74e6d2c575
This adds basic unrolling support for 2 syntax: - `[[gpu::unroll]]` which does full loop unrolling - `[[gpu::unroll(x)]]` which unrolls `x` iteration Nesting is supported. This change is motivated by the added cost in compilation and execution time that some loops have even if they have compile time defined iteration counts. The syntax is inspired by `GL_EXT_control_flow_attributes`. However, we might want to have our own prefix to show it is a blender specific feature and that it differs from the standard. I propose `[[gpu::unroll]]`. In the future, we could extend this to support more directives that can be expanded to backend specific extension / syntax. This would avoid readability issue an error prone copy paste of large amount of preprocessor directives. Currently, given that GL's GLSL flavor doesn't support any of these attributes, the preprocessor does some copy-pasting that does the unrolling at the source level. Note that the added `#line` allow for correct error logging. For the `[[gpu::unroll]]` syntax, the `for` declaration needs to follow a specific syntax to deduce the number of loop iteration. This variant removes the continue condition between iteration, so all iterations are evaluated. This could be modified using a special keyword. For the `[[gpu::unroll(n)]]` syntax, the usercode needs to make sure that `n` is large enough to cover all iterations as the loop is completely removed. We could add shader `assert` to make sure that there is never a remaining iteration. This behavior is usually different from what you see in other implementation as we do not keep a loop at all. Usually, compilers still keep the loop if it is not unrolled fully. But given we don't have IR, this is the best we can do. `break` and `continue` statement are forbidden at the unrolled loop scope level. Nested loop and switch can contain these keywords. This is accounted for by checks in the pre-processor. Only `for` loops are supported for now. There are no real incentive to add support for `while` given how rare it is in the shader codebase. Rel #137446 Pull Request: https://projects.blender.org/blender/blender/pulls/137444
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