From d35b4d4bc62beebaa56b949a1ac59470192b5dfc Mon Sep 17 00:00:00 2001
From: Tim Foley <tfoley@nvidia.com>
Date: Fri, 7 Jul 2017 08:48:45 -0700
Subject: 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.
---
 source/slang/type-layout.cpp | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

(limited to 'source')

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
-- 
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