> 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? Honza > > Richard.
