On Thu, 12 Jun 2014, Jan Hubicka wrote: > > On Thu, 12 Jun 2014, Eric Botcazou wrote: > > > > > > If we want to give frontends a way to pass information that address of a > > > > given global object is not taken (apparently useful for Ada and its > > > > alias > > > > attribute), then I do not think we are looking for middle-end only > > > > solution. > > > > > > I don't feel very confortable with doing that in Ada, since everybody > > > seems to > > > be thinking that TRE_PUBLIC/DECL_EXTERNAL objects are implicitly > > > addressable > > > (see for example Steven's reasoning in an earlier message). > > > > > > > If we really do not want to revisit TREE_ADDRESSABLE in frontends, we > > > > can do > > > > the following: > > > > 1) change semantics of addressable flag on global variables in a way > > > > Richard did, document it is initialized only after symbol table is > > > > built 2) > > > > add code to cgraph construction to set TREE_ADDRESSABLE on every global > > > > variable it sees. > > > > IPA visibility is run before early optimizations. I suppose we can > > > > set > > > > it there. I.e. in function_and_variable_visibility whenever we set > > > > externally_visible and we have !in_lto_p > > > > It is bit of hack. > > > > 3) perhaps add some way to avoid 2) on objects we want - apparenlty we > > > > now > > > > have DECL_NONALIASED that may be useful for this. > > > > > > Then how about using DECL_NONALIASED instead of TREE_ADDRESSABLE to > > > achieve > > > the initial goal here? That is to say, may_be_aliased tests > > > DECL_NONALIASED > > > for TREE_PUBLIC/DECL_EXTERNAL DECLs and the LTO front-end sets it > > > properly. > > > > Btw, may_be_aliased already does that. So yes, when LTO promotes sth > > from non-public to public but hidden visibility and TREE_ADDRESSABLE > > was not set it could set DECL_NONALIASED. That would at least preserve > > the aliasing behavior from without using LTO. If the resolution info > > from the linker allows us to make initial public variables hidden > > _and_ some LTO IPA pass proves that the variables address is not taken > > then that pass can set DECL_NONALIASED as well. > > Yep, I suppose each time I clear TREE_ADDRESSABLE flag, i can also set > DECL_NONALIASED. > > > > Of course one issue is that it's impossible to write a verifier that > > checks whether DECL_NONALIASED and TREE_ADDRESSABLE are "out-of-sync" > > (because by design they can be). So it's a bit more fragile > > (we could make the operand scanner that "updates" TREE_ADDRESSABLE > > also unset DECL_NONALIASED of course). > > Hmm,when one would unset it?
When you extract the address and use it. For example when you do auto-parallelization and outline a part of your function it passes arrays as addresses. Or if you start to introduce address induction variables like the vectorizer or IVOPTs does. Richard.
