SPV_NV_raw_access

Name Strings

SPV_NV_raw_access_chains

Contact

To report problems with this extension, please open a new issue at:

https://github.com/KhronosGroup/SPIRV-Headers

Contributors

Hans-Kristian Arntzen, Valve
Jeff Bolz, NVIDIA
Rodrigo Locatti, NVIDIA

Notice

Status

Complete

Version

Last Modified Date

2024-01-17

Revision

Dependencies

This extension is written against the SPIR-V Specification, Version 1.6 Revision 2.

This extension requires SPIR-V 1.0.

Overview

This extension exposes an interface similar to Direct3D structured buffers and byte address buffers, allowing shaders compiled from an HLSL source to generate more efficient code.
This adds the instruction OpRawAccessChainNV under the RawAccessChainsNV capability. An optional operand can be provided to control bounds checking.

Extension Name

To use this extension within a SPIR-V module, the following OpExtension must be present in the module:

OpExtension "SPV_NV_raw_access_chains"

Modifications to the SPIR-V Specification, Version 1.6

Validation Rules

Modify section 2.16.1. Universal Validation Rules:

In the section:

"It is invalid for a pointer to be an operand to any instruction other than:"

Add:

<<OpRawAccessChainNV,*OpRawAccessChainNV*>>

In the section:

"It is invalid for a pointer to be the Result <id> of any instruction other than:"

Add:

<<OpRawAccessChainNV,*OpRawAccessChainNV*>>

Change:

"All indexes in <<OpAccessChain,*OpAccessChain*>> and <<OpInBoundsAccessChain,*OpInBoundsAccessChain*>> that are <<OpConstant,*OpConstant*>> with type of <<OpTypeInt,*OpTypeInt*>> with a 'signedness' of 1 must not have their sign bit set."

To:

"All indexes in <<OpAccessChain,*OpAccessChain*>>, <<OpInBoundsAccessChain,*OpInBoundsAccessChain*>>, and <<OpRawAccessChainNV,*OpRawAccessChainNV*>> that are <<OpConstant,*OpConstant*>> with type of <<OpTypeInt,*OpTypeInt*>> with a 'signedness' of 1 must not have their sign bit set."

Change:

"An <<OpTypePointer, *OpTypePointer*>> pointing to a 16-bit scalar, a 16-bit vector, or a composite containing a 16-bit member can be used as the result type of <<OpVariable, *OpVariable*>>, or <<OpAccessChain, *OpAccessChain*>>, or <<OpInBoundsAccessChain, *OpInBoundsAccessChain*>>."

To:

"An <<OpTypePointer, *OpTypePointer*>> pointing to a 16-bit scalar, a 16-bit vector, or a composite containing a 16-bit member can be used as the result type of <<OpVariable, *OpVariable*>>, or <<OpAccessChain, *OpAccessChain*>>, or <<OpInBoundsAccessChain, *OpInBoundsAccessChain*>>, or <<OpRawAccessChainNV,*OpRawAccessChainNV*>>."

Change:

"An <<OpTypePointer, *OpTypePointer*>> pointing to an 8-bit scalar, an 8-bit vector, or a composite containing an 8-bit member can be used as the result type of <<OpVariable, *OpVariable*>>, or <<OpAccessChain, *OpAccessChain*>>, or <<OpInBoundsAccessChain, *OpInBoundsAccessChain*>>."

To:

"An <<OpTypePointer, *OpTypePointer*>> pointing to an 8-bit scalar, an 8-bit vector, or a composite containing an 8-bit member can be used as the result type of <<OpVariable, *OpVariable*>>, or <<OpAccessChain, *OpAccessChain*>>, or <<OpInBoundsAccessChain, *OpInBoundsAccessChain*>>, or <<OpRawAccessChainNV,*OpRawAccessChainNV*>>."

Modify section 2.6.2. Validation Rules for Shader Capabilities:

Add text to the decorations Restrict, Aliased, Volatile, Coherent, NonWritable, and NonReadable specifying that they can also be used on the result of OpRawAccessChainNV.

Capabilities

Modify Section 3.31, "Capability", adding these rows to the Capability table:

Capability	Depends On
5414	RawAccessChainNV Allow the instruction OpRawAccessChainNV, and raw access chain operands RawAccessChainRobustnessPerComponentNV and RawAccessChainRobustnessPerElementNV.	Shader

Capability

Depends On

5414

RawAccessChainNV
Allow the instruction OpRawAccessChainNV, and raw access chain operands RawAccessChainRobustnessPerComponentNV and RawAccessChainRobustnessPerElementNV.

Shader

Instructions

Add the following instruction:

OpRawAccessChainNV

Create a pointer into a composite object with a type different than Base's pointer type.
Result type must be an OpTypePointer. Its Storage Class operand must be the same as the Storage Class of Base. The pointee type of Result type must also not be OpTypeArray, OpTypeMatrix, or OpTypeStruct.
Base's type must be an OpTypePointer. The storage class must be StorageBuffer, PhysicalStorageBuffer, or Uniform. If StorageBuffer, the pointee type of Base, or if it points to an OpTypeArray, the pointee type of the array it points to, must be decorated with Block. If Uniform, they must be decorated with BufferBlock.
Stride must be a scalar OpConstant of integer type. It is treated as an unsigned value.
Index and Offset must be a scalar integer type with a 32-bit width. They are treated as unsigned values.
If the product of Stride and Index would overflow, or the addition of Offset to that result would overflow, the behavior is implementation defined but it is not allowed to fault. If Stride is not zero, Offset plus the size of the pointee type must be less than or equal to Stride.
The returned pointer is calculated using Base’s byte address, adding Offset, and the product of Stride and Index.
If the optional operand Raw Access Chain Operands is not provided, the default value of None is used.
The result must only be consumed by OpLoad and OpStore.

Capability:
RawAccessChainsNV

5398

<id>
Result Type

Result <id>

<id>
Base

<id>
Stride

<id>
Index

<id>
Offset

Optional
Raw Access Chain
Operands

Modify the OpLoad and OpStore instructions to add:

If Pointer is the result of a OpRawAccessChainNV instruction, a valid Aligned memory operand must be defined.
This alignment must be at least the size of the component.

Raw Access Chain Operands

At the end of Section 3 "Binary Form", add:

Raw Access Chain Operands	Enabling Capabilities
0x0	None
0x1	RobustnessPerComponentNV Consumers of this access chain will bounds check each scalar using the computed offset. The behavior for out of bounds accesses is specified by the host environment. Must not be used on a pointer with storage class PhysicalStorageBuffer. Must not be used with RobustnessPerElementNV.	RawAccessChainsNV
0x2	RobustnessPerElementNV Consumers of this access chain will bounds check using the product of Stride and Index as access offset for the whole operation. Stride must not be zero. The behavior for out of bounds accesses is specified by the host environment. The implementation may assume any offset within Index is either all in-bounds or all out-of-bounds. Must not be decorated on a pointer with storage class PhysicalStorageBuffer. Must not be used with RobustnessPerComponentNV.	RawAccessChainsNV

Raw Access Chain Operands

Enabling Capabilities

0x0

None

0x1

RobustnessPerComponentNV
Consumers of this access chain will bounds check each scalar using the computed offset.
The behavior for out of bounds accesses is specified by the host environment.
Must not be used on a pointer with storage class PhysicalStorageBuffer.
Must not be used with RobustnessPerElementNV.

RawAccessChainsNV

0x2

RobustnessPerElementNV
Consumers of this access chain will bounds check using the product of Stride and Index as access offset for the whole operation. Stride must not be zero.
The behavior for out of bounds accesses is specified by the host environment.
The implementation may assume any offset within Index is either all in-bounds or all out-of-bounds.
Must not be decorated on a pointer with storage class PhysicalStorageBuffer.
Must not be used with RobustnessPerComponentNV.

RawAccessChainsNV

Revision History

Rev	Date	Author	Changes
1	2023-07-03	Rodrigo Locatti	Initial revision.
2	2023-07-06	Hans-Kristian Arntzen	Misc refinements.
3	2023-07-07	Rodrigo Locatti	Add support for memory decorations to OpRawAccessChainEXT. Clarify stride restrictions for bounds checking decorations. Define behavior for overflows. Offset must be smaller than Stride.
4	2023-07-12	Rodrigo Locatti	Define overflows on Offset.
5	2023-08-29	Rodrigo Locatti	Rename extension to SPV_NV_raw_access_chains.
6	2023-09-29	Rodrigo Locatti	Allow per-component bounds checking for non-zero strides. Move robustness decorations to an operand in the OpRawAccessChainNV. Remove support for ignoring Index if Stride is zero. Define enums.
7	2024-01-17	Rodrigo Locatti	Remove mentions to old decorations. Fix OpRawAccessChainsNV pointer type rules for Base. Explicitly mention overflow does not allow faulting. Give the optional operand a default value. Minor wording changes.