From: Chandler Carruth <chandlerc@gmail.com>
Date: Thu, 13 Aug 2015 09:56:20 +0000 (+0000)
Subject: [LIR] Start leveraging the fundamental guarantees of a loop in
X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=commitdiff_plain;h=0e593a62f657547c6a7af6dc9710fc055a9c4da0;ds=sidebyside

[LIR] Start leveraging the fundamental guarantees of a loop in
simplified form to remove redundant checks and simplify the code for
popcount recognition. We don't actually need to handle all of these
cases.

I've left a FIXME for one in particular until I finish inspecting to
make sure we don't actually *rely* on the predicate in any way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244879 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index e3fe2530c2b..f2f37eec53b 100644
--- a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -830,29 +830,32 @@ bool LoopIdiomRecognize::recognizePopcount() {
   // non-compact loop. Therefore, recognizing popcount idiom only makes sense
   // in a compact loop.
 
-  // Give up if the loop has multiple blocks or multiple backedges.
-  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
+  assert(CurLoop->isLoopSimplifyForm() &&
+         "Loop passes require simplified form!");
+
+  // Give up if the loop has multiple blocks.
+  if (CurLoop->getNumBlocks() != 1)
     return false;
 
-  BasicBlock *LoopBody = *(CurLoop->block_begin());
-  if (LoopBody->size() >= 20) {
-    // The loop is too big, bail out.
+  // If the loop is too big, bail out.
+  BasicBlock &LoopBB = *CurLoop->getHeader();
+  if (LoopBB.size() >= 20)
     return false;
-  }
 
   // It should have a preheader containing nothing but an unconditional branch.
-  BasicBlock *PH = CurLoop->getLoopPreheader();
-  if (!PH)
-    return false;
-  if (&PH->front() != PH->getTerminator())
+  BasicBlock &PH = *CurLoop->getLoopPreheader();
+  if (&PH.front() != PH.getTerminator())
     return false;
-  auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator());
+  // FIXME: Technically, it shouldn't matter what instruction we use as
+  // a terminator, the only property needed is the definition of a preheader:
+  // a single loop predecessor whose only successor is the loop header.
+  auto *EntryBI = dyn_cast<BranchInst>(PH.getTerminator());
   if (!EntryBI || EntryBI->isConditional())
     return false;
 
   // It should have a precondition block where the generated popcount instrinsic
   // function can be inserted.
-  auto *PreCondBB = PH->getSinglePredecessor();
+  auto *PreCondBB = PH.getSinglePredecessor();
   if (!PreCondBB)
     return false;
   auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());