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// Copyright 2021-2024 The Khronos Group, Inc.
//
// SPDX-License-Identifier: CC-BY-4.0
= VK_KHR_shader_expect_assume
:toc: left
:refpage: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/
:sectnums:
This document proposes adding support for expect/assume SPIR-V instructions
to guide shader program optimizations.
== Problem Statement
Shader writers or generators as well as other SPIR-V producers (e.g. Machine
Learning compilers) often have access to information that could enable the SPIR-V
consumers in Vulkan implementations to make better optimization decisions, such
as knowledge of the likely value of objects or whether a given condition holds,
but which they cannot communicate to a Vulkan SPIR-V consumer using existing features.
== Solution Space
SPIR-V already provides some mechanisms for producers to give hints to consumers
in a limited number of scenarios:
- `OpBranchConditional` can accept branch weights that enable producers to
indicate the likelihood of each path. This does not however generalize
to `OpSwitch` constructs.
- Various so called _Loop Controls_ make it possible for producers to provide
metadata about the iteration count of loops or desired unrolling behaviour.
There is however no exposed generic mechanism for SPIR-V producers to communicate
optimisation information to consumers. SPIR-V does support dedicated instructions,
introduced by the
http://htmlpreview.github.io/?https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/KHR/SPV_KHR_expect_assume.html[SPV_KHR_expect_assume]
extension, that make it possible for producers to communicate to consumers the
likely value of an object or whether a given condition holds, but this extension
is currently not exposed in Vulkan.
== Proposal
Expose the
http://htmlpreview.github.io/?https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/KHR/SPV_KHR_expect_assume.html[SPV_KHR_expect_assume]
extension in Vulkan.
The `SPV_KHR_expect_assume` extension introduces two new instructions:
- `OpExpectKHR` makes it possible to state the most probable value of its input.
- `OpAssumeTrueKHR` enables the optimizer to assume that the provided condition is
always true.
== Examples
As an illustration, consider the following pseudocode example:
[source]
----
c = 20
d = 2
b = c / d
if (a - b > 0) {
...
} else {
...
}
----
The writer or producer may know that a > 10. This knowledge makes it possible
to completely remove the `else` branch. In this case, the producer could perform
that optimisation alone. However, if the producer only knows that `a` is greater
than _some_ value provided, say with a specialization constant, it can no longer
perform the optimisation. Adding that information to the SPIR-V module would
enable the SPIR-V consumer to do it.
Another possible use could be to provide guarantees that a particular value
is not NaN or infinite:
[source]
----
value = load(...)
assume(!isnan(value))
----
== Issues
1) What shader stages should the instructions introduced by this extension
be allowed in?
*PROPOSED*: No restrictions are placed on the shader stages the instructions can
be used in.
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