[llvm] [Uniformity] Fixed control-div early stop (PR #139667)

[Junjie Gu via llvm-commits]

jgu222 wrote:

This changes seems handling the cases that IPD cannot handle.  With your comments, I looked into it further and conclude that 

> > Note that the IPD is the exact place where the propagation of labels should stop, because that is the last join point of `DivTermBlock`.

This is a little tricky.  If the execution favors A->E1->E2 cycles,  ie, if there is any active lanes going to E1, the code will execute E1.  The execution goes from A to B only if all active lanes  at A goes to B.   If so,  E1 is not a join point.  This is more like a lit test under AMDGPU/irreducible/irreducible-2.ll::@natural_loop_two_backedges(), which has the following CFG:

```
;          entry
;            |
;            H<-|<-|
;            |    |   |
;            B->     |
;            |        |
;            C----->
;            |
;            X
```

B is DivTermBlock,  is ph in H uniform ?   If the execution will honor the inner loop first, the phi in H is uniform. If the execution at B favors C (outer loop),  then phi in H is divergent.

Unless we are sure the inner loop is always favored,  we have to treat phi in H as divergent.  This also means that IPD isn't necessarily convergence point.   If so,  IPD should not be used.   Is IPD the max-convergence ?

This patch (139667) does not assume IPD is a convergence point.  It will show E1 is a join if making this example a reducible CFG by removing entry->E2 edge.


https://github.com/llvm/llvm-project/pull/139667