zturner added a comment. In https://reviews.llvm.org/D39436#912902, @hintonda wrote:
> In https://reviews.llvm.org/D39436#912829, @clayborg wrote: > > > In https://reviews.llvm.org/D39436#912828, @zturner wrote: > > > > > In https://reviews.llvm.org/D39436#912810, @clayborg wrote: > > > > > > > I was unhappy when we went over two pointers for a FileSpec when > > > > m_syntax was added due to the extra size. Anything we can do to make > > > > this smaller would be great, so the type on the enum would work, but as > > > > you say the alignment will nullify that. The two ConstString members > > > > contain a pointer which isn't aligned so we can't use any bits from the > > > > low end of the pointer. Are there any classes that take advantage of > > > > high bits in pointers? Most if not all OS's don't use the entire 64 bit > > > > address space... It would be great to get lldb_private::FileSpec down > > > > to just 2 pointers again. > > > > > > > > > `ConstString` doesn't *currently* contain aligned pointers, but there's > > > no reason we couldn't make it contain aligned pointers. Then we could > > > use `llvm::PointerUnion`. > > > > > > I would be fine with that. > > > > > That said, I want to state again that I think this change is the wrong > > > direction. I don't think we need this functionality in `FileSpec`, or > > > even in another class. I think it is better served in the script. > > > > Agreed as well. I do like the idea of source path regexes, but only if we > > have a real need to add it to the API. > > > I haven't had time to really look into this, but it seems that maintaining > two independent strings, one for directory and one for basename, is just for > convenience. We could easily keep it in a single string with an index to > basename. When the directory portion is updated, e.g., resolved, etc., we > just overwrite the string and adjust the index. And adjust accessors as > needed. The reason it's two strings is for memory efficiency and de-duplication. Suppose you make `FileSpec` instances from `foo/bar/baz/` and `foo/bar/buzz`. This gets separated into 4 instances of `ConstString`. `foo/bar`, `baz`, `foo/bar`, and `buzz`. Because `ConstString`s are pooled, there's actually only 3 strings here. `foo/bar`, `baz`, and `buzz`. It probably doesn't seem like a lot, but over the course of thousands and thousands of files (which debuggers often examine and which often share a common parent directory) this is a large memory savings. https://reviews.llvm.org/D39436 _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
