Don't allocate a default space for a VK push-constant buffer (#1231)

When a shader only uses `ParameterBlock`s plus a single buffer for root constants:

```hlsl
ParameterBlock<A> a;
ParameterBlock<B> b;

[[vk::push_constant]] cbuffer Stuff { ... }
```

we expect the push-constant buffer should not affect the `space` allocated to the parameter blocks (so `a` should get `space=0`).

This behavior wasn't being implemented correctly in `slang-parameter-binding.cpp`. There was logic to ignore certain resource kinds in entry-point parameter lists when computing whether a default space is needed, but the equivalent logic for the global scope only considered parameters that consuem whole register spaces/sets.

This change shuffles the code around and makes sure it considers a global push-constant buffer as *not* needing a default space/set.

Note that this change will have no impact on D3D targets, where `Stuff` above would always get put in `space0` because for D3D targets a push-constant buffer is no different from any other constant buffer in terms of register/space allocation.

One unrelated point that this change brings to mind is the `[[vk::push_constant]]` should ideally also be allowed to apply to an entry point (where it would modify the default/implicit constant buffer). In fact, it could be argued that push-constant allocation should be the *default* for (non-RT) entry point `uniform` parameters (while `[[vk::shader_record]]` should be the default for RT entry point `uniform` parameters).

1 files changed, 16 insertions, 8 deletions

diff --git a/source/slang/slang-parameter-binding.cpp b/source/slang/slang-parameter-binding.cpp
index 5cd89bd95..0268e1850 100644
--- a/source/slang/slang-parameter-binding.cpp
+++ b/source/slang/slang-parameter-binding.cpp
@@ -3343,14 +3343,7 @@ RefPtr<ProgramLayout> generateParameterBindings(
             //
             for (auto resInfo : varLayout->typeLayout->resourceInfos)
             {
-                // We don't care about whole register spaces/sets, since
-                // we don't need to allocate a default space/set for a parameter
-                // that itself consumes a whole space/set.
-                //
-                if( resInfo.kind == LayoutResourceKind::RegisterSpace )
-                    continue;
-
-                // We also don't want to consider resource kinds for which
+                // We don't want to consider resource kinds for which
                 // the variable already has an (explicit) binding, since
                 // the space from the explicit binding will be used, so
                 // that a default space isn't needed.
@@ -3358,6 +3351,21 @@ RefPtr<ProgramLayout> generateParameterBindings(
                 if( parameterInfo->bindingInfo[resInfo.kind].count != 0 )
                     continue;
 
+                // We also want to exclude certain resource kinds from
+                // consideration, since parameters using those resource
+                // kinds wouldn't be allocated into the default space
+                // anyway.
+                //
+                switch( resInfo.kind )
+                {
+                case LayoutResourceKind::RegisterSpace:
+                case LayoutResourceKind::PushConstantBuffer:
+                    continue;
+
+                default:
+                    break;
+                }
+
                 // Otherwise, we have a shader parameter that will need
                 // a default space or set to live in.
                 //