[PATCH] D48239: [LangRef] Clarify meaning of "dereferencable" attribute/metadata.

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added inline comments.


================
Comment at: docs/LangRef.rst:1133
+    returned value is dereferenceable for the duration of the program.
+    If the pointer cannot be dereferenced, the behavior is undefined. This
     attribute may only be applied to pointer typed parameters. A pointer that
----------------
sanjoy wrote:
> DeadArgElim transforms
> 
> ```
> define void @f(i8* dereferenceable(16) %ptr) {
>   ret void
> }
> 
> define void @caller(i8* %v) {
>   call void @f(i8* %v)
>   ret void
> }
> ```
> 
> to
> 
> ```
> define void @f(i8* dereferenceable(16) %ptr) {
>   ret void
> }
> 
> define void @caller(i8* %v) {
>   call void @f(i8* undef)
>   ret void
> }
> ```
> 
> Given this semantic this transform is wrong, right?
Nice example :)
I guess DeadArgElim could be fixed to drop the dereferenceable tag?

I think being UB when the ptr is not dereferenceable sounds reasonable to me.


Repository:
  rL LLVM

https://reviews.llvm.org/D48239