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).

2 files changed, 51 insertions, 0 deletions

diff --git a/tests/cross-compile/vk-push-constant-set.slang b/tests/cross-compile/vk-push-constant-set.slang
new file mode 100644
index 000000000..ee9c68ed8
--- /dev/null
+++ b/tests/cross-compile/vk-push-constant-set.slang
@@ -0,0 +1,23 @@
+// vk-push-constant.slang
+
+// Test to confirm that a `[[vk::push_constant]]` buffer
+// doesn't end up reserving `space=0` for global scope
+// parameters and shifting a parameer block over to
+// `space=1`.
+
+//TEST:CROSS_COMPILE:-target spirv-assembly -entry main -stage fragment
+
+struct S
+{
+	float4 v;
+}
+
+[[vk::push_constant]]
+ConstantBuffer<S> x;
+
+ParameterBlock<S> y;
+
+float4 main() : SV_Target
+{
+    return x.v + y.v;
+}
diff --git a/tests/cross-compile/vk-push-constant-set.slang.glsl b/tests/cross-compile/vk-push-constant-set.slang.glsl
new file mode 100644
index 000000000..189b14caf
--- /dev/null
+++ b/tests/cross-compile/vk-push-constant-set.slang.glsl
@@ -0,0 +1,28 @@
+// vk-push-constant.slang.glsl
+#version 450
+
+struct S_0
+{
+    vec4 v_0;
+};
+
+layout(push_constant)
+layout(std140) uniform _S1
+{
+    S_0 _data;
+} x_0;
+
+layout(binding = 0, set = 0)
+layout(std140) uniform _S2
+{
+    S_0 _data;
+} y_0;
+
+layout(location = 0)
+out vec4 _S3;
+
+void main()
+{
+    _S3 = x_0._data.v_0 + y_0._data.v_0;
+    return;
+}