commit cc19560253cfba353d118476e344252a1aeb8f7c
author Evgeny Stupachenko <evstupac@gmail.com> Tue May 16 21:44:59 2017 +0000
committer Evgeny Stupachenko <evstupac@gmail.com> Tue May 16 21:44:59 2017 +0000
tree 9088ee42175e86738576797598ba4ba890b01b30
parent c9916d7e97029e22dae2e4fc4d3959cae3bdd2cf

The patch exclude a case from zero check skip in
 CTLZ idiom recognition (r303102).

Summary:

The following case:
i = 1;
if(n)
  while (n >>= 1)
    i++;
use(i);

Was converted to:

i = 1;
if(n)
  i += builtin_ctlz(n >> 1, false);
use(i);

Which is not correct. The patch make it:

i = 1;
if(n)
  i += builtin_ctlz(n >> 1, true);
use(i);

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 303212

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

diff --git a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index 6693a26..cb6223b 100644
--- a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -1292,13 +1292,15 @@
   BasicBlock *PH = CurLoop->getLoopPreheader();
   Value *InitX = PhiX->getIncomingValueForBlock(PH);
   // If we check X != 0 before entering the loop we don't need a zero
-  // check in CTLZ intrinsic.
-  if (BasicBlock *PreCondBB = PH->getSinglePredecessor())
-    if (BranchInst *PreCondBr =
-        dyn_cast<BranchInst>(PreCondBB->getTerminator())) {
-      if (matchCondition(PreCondBr, PH) == InitX)
-        ZeroCheck = true;
-    }
+  // check in CTLZ intrinsic, but only if Cnt Phi is not used outside of the
+  // loop (if it is used we count CTLZ(X >> 1)).
+  if (!IsCntPhiUsedOutsideLoop)
+    if (BasicBlock *PreCondBB = PH->getSinglePredecessor())
+      if (BranchInst *PreCondBr =
+          dyn_cast<BranchInst>(PreCondBB->getTerminator())) {
+        if (matchCondition(PreCondBr, PH) == InitX)
+          ZeroCheck = true;
+      }
 
   // Check if CTLZ intrinsic is profitable. Assume it is always profitable
   // if we delete the loop (the loop has only 6 instructions):