Unaligned pointers in perconaFT

In omt.cc, we take an address of a data member in a templated function:

https://github.com/percona/PerconaFT/blob/416a28e514ddd8918465d186c44b408f426f621c/util/omt.cc#L762?

This function is instantiated with an instance of omt_node_templated:

https://github.com/percona/PerconaFT/blob/91273dbcebaa682c72b48bd0917ad2e4efc291d8/util/omt.h#L195

Where the omtdata_t member is a referenced_xid_tuple, which is an 8 byte aligned type currently.

Because of the packing/alignment of omt_node_templated, the value member is potentially unaligned, and taking the address of an unaligned data member is undefined behavior and a potential crash source on some platforms. Clang reports a warning about this.

We have several possible solutions:

• Silence the warning, as this shouldn't cause any issues on x86
• Remove the packing/alignment attributes from omt_node_templated
• Relax the alignment requirements of referenced_xid_tuple
• ?