spirv-opt: fix copy-propagate-arrays index opti on structs.

As per SPIR-V spec:
OpAccessChain indices must be OpConstant when indexing into a structure.

This optimization tried to remove load cascade. But in some scenario
failed:

```c
cbuffer MyStruct {
    uint my_field;
};

uint main(uint index) {
    const uint my_array[1] = { my_field };
    return my_array[index]
}
```

This is valid as the struct is indexed with a constant index, and then
the array is indexed using a dynamic index.
The optimization would consider the local array to be useless and
generated a load directly into the struct.

Fixes #4887

Signed-off-by: Nathan Gauër <brioche@google.com>

source/opt/copy_prop_arrays.cpp

@@ -523,6 +523,12 @@ bool CopyPropagateArrays::CanUpdateUses(Instruction* original_ptr_inst,
             // Variable index means the type is a type where every element
             // is the same type.  Use element 0 to get the type.
             access_chain.push_back(0);
+
+            // We are trying to access a struct with variable indices.
+            // This cannot happen.
+            if (pointee_type->kind() == analysis::Type::kStruct) {
+              return false;
+            }
           }
         }
 

test/opt/copy_prop_array_test.cpp

@@ -1884,6 +1884,65 @@ OpFunctionEnd
   SetTargetEnv(SPV_ENV_UNIVERSAL_1_4);
   SinglePassRunAndMatch<CopyPropagateArrays>(before, false);
 }
+
+// As per SPIRV spec, struct cannot be indexed with non-constant indices
+// through OpAccessChain, only arrays.
+// The copy-propagate-array pass tries to remove superfluous copies when the
+// original array could be indexed instead of the copy.
+//
+// This test verifies we handle this case:
+//  struct SRC { int field1; ...; int fieldN }
+//  int tmp_arr[2] = { SRC.field1, ..., SRC.fieldN }
+//  return tmp_arr[index];
+//
+// In such case, we cannot optimize the access: this array was added to allow
+// dynamic indexing in the struct.
+TEST_F(CopyPropArrayPassTest, StructIndexCannotBecomeDynamic) {
+  const std::string text = R"(OpCapability Shader
+OpMemoryModel Logical GLSL450
+OpEntryPoint Vertex %1 "main"
+OpDecorate %2 DescriptorSet 0
+OpDecorate %2 Binding 0
+OpMemberDecorate %_struct_3 0 Offset 0
+OpDecorate %_struct_3 Block
+%int = OpTypeInt 32 1
+%int_0 = OpConstant %int 0
+%float = OpTypeFloat 32
+%v4float = OpTypeVector %float 4
+%_struct_3 = OpTypeStruct %v4float
+%_ptr_Uniform__struct_3 = OpTypePointer Uniform %_struct_3
+%uint = OpTypeInt 32 0
+%void = OpTypeVoid
+%11 = OpTypeFunction %void
+%_ptr_Function_uint = OpTypePointer Function %uint
+%13 = OpTypeFunction %v4float %_ptr_Function_uint
+%uint_1 = OpConstant %uint 1
+%_arr_v4float_uint_1 = OpTypeArray %v4float %uint_1
+%_ptr_Function__arr_v4float_uint_1 = OpTypePointer Function %_arr_v4float_uint_1
+%_ptr_Function_v4float = OpTypePointer Function %v4float
+%_ptr_Uniform_v4float = OpTypePointer Uniform %v4float
+%2 = OpVariable %_ptr_Uniform__struct_3 Uniform
+%19 = OpUndef %v4float
+%1 = OpFunction %void None %11
+%20 = OpLabel
+OpReturn
+OpFunctionEnd
+%21 = OpFunction %v4float None %13
+%22 = OpFunctionParameter %_ptr_Function_uint
+%23 = OpLabel
+%24 = OpVariable %_ptr_Function__arr_v4float_uint_1 Function
+%25 = OpAccessChain %_ptr_Uniform_v4float %2 %int_0
+%26 = OpLoad %v4float %25
+%27 = OpCompositeConstruct %_arr_v4float_uint_1 %26
+OpStore %24 %27
+%28 = OpLoad %uint %22
+%29 = OpAccessChain %_ptr_Function_v4float %24 %28
+OpReturnValue %19
+OpFunctionEnd
+)";
+
+  SinglePassRunAndCheck<CopyPropagateArrays>(text, text, false);
+}
 }  // namespace
 }  // namespace opt
 }  // namespace spvtools