gosl

IMPORTANT UPDATE: Cogent Core now has an improved version of gosl in its gpu/gosl package and associated sub-packages. This version will not be further maintained or developed. The v1 version is still needed for the v1 version of emergent.

gosl implements Go as a shader language for GPU compute shaders: converts Go code to HLSL, and then uses the glslc compiler (e.g., from a vulkan package) to compile into an .spv SPIR-V file that can be loaded into a vulkan compute shader. glslc must be installed!

Thus, gosl enables the same CPU-based Go code to also be run on the GPU. The relevant subsets of Go code to use are specifically marked using //gosl: comment directives, and this code must only use basic expressions and concrete types that will compile correctly in a shader (see Restrictions below). Method functions and pass-by-reference pointer arguments to struct types are supported and incur no additional compute cost due to inlining (see notes below for more detail).

See examples/basic and examples/axon examples (simple and much more complicated, respectively), using the vgpu Vulkan-based GPU compute shader system.

You must also install goimports which is used on the extracted subset of Go code:

$ go install golang.org/x/tools/cmd/goimports@latest

To install the gosl command, do:

$ go install github.com/emer/gosl/v2@latest

In your Go code, use these comment directives:

//gosl: start <filename>

< Go code to be translated >

//gosl: end <filename>

to bracket code to be processed. The resulting converted code is copied into a shaders subdirectory created under the current directory where the gosl command is run, using the filenames specified in the comment directives. Each such filename should correspond to a complete shader program (i.e., a "kernel"), or a file that can be included into other shader programs. Code is appended to the target file names in the order of the source .go files on the command line, so multiple .go files can be combined into one resulting HLSL file.

HLSL specific code, e.g., for the main compute function or to specify #include files, can be included either by specifying files with a .hlsl extension as arguments to the gosl command, or by using a //gosl: hlsl comment directive as follows:

//gosl: hlsl <filename>

// <HLSL shader code to be copied>

//gosl: end <filename>

where the HLSL shader code is commented out in the .go file -- it will be copied into the target filename and uncommented. The HLSL code can be surrounded by /* */ comment blocks (each on a separate line) for multi-line code (though using a separate .hlsl file is preferable in this case).

For .hlsl files, their filename is used to determine the shaders destination file name, and they are automatically appended to the end of the corresponding .hlsl file generated from the Go files -- this is where the main function and associated global variables should be specified.

IMPORTANT: all .go, .hlsl, and .spv files are removed from the shaders directory prior to processing to ensure everything there is current -- always specify a different source location for any custom .hlsl files that are included.

Usage

gosl [flags] [path ...]

The flags are:

-exclude string
	comma-separated list of names of functions to exclude from exporting to HLSL (default "Update,Defaults")
-out string
	output directory for shader code, relative to where gosl is invoked (default "shaders")
-keep
	keep temporary converted versions of the source files, for debugging

Note: any existing .go files in the output directory will be removed prior to processing, because the entire directory is built to establish all the types, which might be distributed across multiple files. Any existing .hlsl files with the same filenames as those extracted from the .go files will be overwritten. Otherwise, you can maintain other custom .hlsl files in the shaders directory, although it is recommended to treat the entire directory as automatically generated, to avoid any issues.

gosl path args can include filenames, directory names, or Go package paths (e.g., cogentcore.org/core/math32/fastexp.go loads just that file from the given package) -- files without any //gosl: comment directives will be skipped up front before any expensive processing, so it is not a problem to specify entire directories where only some files are relevant. Also, you can specify a particular file from a directory, then the entire directory, to ensure that a particular file from that directory appears first -- otherwise alphabetical order is used. gosl ensures that only one copy of each file is included.

Any struct types encountered will be checked for 16-byte alignment of sub-types and overall sizes as an even multiple of 16 bytes (4 float32 or int32 values), which is the alignment used in HLSL and glsl shader languages, and the underlying GPU hardware presumably. Look for error messages on the output from the gosl run. This ensures that direct byte-wise copies of data between CPU and GPU will be successful. The fact that gosl operates directly on the original CPU-side Go code uniquely enables it to perform these alignment checks, which are otherwise a major source of difficult-to-diagnose bugs.

Restrictions

In general shader code should be simple mathematical expressions and data types, with minimal control logic via if, for statements, and only using the subset of Go that is consistent with C. Here are specific restrictions:

Types

• Can only use float32, [u]int32, and their 64 bit versions for basic types, and struct types composed of these same types -- no other Go types (i.e., map, slices, string, etc) are compatible. There are strict alignment restrictions on 16 byte (e.g., 4 float32's) intervals that are enforced via the alignsl sub-package.

• Use slbool.Bool instead of bool -- it defines a Go-friendly interface based on a int32 basic type. Using a bool in a uniform struct causes an obscure glslc compiler error: shaderc: internal error: compilation succeeded but failed to optimize: OpFunctionCall Argument <id> '73[%73]'s type does not match Function

• Alignment and padding of struct fields is key -- this is automatically checked by gosl.

• HLSL does not support enum types, but standard go const declarations will be converted. Use an int32 or uint32 data type. It will automatically deal with the simple incrementing iota values, but not more complex cases. Also, for bitflags, define explicitly, not using bitflags package.

• HLSL does not do multi-pass compiling, so all dependent types must be specified before being used in other ones, and this also precludes referencing the current type within itself. todo: can you just use a forward declaration?

• HLSL does not provide the same auto-init-to-zero for declared variables -- safer to initialize directly:

    val := float32(0) // guaranteed 0 value
    var val float32 // not guaranteed to be 0!  avoid!

Syntax

• Cannot use multiple return values, or multiple assignment of variables in a single = expression.

• Can use multiple variable names with the same type (e.g., min, max float32) -- this will be properly converted to the more redundant C form with the type repeated.

Random numbers: slrand

See slrand for a shader-optimized random number generation package, which is supported by gosl -- it will convert slrand calls into appropriate HLSL named function calls. gosl will also copy the slrand.hlsl file, which contains the full source code for the RNG, into the destination shaders directory, so it can be included with a simple local path:

//gosl: hlsl mycode
// #include "slrand.hlsl"
//gosl: end mycode

Performance

With sufficiently large N, and ignoring the data copying setup time, around ~80x speedup is typical on a Macbook Pro with M1 processor. The rand example produces a 175x speedup!

Implementation / Design Notes

HLSL is very C-like and provides a much better target for Go conversion than glsl. See examples/basic/shaders/basic_nouse.glsl vs the .hlsl version there for the difference. Only HLSL supports methods in a struct, and performance is the same as writing the expression directly -- it is suitably inlined.

While there aren't any pointers allowed in HLSL, the inlining of methods, along with the use of the inout InputModifier, effectively supports pass-by-reference. The stackoverflow on this is a bit unclear but the basic example demonstrates that it all goes through.

Links

Key docs for HLSL as compute shaders:

• https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst

• https://www.saschawillems.de/blog/2020/05/23/shaders-for-vulkan-samples-now-also-available-in-hlsl/