[PATCH] D85604: SimplifyCFG: prevent certain transforms on convergent operations

[Nicolai Hähnle via Phabricator via llvm-commits]

nhaehnle added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:4474
+    if (CI->isConvergent())
+      return false;
+
----------------
efriedma wrote:
> I can't imagine a function that makes sense to mark both convergent and speculatable.
An example would be a `shuffle` operation:
```
  if (cond1) {
    y = shuffle(x, srcIdx, token);
    foo(y);
  } else if (cond2) {
    y = shuffle(x, srcIdx, token);
    bar(y)
  }
```
If `any(cond1 || cond2)` is sufficiently likely, the following is more efficient:
```
  y = shuffle(x, srcIdx, token);
  if (cond1) {
    foo(y);
  } else if (cond2) {
    bar(y)
  }
```
This transform is correct for the underlying hardware implementation if `srcIdx` can be out-of-range without causing undefined behavior.

`convergent speculatable` would still indicate that the transform is forbidden, because it changes the set of communicating threads, but I think we're going to want attributes that partially relax `convergent` again. In this case, we'd allow changing the set of communicating threads to a superset.


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https://reviews.llvm.org/D85604