba8Snorm

VK_FORMAT_R8G8B8A8_SNORM

Rg32f

VK_FORMAT_R32G32_SFLOAT

Rg16f

VK_FORMAT_R16G16_SFLOAT

R11fG11fB10f

VK_FORMAT_B10G11R11_UFLOAT_PACK32

R16f

VK_FORMAT_R16_SFLOAT

Rgba16

VK_FORMAT_R16G16B16A16_UNORM

Rgb10A2

VK_FORMAT_A2B10G10R10_UNORM_PACK32

Rg16

VK_FORMAT_R16G16_UNORM

Rg8

VK_FORMAT_R8G8_UNORM

R16

VK_FORMAT_R16_UNORM

R8

VK_FORMAT_R8_UNORM

Rgba16Snorm

VK_FORMAT_R16G16B16A16_SNORM

Rg16Snorm

VK_FORMAT_R16G16_SNORM

Rg8Snorm

VK_FORMAT_R8G8_SNORM

R16Snorm

VK_FORMAT_R16_SNORM

R8Snorm

VK_FORMAT_R8_SNORM

Rgba32i

VK_FORMAT_R32G32B32A32_SINT

Rgba16i

VK_FORMAT_R16G16B16A16_SINT

Rgba8i

VK_FORMAT_R8G8B8A8_SINT

R32i

VK_FORMAT_R32_SINT

Rg32i

VK_FORMAT_R32G32_SINT

Rg16i

VK_FORMAT_R16G16_SINT

Rg8i

VK_FORMAT_R8G8_SINT

R16i

VK_FORMAT_R16_SINT

R8i

VK_FORMAT_R8_SINT

Rgba32ui

VK_FORMAT_R32G32B32A32_UINT

Rgba16ui

VK_FORMAT_R16G16B16A16_UINT

Rgba8ui

VK_FORMAT_R8G8B8A8_UINT

R32ui

VK_FORMAT_R32_UINT

Rgb10a2ui

VK_FORMAT_A2B10G10R10_UINT_PACK32

Rg32ui

VK_FORMAT_R32G32_UINT

Rg16ui

VK_FORMAT_R16G16_UINT

Rg8ui

VK_FORMAT_R8G8_UINT

R16ui

VK_FORMAT_R16_UINT

R8ui

VK_FORMAT_R8_UINT