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 {api-version} implementation must support the {spirv-versions} of SPIR-V and the 1.0 version of the SPIR-V Extended Instructions for GLSL. If the VK_KHR_spirv_1_4 extension is enabled, the implementation must additionally support the 1.4 version of SPIR-V.

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 table below lists the set of SPIR-V capabilities that may be supported in Vulkan implementations. The application must not use any of these capabilities in SPIR-V passed to vkCreateShaderModule unless one of the following conditions is met for the VkDevice specified in the device parameter of vkCreateShaderModule:

• The corresponding field in the table is blank.
• Any corresponding Vulkan feature is enabled.
• Any corresponding Vulkan extension is enabled.
• Any corresponding Vulkan property is supported.
• The corresponding core version is supported (as returned by VkPhysicalDeviceProperties::apiVersion).

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.

SPIR-V Extensions

The following table lists SPIR-V extensions that implementations may support. The application must not pass a SPIR-V module to vkCreateShaderModule that uses the following SPIR-V extensions unless one of the following conditions is met for the VkDevice specified in the device parameter of vkCreateShaderModule:

• Any corresponding Vulkan extension is enabled.
• The corresponding core version is supported (as returned by VkPhysicalDeviceProperties::apiVersion).

Validation Rules Within a Module

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

Standalone SPIR-V Validation

Runtime SPIR-V Validation

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 table below.
• 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 set of operations OpPhi, OpSelect, OpFunctionCall, OpReturnValue, OpVectorExtractDynamic, OpVectorInsertDynamic, OpVectorShuffle, OpCompositeConstruct, OpCompositeExtract, OpCompositeInsert, OpTranspose, OpCopyObject, OpCopyLogical, OpCopyMemory, OpSubgroupReadInvocationKHR, OpSubgroupFirstInvocationKHR, OpGroupNonUniformRotateKHR, OpCooperativeMatrixLoadKHR, OpCooperativeMatrixStoreKHR, OpCooperativeMatrixLoadNV, OpCooperativeMatrixStoreNV, OpAtomicLoad, OpAtomicStore, OpAtomicExchange, OpStore, and OpLoad are referred to as bit-preserving operations.
• The floating-point environment used for an instruction can be determined as follows:
  • If the SPIR-V specifies it explicitly using the FPFastMath decoration or FPFastMathDefault Execution Mode then that is used.
  • If the environment is not specified in the SPIR-V then it is determined as follows:
    • If the operation is not decorated NoContraction then the flags AllowContract, AllowReassoc, AllowRecip, and AllowTransform are assumed.
    • If any of the following conditions are true then the flags NSZ, NotInf, and NotNaN are assumed:
      • The entry point does not use the Execution Mode
SignedZeroInfNanPreserve with a bit-width corresponding to one of the operands or to the result type.
      • The operation is not a bit-preserving operation and is not one of OpFConvert, OpFNegate, OpFAdd, OpFSub, OpFMul, OpFDiv, OpIsNan, or OpIsInf.
      • The operation is an OpLoad from the Input Storage Class in the fragment shader stage.
• All bit-preserving operations and the following instructions must not flush denormalized values: OpConstant, OpConstantComposite, OpSpecConstant, OpSpecConstantComposite, and OpBitcast.
• Denormalized values are supported.
  • By default, any half, single, or double-precision denormalized value input into a shader or potentially generated by any instruction (except those listed above) or any extended instructions for GLSL in a shader may be flushed to zero.
  • If the entry point is declared with the DenormFlushToZero
Execution Mode then for the affected instructions the denormalized result must be flushed to zero and the denormalized operands may be flushed to zero. Denormalized values obtained via unpacking an integer into a vector of values with smaller bit width and interpreting those values as floating-point numbers must be flushed to zero.
  • The following core SPIR-V instructions must respect the DenormFlushToZero Execution Mode: OpSpecConstantOp (with opcode OpFConvert), OpFConvert, OpFNegate, OpFAdd, OpFSub, OpFMul, OpFDiv, OpFRem, OpFMod, OpVectorTimesScalar, OpMatrixTimesScalar, OpVectorTimesMatrix, OpMatrixTimesVector, OpMatrixTimesMatrix, OpOuterProduct, OpDot; and the following extended instructions for GLSL: Round, RoundEven, Trunc, FAbs, Floor, Ceil, Fract, Radians, Degrees, Sin, Cos, Tan, Asin, Acos, Atan, Sinh, Cosh, Tanh, Asinh, Acosh, Atanh, Atan2, Pow, Exp, Log, Exp2, Log2, Sqrt, InverseSqrt, Determinant, MatrixInverse, Modf, ModfStruct, FMin, FMax, FClamp, FMix, Step, SmoothStep, Fma, UnpackHalf2x16, Length, Distance, Cross, Normalize, FaceForward, Reflect, Refract, NMin, NMax, and NClamp.
  • The following core SPIR-V instructions must respect the DenormPreserve Execution Mode: OpSpecConstantOp, OpFConvert, OpFNegate, OpFAdd, OpFSub, OpFMul, OpVectorTimesScalar, OpMatrixTimesScalar, OpVectorTimesMatrix, OpMatrixTimesVector, OpMatrixTimesMatrix, OpOuterProduct, OpDot, OpFOrdEqual, OpFUnordEqual, OpFOrdNotEqual, OpFUnordNotEqual, OpFOrdLessThan, OpFUnordLessThan, OpFOrdGreaterThan, OpFUnordGreaterThan, OpFOrdLessThanEqual, OpFUnordLessThanEqual, OpFOrdGreaterThanEqual, OpFUnordGreaterThanEqual, OpSubgroupAllEqualKHR, OpGroupNonUniformAllEqual; and the following extended instructions for GLSL: FAbs, FSign, Radians, Degrees, FMin, FMax, FClamp, FMix, Fma, PackHalf2x16, PackDouble2x32, UnpackHalf2x16, UnpackDouble2x32, NMin, NMax, and NClamp.

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

The precision of individual operations is defined in Precision of Individual Operations. Subject to the constraints below, however, implementations may reorder or combine operations, resulting in expressions exhibiting different precisions than might be expected from the constituent operations.

Evaluation of Expressions

Implementations may rearrange floating-point operations using any of the mathematical properties governing the expressions in precise arithmetic, even where the floating- point operations do not share these properties. This includes, but is not limited to, associativity and distributivity, and may involve a different number of rounding steps than would occur if the operations were not rearranged. In shaders that use the SignedZeroInfNanPreserve Execution Mode the values must be preserved if they are generated after any rearrangement but the Execution Mode does not change which rearrangements are valid. This rearrangement can be prevented for particular operations by using the NoContraction decoration.

Precision of Individual Operations

The precision of individual operations is defined either in terms of rounding (correctly rounded), as an error bound in ULP, or as inherited from a formula as follows:

Correct Result

Operations that are described as returning the "correct result" will return the infinitely precise result which, due to the nature of the operation, will not need rounding.

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 is not specified, unless the entry point is declared with the RoundingModeRTE or the RoundingModeRTZ Execution Mode. These execution modes affect only correctly rounded SPIR-V instructions. These execution modes do not affect OpQuantizeToF16 and PackHalf2x16. If the rounding mode is not specified then this rounding is implementation specific, subject to 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, finite 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 non-equal, 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 F_max and the smallest be F_min. The operation must return a value in the range [x - E, x + E] where $E = \mathrm{max} \left( | x - F\_{\mathrm{min}} |, | x - F\_{\mathrm{max}} | \right)$. If the entry point is declared with the DenormFlushToZero execution mode, then any intermediate denormal value(s) while evaluating the formula may be flushed to zero. Denormal final results must be flushed to zero. If the entry point is declared with the DenormPreserve Execution Mode, then denormals must be preserved throughout the formula.

For half- (16 bit) and single- (32 bit) precision instructions, precisions are required to be at least as follows:

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:.

OpCooperativeMatrixMulAddNV performs its operations in an implementation-dependent order and internal precision.

OpCooperativeMatrixMulAddKHR performs its operations in an implementation-dependent order and internal precision.

Signedness of SPIR-V Image Accesses

SPIR-V associates a signedness with all integer image accesses. This is required in certain parts of the SPIR-V and the Vulkan image access pipeline to ensure defined results. The signedness is determined from a combination of the access instruction’s Image Operands and the underlying image’s Sampled Type as follows:

1. If the instruction’s Image Operands contains the SignExtend operand then the access is signed.
2. If the instruction’s Image Operands contains the ZeroExtend operand then the access is unsigned.
3. Otherwise, the image accesses signedness matches that of the Sampled Type of the OpTypeImage being accessed.

Image Format and Type Matching

When specifying the Image Format of an OpTypeImage, the converted bit width and type, as shown in the table below, must match the Sampled Type. The signedness must match the signedness of any access to the image.

For a given Image Format, the Sampled Type must be the type described in the Type column of the below table, with its Literal Width set to that in the Bit Width column. Every access that is made to the image must have a signedness equal to that in the Signedness column (where applicable).

The SPIR-V Type is defined by an instruction in SPIR-V, declared with the Type-Declaration Instruction, Bit Width, and Signedness from above.

Compatibility Between SPIR-V Image Dimensions and Vulkan ImageView Types

SPIR-V Image Dim values are compatible with VkImageView

viewType values as defined below:

Compatibility Between SPIR-V Image Formats and Vulkan Formats

SPIR-V Image Format values are compatible with VkFormat values as defined below:

Ray Query Precision and Operation

The values returned by OpRayQueryGetIntersectionTriangleVertexPositionsKHR are transformed by the geometry transform, which is performed at standard floating-point precision, but without a specifically defined order of floating-point operations to perform the matrix multiplication.