Name Strings

SPV_EXT_demote_to_helper_invocation

Contact

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

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

Contributors

  • Jeff Bolz, NVIDIA Corporation

  • Alan Baker, Google LLC

Notice

Copyright (c) 2019 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html

Status

  • Complete.

Version

Last Modified Date

2019-06-06

Revision

1

Dependencies

This extension is written against the SPIR-V Specification, Version 1.4 Revision 1.

This extension requires SPIR-V 1.0.

Overview

This extension adds a new OpDemoteToHelperInvocationEXT instruction which is similar to OpKill in that it suppresses subsequent stores and writes to outputs, but is not a flow control instruction and does not necessarily terminate the shader invocation. This is a better match for D3D’s discard instruction, and preserves the ability to rely on uniform flow control for derivatives after the discard.

This extension also adds a new OpIsHelperInvocationEXT instruction which returns whether the invocation is currently a helper invocation. That is, at the beginning of a fragment shader invocation it returns the same value as the HelperInvocation input, and after demotion it returns true. The HelperInvocation builtin decoration is used on a variable in the Input storage class, and it wouldn’t make sense for an input variable’s value to change over the course of the invocation’s execution.

Extension Name

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

OpExtension "SPV_EXT_demote_to_helper_invocation"

Modifications to the SPIR-V Specification, Version 1.4

Capabilities

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

Capability Enabling Capabilities

5379

DemoteToHelperInvocationEXT
Allow the OpDemoteToHelperInvocationEXT and OpIsHelperInvocationEXT instructions

Shader

Instructions

Modify Section 3.32.1, "Miscellaneous Instructions", adding the new instructions:

OpDemoteToHelperInvocationEXT

Demote fragment shader invocation to a helper invocation. Any stores to memory after this instruction are suppressed and the fragment does not write outputs to the framebuffer.

Unlike the OpKill instruction, this does not necessarily terminate the invocation. It is not considered a flow control instruction (flow control does not become non-uniform) and does not terminate the block. The implementation may terminate helper invocations before the end of the shader as an optimization, but doing so must not affect derivative calculations and does not make control flow non-uniform.

After this instruction executes, the value of a HelperInvocation builtin variable is undefined. Use OpIsHelperInvocationEXT to determine whether invocations are helper invocations in the presence of OpDemoteToHelperInvocationEXT.

This instruction is only valid in the Fragment Execution Model.

Capability:
DemoteToHelperInvocationEXT

1

5380

OpIsHelperInvocationEXT

Result is true if the invocation is currently a helper invocation, otherwise result is false. An invocation is currently a helper invocation if it was originally invoked as a helper invocation or if it has been demoted to a helper invocation by OpDemoteToHelperInvocationEXT.

Result Type must be a Boolean type scalar.

This instruction is only valid in the Fragment Execution Model.

Capability:
DemoteToHelperInvocationEXT

3

5381

<id> Result Type

<id> Result

Issues

None.

Revision History

Rev Date Author Changes

1

2019-06-06

Jeff Bolz

Initial revision