116 lines
5.0 KiB
Markdown
116 lines
5.0 KiB
Markdown
# SPIR-V translation of WGSL pointers and references
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WGSL was updated to have two kinds of memory views: pointers and references.
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See https://github.com/gpuweb/gpuweb/pull/1569
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In summary:
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* Reference types are never explicitly mentioned in WGSL source.
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* A use of a variable is a value of reference type corresponding
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to the reference memory view of the storage allocated for the
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variable.
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* Let-declared constants can be of pointer type, but not reference
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type.
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* Function parameter can be of pointer type, but not reference type.
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* A variable's store type is never a pointer type, and never a
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reference type.
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* The "Load Rule" allows a reference to decay to the underlying
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store type, by issuing a load of the value in the underlying memory.
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* For an assignment:
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* The right-hand side evaluates to a non-reference type (atomic-free
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plain type).
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* The left-hand side evaluates to a reference type, whose store
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type is the same as the result of evaluating the right hand side.
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* The address-of (unary `&`) operator converts a reference to a
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pointer.
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* The dereference (unary `*`) operator converts a pointer to a
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reference.
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TODO: Passing textures and samplers to helper functions might be
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done by "handler value", or by pointer-to-handle.
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## Writing SPIR-V from WGSL
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The distinction in WGSL between reference and pointer disappears
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at the SPIR-V level. Both types map into pointer types in SPIR-V.
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To translate a valid WGSL program to SPIR-V:
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* The dereference operator (unary `*`) is the identity operation.
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* The address-of operator (unary `&`) is the identity operation.
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* Assignment maps to OpStore.
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* The Load Rule translates to OpLoad.
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## Reading SPIR-V to create WGSL
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The main changes to the SPIR-V reader are:
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* When translating a SPIR-V pointer expression, track whether the
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corresponding WGSL expression is of corresponding WGSL pointer
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type or correspoinding WGSL type.
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* Insert dereference (unary-`*`) or address-of (unary-`&`) operators
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as needed to generate valid WGSL expressions.
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The choices can be made deterministic, as described below.
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The SPIR-V reader only supports baseline functionality in Vulkan.
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Therefore we assume no VariablePointers or VariablePointersStorageBuffer
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capabilities. All pointers are
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[SPIR-V logical pointers](https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#LogicalPointerType).
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The [SPIR-V Universal Validation Rules](https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#_universal_validation_rules)
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specify where logical pointers can appear as results of instructions
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or operands of instructions.
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Each SPIR-V pointer result expression is a logical pointer, and
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therefore is one of:
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* OpVariable: map to the reference type.
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* OpFunctionParameter: map to the pointer type.
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* OpCopyObject:
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* When these only have one use, then these often fold away.
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Otherwise, they map to a a let-declared constant.
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* Map to the pointer type.
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* OpAccessChain, OpInBoundsAccessChain:
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* This could map to either pointer or reference, and adjustments
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in other areas could make it work. However, we recommend mapping
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this to the reference type.
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* OpImageTexelPointer is not supported in WGSL.
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It is used to get a pointer into a storage texture, for use with
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atomic instructions. But image atomics is not supported in
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WebGPU/WGSL.
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Each SPIR-V pointer operand is also a logical pointer, and is an
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operand to one of:
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* OpLoad Pointer operand:
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* Map to reference, inserting a dereference operator if needed.
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* OpStore Pointer operand:
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* Map to reference, inserting a dereference operator if needed.
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* OpStore Pointer operand:
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* OpAccessChain, OpInBoundsAccessChain Base operand:
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* WGSL array-access and subfield access only works on references.
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* [Gpuweb issue 1530](https://github.com/gpuweb/gpuweb/issues/1530)
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is filed to allow those operations to work on pointers.
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* Map to reference, inserting a dereference operator if needed.
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* OpFunctionCall function argument pointer operands
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* Function operands can't be references.
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* Map to pointer, inserting an address-of operator if needed.
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* OpAtomic instruction Pointer operand
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* These map to WGSL atomic builtins.
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* Map to pointer, inserting an address-of operator if needed.
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* Note: As of this writing, the atomic instructions are not supported
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by the SPIR-V reader.
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* OpCopyObject source operand
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* This could have been mapped either way, but it's easiest to
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map to pointer, to match the choice for OpCopyObject result type.
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* Map to pointer, inserting an address-of operator if needed.
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* OpCopyMemory, source and destination operands
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* This acts as an assignment.
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* Map both source and destination to reference, inserting dereference
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operators if needed.
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* Note: As of this writing, OpCopyMemory is not supported by the
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SPIR-V reader.
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* Extended instruction set instructions Modf and Frexp
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* These map to builtins.
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* Map the pointer operand to pointer, inserting an address-of
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operator if needed.
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