[SCEV] Make computeExitLimit more simple and more powerful

Current implementation of `computeExitLimit` has a big piece of code
the only purpose of which is to prove that after the execution of this
block the latch will be executed. What it currently checks is actually a
subset of situations where the exiting block dominates latch.

This patch replaces all these checks for simple particular cases with
domination check over loop's latch which is the only necessary condition
of taking the exiting block into consideration. This change allows to
calculate exact loop taken count for simple loops like

  for (int i = 0; i < 100; i++) {
    if (cond) {...} else {...}
    if (i > 50) break;
    . . .
  }

Differential Revision: https://reviews.llvm.org/D44677
Reviewed By: efriedma

llvm-svn: 329047

1 files changed, 17 insertions, 58 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index a6b3dce0a0d..d8a41944acb 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6884,63 +6884,12 @@ ScalarEvolution::computeBackedgeTakenCount(const Loop *L,
 ScalarEvolution::ExitLimit
 ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
                                       bool AllowPredicates) {
-  // 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();
-  }
+  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();
 
   bool IsOnlyExit = (L->getExitingBlock() != nullptr);
   TerminatorInst *Term = ExitingBlock->getTerminator();
@@ -6955,9 +6904,19 @@ ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
         /*ControlsExit=*/IsOnlyExit, AllowPredicates);
   }
 
-  if (SwitchInst *SI = dyn_cast<SwitchInst>(Term))
+  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");
     return computeExitLimitFromSingleExitSwitch(L, SI, Exit,
                                                 /*ControlsExit=*/IsOnlyExit);
+  }
 
   return getCouldNotCompute();
 }