[PATCH] D54189: [HotColdSplitting] Ensure PHIs have unique incoming values

[Vedant Kumar via Phabricator via llvm-commits]

vsk added inline comments.


================
Comment at: llvm/lib/Transforms/IPO/HotColdSplitting.cpp:411
+          // Ignore successor PHIs without an incoming value from BB.
+          int BBIndex = SuccPhi.getBasicBlockIndex(&BB);
+          if (BBIndex == -1)
----------------
vsk wrote:
> vsk wrote:
> > hiraditya wrote:
> > > When can we have a case where PHI does not have incoming value from immediate predecessor?
> > When the successor block has no uses of values defined in the predecessor. A special case of this is when the successor block just contains a terminator:
> > 
> > ```
> > foo:
> >   %sink = ...
> >   br label %bar
> > bar:
> >   br label %baz
> > ```
> Sorry, that example is incorrect because there isn't a phi there at all. How about:
> ```
> foo:
>   %sink = ...
>   br label %bar
> bar:
>   %phi = phi [ 0, %p1 ], [ 1, %p2 ]
>   br i1 undef, label %p1, label %p2
> p1:
>   br i1 undef, label %bar, %p1
> p2:
>   br i1 undef, label %bar, %p2
> ```
> 
> I think foo dominates bar here, but bar has a phi without an incoming value from foo.
No, my example is wrong again. A phi has to have an incoming value from each of its parent's predecessor blocks.


https://reviews.llvm.org/D54189