[cfe-dev] __maybe_unused for conditional struct field?

[Vincent Li via cfe-dev]

Hi,

I could be not making  sense here since I am newbie to compiler and
not even sure if this is a problem that compiler should care to
address

I ran into this
https://github.com/cilium/cilium/pull/17370#discussion_r731170618

where the suggestion to add conditional field like below,


 struct remote_endpoint_info {
        __u32           sec_label;
        __u32           tunnel_endpoint;
#ifdef ENABLE_VTEP
        mac_t           vtep_mac;
#endif
        __u8            key;
 };

but it cause misalignment issue with golang struct below:

// RemoteEndpointInfo implements the bpf.MapValue interface. It contains the
// security identity of a remote endpoint.
// +k8s:deepcopy-gen=true
   +k8s:deepcopy-gen:interfaces=github.com/cilium/cilium/pkg/bpf.MapValue
type RemoteEndpointInfo struct {
        SecurityIdentity uint32        `align:"sec_label"`
        TunnelEndpoint   types.IPv4    `align:"tunnel_endpoint"`
        VtepMAC          mac.Uint64MAC `align:"vtep_mac"`
        Key              uint8         `align:"key"`
}

so out of my imagination without understanding things going on
underneath, I attempted to use __maybe_unused since I saw it being
used for stack variable,

 struct remote_endpoint_info {
        __u32           sec_label;
        __u32           tunnel_endpoint;
       mac_t           vtep_mac __maybe_unused;
        __u8            key;
 };

the clang llvm did not complain error, but it has no effect on the
struct memory layout neither, so I wonder if __maybe_unused to struct
field is a thing, or if I am looking for wrong solution for the
problem, appreciate any input

Thanks

Vincent