[PATCH] [LCSSA] Handle PHI insertion in disjoint loops

[Bruno Cardoso Lopes]

================
Comment at: lib/Transforms/Utils/LCSSA.cpp:144-146
@@ -134,1 +143,5 @@
+    // LoopSimplify gets improved.
+    Loop *OtherLoop = LI->getLoopFor(ExitBB);
+    if (OtherLoop && !L.contains(OtherLoop))
+      PostProcessPHIs.push_back(PN);
   }
----------------
chandlerc wrote:
> I would write this as:
> 
>   if (auto OtherLoop = LI->getLoopFor(ExitBB))
>     if (!L.contains(OtherLoop))
>       PostProcessPHIs.push_back(PN)
------
Neat!

================
Comment at: lib/Transforms/Utils/LCSSA.cpp:177-181
@@ +176,7 @@
+      continue;
+    for (unsigned j = 0, ee = PostProcessPHIs.size(); j != ee; ++j)
+      if (PostProcessPHIs[j] == AddedPHIs[i]) {
+        PostProcessPHIs.erase(PostProcessPHIs.begin()+j);
+        break;
+      }
+    AddedPHIs[i]->eraseFromParent();
----------------
chandlerc wrote:
> Ow. This is quadratic in all cases AFAICT. Either we move all remaining elements in PostProcessPHIs down a slot, or we skip over an element in every trip throuh AddedPHIs (AddedPHIs is a superset and ordered the same).
> 
> Instead of all this, how about we do the post processing first, and then this loop? You can add a similar guard to skip post processing of phis with empty use lists. That avoids all the complexity here.
----------
Yep, I was trying to avoid it by using SmallPtrSet in the first patch, thought you had noticed the tradeoff, should have made that explicit. But this solution looks better anyway. Thanks again. :-)

http://reviews.llvm.org/D6624

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