Compute estimated trip counts for multiple exit loops

This change allows us to estimate trip count from profile metadata for all multiple exit loops. We still do the estimate only from the latch, but that's fine as it causes us to over estimate the trip count at worst.

Reviewing the uses of the API, all but one are cases where we restrict a loop transformation (unroll, and vectorize respectively) when we know the trip count is short enough. So, as a result, the change makes these passes strictly less aggressive. The test change illustrates a case where we'd previously have runtime unrolled a loop which ran fewer iterations than the unroll factor. This is definitely unprofitable.

The one case where an upper bound on estimate trip count could drive a more aggressive transform is peeling, and I duplicated the logic being removed from the generic estimation there to keep it the same. The resulting heuristic makes no sense and should probably be immediately removed, but we can do that in a separate change.

This was noticed when analyzing regressions on D113939.

I plan to come back and incorporate estimated trip counts from other exits, but that's a minor improvement which can follow separately.

Differential Revision: https://reviews.llvm.org/D115362

2 files changed, 36 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
index f3cf42b..5ce392d 100644
--- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp
@@ -333,6 +333,31 @@ static unsigned countToEliminateCompares(Loop &L, unsigned MaxPeelCount,
   return DesiredPeelCount;
 }
 
+/// This "heuristic" exactly matches implicit behavior which used to exist
+/// inside getLoopEstimatedTripCount.  It was added here to keep an
+/// improvement inside that API from causing peeling to become more agressive.
+/// This should probably be removed.
+static bool violatesLegacyMultiExitLoopCheck(Loop *L) {
+  BasicBlock *Latch = L->getLoopLatch();
+  if (!Latch)
+    return true;
+
+  BranchInst *LatchBR = dyn_cast<BranchInst>(Latch->getTerminator());
+  if (!LatchBR || LatchBR->getNumSuccessors() != 2 || !L->isLoopExiting(Latch))
+    return true;
+
+  assert((LatchBR->getSuccessor(0) == L->getHeader() ||
+          LatchBR->getSuccessor(1) == L->getHeader()) &&
+         "At least one edge out of the latch must go to the header");
+
+  SmallVector<BasicBlock *, 4> ExitBlocks;
+  L->getUniqueNonLatchExitBlocks(ExitBlocks);
+  return any_of(ExitBlocks, [](const BasicBlock *EB) {
+      return !EB->getTerminatingDeoptimizeCall();
+    });
+}
+
+
 // Return the number of iterations we want to peel off.
 void llvm::computePeelCount(Loop *L, unsigned LoopSize,
                             TargetTransformInfo::PeelingPreferences &PP,
@@ -436,6 +461,8 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
   // We only do this in the presence of profile information, since otherwise
   // our estimates of the trip count are not reliable enough.
   if (L->getHeader()->getParent()->hasProfileData()) {
+    if (violatesLegacyMultiExitLoopCheck(L))
+      return;
     Optional<unsigned> PeelCount = getLoopEstimatedTripCount(L);
     if (!PeelCount)
       return;
diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index c8e42ac..5af12fe 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -773,8 +773,8 @@ void llvm::breakLoopBackedge(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
 }
 
 
-/// Checks if \p L has single exit through latch block except possibly
-/// "deoptimizing" exits. Returns branch instruction terminating the loop
+/// Checks if \p L has an exiting latch branch.  There may also be other
+/// exiting blocks.  Returns branch instruction terminating the loop
 /// latch if above check is successful, nullptr otherwise.
 static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
   BasicBlock *Latch = L->getLoopLatch();
@@ -789,21 +789,16 @@ static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
           LatchBR->getSuccessor(1) == L->getHeader()) &&
          "At least one edge out of the latch must go to the header");
 
-  SmallVector<BasicBlock *, 4> ExitBlocks;
-  L->getUniqueNonLatchExitBlocks(ExitBlocks);
-  if (any_of(ExitBlocks, [](const BasicBlock *EB) {
-        return !EB->getTerminatingDeoptimizeCall();
-      }))
-    return nullptr;
-
   return LatchBR;
 }
 
 Optional<unsigned>
 llvm::getLoopEstimatedTripCount(Loop *L,
                                 unsigned *EstimatedLoopInvocationWeight) {
-  // Support loops with an exiting latch and other existing exists only
-  // deoptimize.
+  // Currently we take the estimate exit count only from the loop latch,
+  // ignoring other exiting blocks.  This can overestimate the trip count
+  // if we exit through another exit, but can never underestimate it.
+  // TODO: incorporate information from other exits
   BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
   if (!LatchBranch)
     return None;
@@ -834,8 +829,9 @@ llvm::getLoopEstimatedTripCount(Loop *L,
 
 bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
                                      unsigned EstimatedloopInvocationWeight) {
-  // Support loops with an exiting latch and other existing exists only
-  // deoptimize.
+  // At the moment, we currently support changing the estimate trip count of
+  // the latch branch only.  We could extend this API to manipulate estimated
+  // trip counts for any exit.
   BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
   if (!LatchBranch)
     return false;