When rotating loops, put the original header at the bottom of the
loop, making the resulting loop significantly less ugly. Also, zap
its trivial PHI nodes, since it's easy.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111255 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/LoopRotation.cpp b/lib/Transforms/Scalar/LoopRotation.cpp
index 1e4bfa9..31957dd 100644
--- a/lib/Transforms/Scalar/LoopRotation.cpp
+++ b/lib/Transforms/Scalar/LoopRotation.cpp
@@ -261,6 +261,26 @@
// NewHeader is now the header of the loop.
L->moveToHeader(NewHeader);
+ // Move the original header to the bottom of the loop, where it now more
+ // naturally belongs. This isn't necessary for correctness, and CodeGen can
+ // usually reorder blocks on its own to fix things like this up, but it's
+ // still nice to keep the IR readable.
+ //
+ // The original header should have only one predecessor at this point, since
+ // we checked that the loop had a proper preheader and unique backedge before
+ // we started.
+ assert(OrigHeader->getSinglePredecessor() &&
+ "Original loop header has too many predecessors after loop rotation!");
+ OrigHeader->moveAfter(OrigHeader->getSinglePredecessor());
+
+ // Also, since this original header only has one predecessor, zap its
+ // PHI nodes, which are now trivial.
+ FoldSingleEntryPHINodes(OrigHeader);
+
+ // TODO: We could just go ahead and merge OrigHeader into its predecessor
+ // at this point, if we don't mind updating dominator info.
+
+ // Establish a new preheader, update dominators, etc.
preserveCanonicalLoopForm(LPM);
++NumRotated;