On Thu, Jun 24, 2021 at 6:28 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes: > > On Wed, Jun 23, 2021 at 12:22 PM Richard Sandiford > > <richard.sandif...@arm.com> wrote: > >> > >> Richard Biener <richard.guent...@gmail.com> writes: > >> > On Wed, Jun 23, 2021 at 7:23 AM Trevor Saunders <tbsau...@tbsaunde.org> > >> > wrote: > >> >> > >> >> On Tue, Jun 22, 2021 at 02:01:24PM -0600, Martin Sebor wrote: > >> >> > On 6/21/21 1:15 AM, Richard Biener wrote: > >> > [...] > >> >> > > > >> >> > > But maybe I'm misunderstanding C++ too much :/ > >> >> > > > >> >> > > Well, I guess b) from above means auto_vec<> passing to > >> >> > > vec<> taking functions will need changes? > >> >> > > >> >> > Converting an auto_vec object to a vec slices off its data members. > >> >> > The auto_vec<T, 0> specialization has no data members so that's not > >> >> > a bug in and of itself, but auto_vec<T, N> does have data members > >> >> > so that would be a bug. The risk is not just passing it to > >> >> > functions by value but also returning it. That risk was made > >> >> > worse by the addition of the move ctor. > >> >> > >> >> I would agree that the conversion from auto_vec<> to vec<> is > >> >> questionable, and should get some work at some point, perhaps just > >> >> passingauto_vec references is good enough, or perhaps there is value in > >> >> some const_vec view to avoid having to rely on optimizations, I'm not > >> >> sure without looking more at the usage. > >> > > >> > We do need to be able to provide APIs that work with both auto_vec<> > >> > and vec<>, I agree that those currently taking a vec<> by value are > >> > fragile (and we've had bugs there before), but I'm not ready to say > >> > that changing them all to [const] vec<>& is OK. The alternative > >> > would be passing a const_vec<> by value, passing that along to > >> > const vec<>& APIs should be valid then (I can see quite some API > >> > boundary cleanups being necessary here ...). > >> > >> FWIW, as far as const_vec<> goes, we already have array_slice, which is > >> mostly a version of std::span. (The only real reason for not using > >> std::span itself is that it requires a later version of C++.) > >> > >> Taking those as arguments has the advantage that we can pass normal > >> arrays as well. > > > > It's not really a "const" thing it seems though it certainly would not > > expose > > any API that would reallocate the vector (which is the problematic part > > of passing vec<> by value, not necessarily changing elements in-place). > > > > Since array_slice doesn't inherit most of the vec<> API transforming an > > API from vec<> to array_slice<> will be also quite some work. But I > > agree it might be useful for generic API stuff. > > Which API functions would be the most useful ones to have? We could > add them if necessary.
I think we'll see when introducing uses. I guess that vec<> to const vec<>& changes will be mostly fine but for vec<> to vec<>& I might prefer vec<> to array_slice<> since that makes clear the caller won't re-allocate. We'll see what those APIs would require then. > There again, for things like searching and sorting, I think it would > be better to use <algorithm> where possible. It should usually be > more efficient than the void* callback approach that the C code tended > to use. Not sure whether <algorithm> really is better, we've specifically replaced libc qsort calls with our own sort to avoid all sorts of host isues and the vec<>::bsearch is inline and thus the callbacks can be inlined. Richard. > Thanks, > Richard
