Revert "[SCEV] Make computeExitLimit more simple and more powerful"

This reverts commit 023c8be90980e0180766196cba86f81608b35d38.

This patch triggers miscompile of zlib on PowerPC platform. Most likely it is
caused by some pre-backend PPC-specific pass, but we don't clearly know the
reason yet. So we temporally revert this patch with intention to return it
once the problem is resolved. See bug 37229 for details.

llvm-svn: 330893

1 files changed, 58 insertions, 17 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 87b0141c850..d6fee9be570 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6890,12 +6890,63 @@ ScalarEvolution::computeBackedgeTakenCount(const Loop *L,
 ScalarEvolution::ExitLimit
 ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
                                       bool AllowPredicates) {
-  assert(L->contains(ExitingBlock) && "Exit count for non-loop block?");
-  // If our exiting block does not dominate the latch, then its connection with
-  // loop's exit limit may be far from trivial.
-  const BasicBlock *Latch = L->getLoopLatch();
-  if (!Latch || !DT.dominates(ExitingBlock, Latch))
-    return getCouldNotCompute();
+  // Okay, we've chosen an exiting block.  See what condition causes us to exit
+  // at this block and remember the exit block and whether all other targets
+  // lead to the loop header.
+  bool MustExecuteLoopHeader = true;
+  BasicBlock *Exit = nullptr;
+  for (auto *SBB : successors(ExitingBlock))
+    if (!L->contains(SBB)) {
+      if (Exit) // Multiple exit successors.
+        return getCouldNotCompute();
+      Exit = SBB;
+    } else if (SBB != L->getHeader()) {
+      MustExecuteLoopHeader = false;
+    }
+
+  // At this point, we know we have a conditional branch that determines whether
+  // the loop is exited.  However, we don't know if the branch is executed each
+  // time through the loop.  If not, then the execution count of the branch will
+  // not be equal to the trip count of the loop.
+  //
+  // Currently we check for this by checking to see if the Exit branch goes to
+  // the loop header.  If so, we know it will always execute the same number of
+  // times as the loop.  We also handle the case where the exit block *is* the
+  // loop header.  This is common for un-rotated loops.
+  //
+  // If both of those tests fail, walk up the unique predecessor chain to the
+  // header, stopping if there is an edge that doesn't exit the loop. If the
+  // header is reached, the execution count of the branch will be equal to the
+  // trip count of the loop.
+  //
+  //  More extensive analysis could be done to handle more cases here.
+  //
+  if (!MustExecuteLoopHeader && ExitingBlock != L->getHeader()) {
+    // The simple checks failed, try climbing the unique predecessor chain
+    // up to the header.
+    bool Ok = false;
+    for (BasicBlock *BB = ExitingBlock; BB; ) {
+      BasicBlock *Pred = BB->getUniquePredecessor();
+      if (!Pred)
+        return getCouldNotCompute();
+      TerminatorInst *PredTerm = Pred->getTerminator();
+      for (const BasicBlock *PredSucc : PredTerm->successors()) {
+        if (PredSucc == BB)
+          continue;
+        // If the predecessor has a successor that isn't BB and isn't
+        // outside the loop, assume the worst.
+        if (L->contains(PredSucc))
+          return getCouldNotCompute();
+      }
+      if (Pred == L->getHeader()) {
+        Ok = true;
+        break;
+      }
+      BB = Pred;
+    }
+    if (!Ok)
+      return getCouldNotCompute();
+  }
 
   bool IsOnlyExit = (L->getExitingBlock() != nullptr);
   TerminatorInst *Term = ExitingBlock->getTerminator();
@@ -6910,19 +6961,9 @@ ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
         /*ControlsExit=*/IsOnlyExit, AllowPredicates);
   }
 
-  if (SwitchInst *SI = dyn_cast<SwitchInst>(Term)) {
-    // For switch, make sure that there is a single exit from the loop.
-    BasicBlock *Exit = nullptr;
-    for (auto *SBB : successors(ExitingBlock))
-      if (!L->contains(SBB)) {
-        if (Exit) // Multiple exit successors.
-          return getCouldNotCompute();
-        Exit = SBB;
-      }
-    assert(Exit && "Exiting block must have at least one exit");
+  if (SwitchInst *SI = dyn_cast<SwitchInst>(Term))
     return computeExitLimitFromSingleExitSwitch(L, SI, Exit,
                                                 /*ControlsExit=*/IsOnlyExit);
-  }
 
   return getCouldNotCompute();
 }