This adds structure splitting, which among other things will enable GS support where input structs
are passed, and thus become input arrays of structs in the GS inputs. That is a common GS case.
The salient points of this PR are:
* Structure splitting has been changed from "always between stages" to "only into the VS and out of
the PS". It had previously happened between stages because it's not legal to pass a struct
containing a builtin IO variable.
* Structs passed between stages are now split into a struct containing ONLY user types, and a
collection of loose builtin IO variables, if any. The user-part is passed as a normal struct
between stages, which is valid SPIR-V now that the builtin IO is removed.
* Internal to the shader, a sanitized struct (with IO qualifiers removed) is used, so that e.g,
functions can work unmodified.
* If a builtin IO such as Position occurs in an arrayed struct, for example as an input to a GS,
the array reference is moved to the split-off loose variable, which is given the array dimension
itself.
When passing things around inside the shader, such as over a function call, the the original type
is used in a sanitized form that removes the builtIn qualifications and makes them temporaries.
This means internal function calls do not have to change. However, the type when returned from
the shader will be member-wise copied from the internal sanitized one to the external type.
The sanitized type is used in variable declarations.
When copying split types and unsplit, if a sub-struct contains only user variables, it is copied
as a single entity to avoid more AST verbosity.
Above strategy arrived at with talks with @johnkslang.
This is a big complex change. I'm inclined to leave it as a WIP until it can get some exposure to
real world cases.
This PR implements recursive type flattening. For example, an array of structs of other structs
can be flattened to individual member variables at the shader interface.
This is sufficient for many purposes, e.g, uniforms containing opaque types, but is not sufficient
for geometry shader arrayed inputs. That will be handled separately with structure splitting,
which is not implemented by this PR. In the meantime, that case is detected and triggers an error.
The recursive flattening extends the following three aspects of single-level flattening:
- Flattening of structures to individual members with names such as "foo[0].samp[1]";
- Turning constant references to the nested composite type into a reference to a particular
flattened member.
- Shadow copies between arrays of flattened members and the nested composite type.
Previous single-level flattening only flattened at the shader interface, and that is unchanged by this PR.
Internally, shadow copies are, such as if the type is passed to a function.
Also, the reasons for flattening are unchanged. Uniforms containing opaque types, and interface struct
types are flattened. (The latter will change with structure splitting).
One existing test changes: hlsl.structin.vert, which did in fact contain a nested composite type to be
flattened.
Two new tests are added: hlsl.structarray.flatten.frag, and hlsl.structarray.flatten.geom (currently
issues an error until type splitting is online).
The process of arriving at the individual member from chained postfix expressions is more complex than
it was with one level. See large-ish comment above HlslParseContext::flatten() for details.
steve-lunarg