[PATCH] D26348: Allow convergent attribute for function arguments

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini added inline comments.


================
Comment at: docs/LangRef.rst:1199
+    Calling a function F that contains a call-site CS with convergent function
+    arguments is only defined behavior if the only data-dependencies of the
+    convergent function arguments of CS are convergent parameters of F. If this
----------------
sanjoy wrote:
> This is a little tricky -- what if the source program is:
> 
> ```
> void f(int convergent i);
> void g(int convergent j, int /* normal */ k) {
>   f(j - j + k);
> }
> ```
> 
> Can we simplify the call site to call `f(k)`?  If so, did we introduce UB in functions that call `g`?
Isn't your source program already UB? You have a call site with an argument that is data-dependent on a non-convergent parameter, `k`.

The kind of annoying and SSA-breaking limitation of convergent would be:

```
void f(int convergent i);
void g(int convergent j, int /* normal */ k) {
  if (j == k)
     f(j);
}
```

You can't replace `f(j);` with `f(k);` IIUC.




https://reviews.llvm.org/D26348