[SPIRV] Round the stride to be a multiple of the alignment. (microsoft#6967)

We currently pick the size of a struct to be the stride of the array
elements when doing scalar layout. This is not correct because this
could cause the struct to not be correctly aligned.

This is fixed by rounding the size of the struct up to a mutliple of the
alignment.

Fixes microsoft#6947

Author: s-perron

tools/clang/lib/SPIRV/AlignmentSizeCalculator.cpp

@@ -22,6 +22,15 @@ inline uint32_t roundToPow2(uint32_t val, uint32_t pow2) {
   return (val + pow2 - 1) & ~(pow2 - 1);
 }
 
+/// Returns the smallest value greater than or equal to |val| that is a multiple
+/// of |multiple|.
+inline uint32_t roundToMultiple(uint32_t val, uint32_t multiple) {
+  if (val == 0)
+    return 0;
+  uint32_t t = (val - 1) / multiple;
+  return (multiple * (t + 1));
+}
+
 /// Returns true if the given vector type (of the given size) crosses the
 /// 4-component vector boundary if placed at the given offset.
 bool improperStraddle(clang::QualType type, int size, int offset) {
@@ -411,7 +420,7 @@ std::pair<uint32_t, uint32_t> AlignmentSizeCalculator::getAlignmentAndSize(
 
     if (rule == SpirvLayoutRule::FxcSBuffer ||
         rule == SpirvLayoutRule::Scalar) {
-      *stride = size;
+      *stride = roundToMultiple(size, alignment);
       // Use element alignment for fxc structured buffers and
       // VK_EXT_scalar_block_layout
       return {alignment, size * elemCount};

tools/clang/test/CodeGenSPIRV/array.scalar.layout.hlsl

@@ -0,0 +1,21 @@
+// RUN: %dxc -T cs_6_2 -E main %s -fvk-use-scalar-layout -spirv | FileCheck %s
+
+// Check that the array stride and offsets are corrects. The uint64_t has alignment
+// 8 and the struct has size 12. So the stride should be the smallest multiple of 8
+// greater than or equal to 12, which is 16.
+
+// CHECK-DAG: OpMemberDecorate %Data 0 Offset 0
+// CHECK-DAG: OpMemberDecorate %Data 1 Offset 8
+// CHECK-DAG: OpDecorate %_runtimearr_Data ArrayStride 16
+// CHECK-DAG: OpMemberDecorate %type_RWStructuredBuffer_Data 0 Offset 0
+struct Data {
+    uint64_t y;
+    uint x;
+};
+RWStructuredBuffer<Data> buffer;
+
+[numthreads(1, 1, 1)]
+void main()
+{
+    buffer[0].x = 5;
+}