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

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy accepted this revision.
sanjoy added inline comments.
This revision is now accepted and ready to land.


================
Comment at: llvm/docs/LangRef.rst:1164
+
+``dereferenceable_in_scope(<n>)``
+    This indicates that the annotated pointer value has the
----------------
jdoerfert wrote:
> 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>)``.
> 
I think this is implied, but how about "the annotated pointer value has the
``dereferenceable(<n>)`` starting from the definition of the value to the termination of the program."?


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