Legalize the Entry-point for WGSL (#5498)

* Legalize the Entry-point for WGSL

The return type of the entry-point needs to be legalized when targeting
WGSL.
This commit flattens the nested-structs of the return type and the input
parameters of the entry-point.

Most of code is copied from the legalization code for Metal. The
following functions are exactly same to the implementation for Metal or
almost same.
- flattenInputParameters() : 136 lines
- reportUnsupportedSystemAttribute() : 7 lines
- ensureResultStructHasUserSemantic() : 46 lines
- struct MapStructToFlatStruct : 176 lines
- flattenNestedStructs() : 95 lines
- maybeFlattenNestedStructs() : 42 lines
- _replaceAllReturnInst() : 19 lines
- _returnNonOverlappingAttributeIndex() : 16 lines
- _replaceAttributeOfLayout() : 23 lines
- tryConvertValue() : 41 lines
- legalizeSystemValueParameters() : 11 lines

They need to be refactored to reduce the duplication later.

The test case, `tests/compute/assoctype-lookup.slang`, had a bug that
the compute shader was trying to use the varying input/output with the
user defined semantics.

This commit removes the user defined semantics, because the compute
shaders cannot use the user defined semantics.

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

2 files changed, 56 insertions, 3 deletions

diff --git a/tests/compute/assoctype-lookup.slang b/tests/compute/assoctype-lookup.slang
index 348391e21..8a032528b 100644
--- a/tests/compute/assoctype-lookup.slang
+++ b/tests/compute/assoctype-lookup.slang
@@ -16,8 +16,8 @@ struct StandardBoneWeightSet : IBoneWeightSet
 {

     struct PackedType

     {

-        uint boneIds : BONEIDS;

-        uint boneWeights : BONEWEIGHTS;

+        uint boneIds;

+        uint boneWeights;

     };

     PackedType field;

 };

@@ -55,4 +55,4 @@ void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
     int inputVal = int(tid);

     int outputVal = test(inputVal);

     gOutputBuffer[tid] = outputVal;

-}
\ No newline at end of file
+}

diff --git a/tests/wgsl/nested-varying-input.slang b/tests/wgsl/nested-varying-input.slang
new file mode 100644
index 000000000..2cdf4f7eb
--- /dev/null
+++ b/tests/wgsl/nested-varying-input.slang
@@ -0,0 +1,53 @@
+//TEST:SIMPLE(filecheck=VERT): -target wgsl -stage vertex -entry vertexMain
+//TEST:SIMPLE(filecheck=FRAG): -target wgsl -stage fragment -entry fragmentMain
+
+// Tests three aspects:
+//  1. Flatten the nested struct for the return type of the entry-point
+//  2. For fragment shader, SV_TARGET index must be emitted as @location(index)
+//  3. For non-fragment shader, the user defined semantics should be emitted as @location(index)
+
+struct FragmentOutput
+{
+    //FRAG: @location(1) color1
+    float4 color1 : SV_TARGET1;
+
+    //FRAG: @location(0) color0
+    float4 color0 : SV_TARGET0;
+};
+
+struct NestedVertexOutput
+{
+    float4 color : COLOR0;
+};
+
+struct VertexOutput
+{
+    //VERT: @builtin(position) position
+    //FRAG: @builtin(position) position
+    float4 position : SV_Position;
+
+    //VERT: @location(0) uv
+    //FRAG: @location(0) uv
+    float2 uv : TEXCOORD0;
+
+    //VERT: @location(1) color
+    //FRAG: @location(1) color
+    NestedVertexOutput nested;
+};
+
+VertexOutput vertexMain()
+{
+    VertexOutput out;
+    out.position = float4(1.0, 1.0, 1.0, 1.0);
+    out.uv = float2(0.5, 0.5);
+    out.nested.color = float4(0.0, 0.0, 0.0, 0.0);
+    return out;
+}
+
+FragmentOutput fragmentMain(VertexOutput input)
+{
+    FragmentOutput out;
+    out.color0 = input.nested.color;
+    out.color1 = input.nested.color;
+    return out;
+}