char * ?

[abellgithub]

Can someone explain why RTLIL::IdString uses char * instead of std::string?

[whitequark]

It's deduplicated: each ID::A refers to the exact same storage, and as such they can be compared by comparing their pointer (extremely cheap) vs comparing their contents (moderately expensive).

[abellgithub]

For this to be of value the pointers have to be identical -- meaning that the strings have to be compile-time constant. When the strings are stored in global_id_storage_ they're dup'd, so the address of the stored string isn't going to be the same as that being looked up unless I'm missing something.

[abellgithub]

(They don't need to be compile-time constant, but they have to be the same memory.)

[QuantamHD]

General technique is called interning https://en.m.wikipedia.org/wiki/String_interning