[PATCH] D61652: [LangRef][Attr] Clarify dereferenceable(_in_scope)

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert marked an inline comment as done.
jdoerfert added inline comments.


================
Comment at: llvm/docs/LangRef.rst:1164
+
+``dereferenceable_in_scope(<n>)``
+    This indicates that the annotated pointer value has the
----------------
sanjoy wrote:
> jdoerfert wrote:
> > sanjoy wrote:
> > > I'm not sure if this is a good semantic property.  Consider (the CFG form of):
> > > 
> > > ```
> > > if (cond) {
> > >   a = dereferenceable_in_scope(4) ...
> > > } else {
> > >   ...
> > > }
> > > side_effect_that_frees_a()
> > > ```
> > > 
> > > I think this program above is well defined because the definition of `a` does not dominate `side_effect_that_frees_a()`.  But that means we can't tail duplicate the program into:
> > > 
> > > ```
> > > if (cond) {
> > >   a = dereferenceable_in_scope(4) ...
> > >   side_effect_that_frees_a()
> > > } else {
> > >   ...
> > >   side_effect_that_frees_a()
> > > }
> > > ```
> > > 
> > > Same applies for inlining.  Any transform that can extend the dominance region of some value needs to be audited.
> > > 
> > > What is the use case for this attribute?  Can we do something more targeted for this use case?
> > > What is the use case for this attribute? Can we do something more targeted for this use case?
> > 
> > My "use case" is basically the clang interpretation of (current) `derferenceable` which I tried to replicate with the new `derferenceable` definition. There are users of (current) `derferenceable` that interpret it as `derferenceable_in_scope` and they should have a way to do so.
> > 
> > > I think this program above is well defined because the definition of a does not dominate 
> > > Same applies for inlining. Any transform that can extend the dominance region of some value needs to be audited.
> > 
> > I agree that there is a problem with the current wording wrt. inlining. I'll try to fix that.
> > Maybe @reames can help me define what they want/need.
> > 
> > For the record, the example above is not sufficient to expose the inlining problem though. Above, a was "accessible" at the location where a was freed, so the `dereferenceable_in_scope` was violated already. Thinking about it, even if we inline, SSA dominance property should be combined with reachability to a user to give you what you want.
> > I agree that there is a problem with the current wording wrt. inlining. I'll try to fix that. Maybe @reames can help me define what they want/need.
> 
> Maybe we need something stronger that says that a pointer is always `dereferenceable(n)`, like a global variable?
> 
> > Thinking about it, even if we inline, SSA dominance property should be combined with reachability to a user to give you what you want.
> 
> I was thinking about something like:
> 
> ```
> void f() {
>   a = dereferenceable_in_scope(4) ...
> }
> 
> void main() {
>   f();
>   free_a();
> }
> 
> ```
> 
> I think this is well defined according to the current wording since the `free_a` does not lie within the scope of `a`?  If yes, then inlining becomes illegal.
What do you think about `dereferenceable_globally(<n>)`, which would be the wording without the "in its scope (asdefined by the SSA dominance property)"?

So:


    This indicates that the annotated pointer value has the
    ``dereferenceable(<n>)`` property *at any program point*. Thus, unlike pointer values
    annotated with ``dereferenceable(<n>)``, ``dereferenceable_in_scope(<n>)``
    pointer values can never lose the ``dereferenceable(<n>)``.



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