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

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added inline comments.


================
Comment at: llvm/docs/LangRef.rst:1164
+
+``dereferenceable_in_scope(<n>)``
+    This indicates that the annotated pointer value has the
----------------
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.


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https://reviews.llvm.org/D61652