Hi, On Wed, 17 Apr 2019, Richard Biener wrote:
> for the C++ FE the LRU cache effectively moves the unqualified > variants first in the variant list. Since we always first > build the unqualified variants before the qualified ones > the unqualified ones tend to be at the end of the list. That's > clearly bad for the C++ pattern of repeatedly looking up the > unqualified type variant from a type. Of course a direct > shortcut would be much cheaper here (but it obviously isn't > the main variant due to TYPE_NAME differences). > > So do you think the change to get_qualified_type is OK? Or > do we absolutely want to avoid changing the variant list from > a function like this? I think changing the variant list in this accessor should be okay. For it not to be okay some callers would have to remember a particular subset of that list and also care about the order of that subset. That would be fragile no matter what. I had the additional idea to only move the non-qualified variant to the front, i.e. not really LRU. By that we would slowly establish the invariant that unqualified variants are early in the list; or alternatively add a combination of build_variant_type_copy+set_type_quals which would establish that invariant directly. But unlike a real LRU cache it's harder to see if this brings similar benefits as the scheme is then lopsided towards the specific case of looking up unqualified variants. Ciao, Michael.
