From fc8042a1225790b3a6de434546623babea08496f Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Tue, 17 Aug 2010 17:39:21 +0000
Subject: [PATCH] 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
---
 lib/Transforms/Scalar/LoopRotation.cpp      | 20 +++++++++++++++++++
 test/Transforms/LoopRotate/phi-duplicate.ll | 22 ++++++++++++++++-----
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/lib/Transforms/Scalar/LoopRotation.cpp b/lib/Transforms/Scalar/LoopRotation.cpp
index 1e4bfa94be5..31957ddac14 100644
--- a/lib/Transforms/Scalar/LoopRotation.cpp
+++ b/lib/Transforms/Scalar/LoopRotation.cpp
@@ -261,6 +261,26 @@ bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
   // 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;
diff --git a/test/Transforms/LoopRotate/phi-duplicate.ll b/test/Transforms/LoopRotate/phi-duplicate.ll
index 9a64e2a9a83..5403e723ee1 100644
--- a/test/Transforms/LoopRotate/phi-duplicate.ll
+++ b/test/Transforms/LoopRotate/phi-duplicate.ll
@@ -27,9 +27,21 @@ for.body:                                         ; preds = %for.cond
 for.end:                                          ; preds = %for.cond
   ret void
 }
-; Should only end up with one phi.
-; CHECK: for.body:
-; CHECK-NEXT: %j.02 = phi i64
-; CHECK-NOT: phi
-; CHECK: ret void
 
+; Should only end up with one phi. Also, the original for.cond block should
+; be moved to the end of the loop so that the new loop header pleasantly
+; ends up at the top.
+
+; CHECK:      define void @test
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   icmp slt i64
+; CHECK-NEXT:   br i1
+; CHECK-NOT:  :
+; CHECK:      bb.nph:
+; CHECK-NEXT:   br label %for.body
+; CHECK-NOT:  :
+; CHECK:      for.body:
+; CHECK-NEXT:   %j.02 = phi i64
+; CHECK-NOT:    phi
+; CHECK:        ret void
+; CHECK-NEXT: }