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Re: [PATCH] Trust TREE_ADDRESSABLE


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.

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).

Richard.


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