The structurizer is implemented along the lines of what is usually called
the "structured program theorem": the control flow is completely
virtualized by mean of an additional TEMP register which stores the
block index which is currently running. The whole program is then
converted to a huge switch construction enclosed in a loop, executing
at each iteration the appropriate block and updating the register
depending on block jump instruction.
The algorithm's generality is also its major weakness: it accepts any
input program, even if its CFG is not reducible, but the output
program lacks any useful convergence information. It satisfies the
letter of the SPIR-V requirements, but it is expected that it will
be very inefficient to run on a GPU (unless a downstream compiler is
able to devirtualize the control flow and do a proper convergence
analysis pass). The algorithm is however very simple, and good enough
to at least pass tests, enabling further development. A better
alternative is expected to be upstreamed incrementally.
Side note: the structured program theorem is often called the
Böhm-Jacopini theorem; Böhm and Jacopini did indeed prove a variation
of it, but their algorithm is different from what is commontly attributed
to them and implemented here, so I opted for not using their name.
At the current moment this is a little odd because for SM1 [test]
directives are skipped, and the [shader] directives are not executed by
the shader_runner_vulkan.c:compile_shader() but by the general
shader_runner.c:compile_shader(). So in principle it is a little weird
that we go through the vulkan runner.
But fret not, because in the future we plan to make the parser agnostic
to the language of the tests, so we will get rid of the general
shader_runner.c:compile_shader() function and instead call a
runner->compile_shader() function, defined for each runner. Granted,
most of these may call a generic implementation that uses native
compiler in Windows, and vkd3d-shader on Linux, but it would be more
conceptually correct.
