Fix support for `std430` layout

Fixes #57

There were a bunch of issues in how `std430` was being implemented, largely due to just stubbing it in without any test cases. This commit adds a reasonably good test case to ensure that we've got things basically working.

3 files changed, 186 insertions, 11 deletions

diff --git a/source/slang/type-layout.cpp b/source/slang/type-layout.cpp
index fbaec3736..10c7327ce 100644
--- a/source/slang/type-layout.cpp
+++ b/source/slang/type-layout.cpp
@@ -155,20 +155,32 @@ struct GLSLConstantBufferLayoutRulesImpl : DefaultConstantBufferLayoutRulesImpl
 {
 };
 
+// The `std140` and `std430` rules require vectors to be aligned to the next power of
+// two up from their size (so a `float2` is 8-byte aligned, and a `float3` is
+// 16-byte aligned).
+//
+// Note that in this case we have a type layout where the size is *not* a multiple
+// of the alignment, so it should be possible to pack a scalar after a `float3`.
+static SimpleLayoutInfo getGLSLVectorLayout(
+    SimpleLayoutInfo elementInfo, size_t elementCount)
+{
+    assert(elementInfo.kind == LayoutResourceKind::Uniform);
+    auto size = elementInfo.size * elementCount;
+    SimpleLayoutInfo vectorInfo(
+        LayoutResourceKind::Uniform,
+        size,
+        RoundUpToPowerOfTwo(size));
+    return vectorInfo;
+}
+
+// The `std140` rules combine the GLSL-specific layout for 3-vectors with the
+// alignment padding for structures and arrays that is common to both HLSL
+// and GLSL constant buffers.
 struct Std140LayoutRulesImpl : GLSLConstantBufferLayoutRulesImpl
 {
-    // The `std140` rules require vectors to be aligned to the next power of two
-    // up from their size (so a `float2` is 8-byte aligned, and a `float3` is
-    // 16-byte aligned).
     SimpleLayoutInfo GetVectorLayout(SimpleLayoutInfo elementInfo, size_t elementCount) override
     {
-        assert(elementInfo.kind == LayoutResourceKind::Uniform);
-        auto size = elementInfo.size * elementCount;
-        SimpleLayoutInfo vectorInfo(
-            LayoutResourceKind::Uniform,
-            size,
-            RoundUpToPowerOfTwo(size));
-        return vectorInfo;
+        return getGLSLVectorLayout(elementInfo, elementCount);
     }
 };
 
@@ -207,8 +219,15 @@ struct HLSLStructuredBufferLayoutRulesImpl : DefaultLayoutRulesImpl
     // TODO: customize these to be correct...
 };
 
-struct Std430LayoutRulesImpl : GLSLConstantBufferLayoutRulesImpl
+// The `std430` rules don't include the array/structure alignment padding that
+// gets applied to constant buffers, but they do include the padding of 3-vectors
+// to be aligned as 4-vectors.
+struct Std430LayoutRulesImpl : DefaultLayoutRulesImpl
 {
+    SimpleLayoutInfo GetVectorLayout(SimpleLayoutInfo elementInfo, size_t elementCount) override
+    {
+        return getGLSLVectorLayout(elementInfo, elementCount);
+    }
 };
 
 struct DefaultVaryingLayoutRulesImpl : DefaultLayoutRulesImpl
diff --git a/tests/reflection/std430-layout.glsl b/tests/reflection/std430-layout.glsl
new file mode 100644
index 000000000..5d4dee88f
--- /dev/null
+++ b/tests/reflection/std430-layout.glsl
@@ -0,0 +1,49 @@
+//TEST(smoke):SIMPLE:-profile ps_4_0 -target reflection-json
+
+// Confirm fix for GitHub issue #55
+
+struct Foo
+{
+	vec2 f;
+};
+
+layout(set = 0, binding = 0)
+buffer SomeBuffer
+{
+	// offset: 0, size: 4, alignment: 4
+    float a;
+
+    // offset: 16, size: 12, alignment: 16
+    //
+    // Note that it can't immediately follow `a`
+    // bcause of its alignment
+    vec3 b;
+
+    // offset: 28, size: 16, alignment: 4, stride: 4
+    //
+    // This array can be densely packed and 4-byte aligned under `std430` rules
+    float c[4];
+
+    // offset: 48, size: 8, alignment: 8
+    //
+    // This nees to be bumped up to a 8-byte aligned boundary
+    vec2 d;
+
+    // offset: 56, size: 8, alignment: 8
+    //
+    // This can come right after `d`, because `struct` types no longer
+    // get an artificial padding out to 16-byte alignment 
+    Foo e;
+
+    // offset: 64, size: 12: alignment: 16
+    vec3 g;
+
+    // offset: 76, size: 4, alignment: 4
+    //
+    // This can fit in the empty space after `g` because we allow
+    // the size of a `vec3` to be smaller than its alignment
+    float h;
+};
+
+void main()
+{}
diff --git a/tests/reflection/std430-layout.glsl.expected b/tests/reflection/std430-layout.glsl.expected
new file mode 100644
index 000000000..701a36909
--- /dev/null
+++ b/tests/reflection/std430-layout.glsl.expected
@@ -0,0 +1,107 @@
+result code = 0
+standard error = {
+}
+standard output = {
+{
+    "parameters": [
+        {
+            "name": "SomeBuffer",
+            "binding": {"kind": "descriptorTableSlot", "index": 0},
+            "type": {
+                "kind": "shaderStorageBuffer",
+                "elementType": {
+                    "kind": "struct",
+                    "fields": [
+                        {
+                            "name": "a",
+                            "type": {
+                                "kind": "scalar",
+                                "scalarType": "float32"
+                            },
+                            "binding": {"kind": "uniform", "offset": 0, "size": 4}
+                        },
+                        {
+                            "name": "b",
+                            "type": {
+                                "kind": "vector",
+                                "elementCount": 3,
+                                "elementType": {
+                                    "kind": "scalar",
+                                    "scalarType": "float32"
+                                }
+                            },
+                            "binding": {"kind": "uniform", "offset": 16, "size": 12}
+                        },
+                        {
+                            "name": "c",
+                            "type": {
+                                "kind": "array",
+                                "elementCount": 4,
+                                "elementType": {
+                                    "kind": "scalar",
+                                    "scalarType": "float32"
+                                },
+                                "uniformStride": 4
+                            },
+                            "binding": {"kind": "uniform", "offset": 28, "size": 16}
+                        },
+                        {
+                            "name": "d",
+                            "type": {
+                                "kind": "vector",
+                                "elementCount": 2,
+                                "elementType": {
+                                    "kind": "scalar",
+                                    "scalarType": "float32"
+                                }
+                            },
+                            "binding": {"kind": "uniform", "offset": 48, "size": 8}
+                        },
+                        {
+                            "name": "e",
+                            "type": {
+                                "kind": "struct",
+                                "fields": [
+                                    {
+                                        "name": "f",
+                                        "type": {
+                                            "kind": "vector",
+                                            "elementCount": 2,
+                                            "elementType": {
+                                                "kind": "scalar",
+                                                "scalarType": "float32"
+                                            }
+                                        },
+                                        "binding": {"kind": "uniform", "offset": 0, "size": 8}
+                                    }
+                                ]
+                            },
+                            "binding": {"kind": "uniform", "offset": 56, "size": 8}
+                        },
+                        {
+                            "name": "g",
+                            "type": {
+                                "kind": "vector",
+                                "elementCount": 3,
+                                "elementType": {
+                                    "kind": "scalar",
+                                    "scalarType": "float32"
+                                }
+                            },
+                            "binding": {"kind": "uniform", "offset": 64, "size": 12}
+                        },
+                        {
+                            "name": "h",
+                            "type": {
+                                "kind": "scalar",
+                                "scalarType": "float32"
+                            },
+                            "binding": {"kind": "uniform", "offset": 76, "size": 4}
+                        }
+                    ]
+                }
+            }
+        }
+    ]
+}
+}