Fix incorrect OpAccessChain for inherited struct members (#8005)

Adds a new test case to verify that the OpAccessChain is generated
correctly for inherited members of a struct.

This fixes an issue where the indices for a base class were being
prepended to the indices of the derived class, resulting in an incorrect
access chain.

Fixes #7968

Author: s-perron

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -8861,6 +8861,11 @@ const Expr *SpirvEmitter::collectArrayStructIndices(
         }
       } else if (castExpr->getCastKind() == CK_UncheckedDerivedToBase ||
                  castExpr->getCastKind() == CK_HLSLDerivedToBase) {
+        // First the indices for the sub expression.
+        const Expr *base =
+            collectArrayStructIndices(castExpr->getSubExpr(), rawIndex,
+                                      rawIndices, indices, isMSOutAttribute);
+
         llvm::SmallVector<uint32_t, 4> BaseIdx;
         getBaseClassIndices(castExpr, &BaseIdx);
         if (rawIndex) {
@@ -8871,8 +8876,7 @@ const Expr *SpirvEmitter::collectArrayStructIndices(
                                                          llvm::APInt(32, Idx)));
         }
 
-        return collectArrayStructIndices(castExpr->getSubExpr(), rawIndex,
-                                         rawIndices, indices, isMSOutAttribute);
+        return base;
       }
       return collectArrayStructIndices(castExpr->getSubExpr(), rawIndex,
                                        rawIndices, indices, isMSOutAttribute);

tools/clang/test/CodeGenSPIRV/spirv-codegen-struct-inheritance.hlsl

@@ -0,0 +1,25 @@
+// RUN: %dxc -T cs_6_0 -E main -spirv %s | FileCheck %s
+
+struct X { int a1; };
+struct Y : X { int2 a2; };
+struct Z {
+  X xs[2];
+  Y y;
+};
+
+cbuffer CBStructs : register(b0) {
+  Z z;
+};
+RWStructuredBuffer<int> Out : register(u1);
+
+[numthreads(1,1,1)]
+void main() {
+// CHECK: OpAccessChain
+// CHECK-SAME: %_ptr_Uniform_int
+// CHECK-SAME: %CBStructs
+// CHECK-SAME: %int_0
+// CHECK-SAME: %int_1
+// CHECK-SAME: %int_0
+// CHECK-SAME: %int_0
+ Out[0] = z.y.a1;
+}