1 files changed, 30 insertions, 29 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index d1ac444bb5b..0cf2b9f15a8 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -114,12 +114,13 @@ static cl::opt<bool>
     EnableIfConversion("enable-if-conversion", cl::init(true), cl::Hidden,
                        cl::desc("Enable if-conversion during vectorization."));
 
-/// We don't vectorize loops with a known constant trip count below this number.
+/// Loops with a known constant trip count below this number are vectorized only
+/// if no scalar iteration overheads are incurred.
 static cl::opt<unsigned> TinyTripCountVectorThreshold(
     "vectorizer-min-trip-count", cl::init(16), cl::Hidden,
-    cl::desc("Don't vectorize loops with a constant "
-             "trip count that is smaller than this "
-             "value."));
+    cl::desc("Loops with a constant trip count that is smaller than this "
+             "value are vectorized only if no scalar iteration overheads "
+             "are incurred."));
 
 static cl::opt<bool> MaximizeBandwidth(
     "vectorizer-maximize-bandwidth", cl::init(false), cl::Hidden,
@@ -7801,8 +7802,25 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     return false;
   }
 
-  // Check the loop for a trip count threshold:
-  // do not vectorize loops with a tiny trip count.
+  PredicatedScalarEvolution PSE(*SE, *L);
+
+  // Check if it is legal to vectorize the loop.
+  LoopVectorizationRequirements Requirements(*ORE);
+  LoopVectorizationLegality LVL(L, PSE, DT, TLI, AA, F, TTI, GetLAA, LI, ORE,
+                                &Requirements, &Hints);
+  if (!LVL.canVectorize()) {
+    DEBUG(dbgs() << "LV: Not vectorizing: Cannot prove legality.\n");
+    emitMissedWarning(F, L, Hints, ORE);
+    return false;
+  }
+
+  // Check the function attributes to find out if this function should be
+  // optimized for size.
+  bool OptForSize =
+      Hints.getForce() != LoopVectorizeHints::FK_Enabled && F->optForSize();
+
+  // Check the loop for a trip count threshold: vectorize loops with a tiny trip
+  // count by optimizing for size, to minimize overheads.
   unsigned ExpectedTC = SE->getSmallConstantMaxTripCount(L);
   bool HasExpectedTC = (ExpectedTC > 0);
 
@@ -7816,36 +7834,19 @@ bool LoopVectorizePass::processLoop(Loop *L) {
 
   if (HasExpectedTC && ExpectedTC < TinyTripCountVectorThreshold) {
     DEBUG(dbgs() << "LV: Found a loop with a very small trip count. "
-                 << "This loop is not worth vectorizing.");
+                 << "This loop is worth vectorizing only if no scalar "
+                 << "iteration overheads are incurred.");
     if (Hints.getForce() == LoopVectorizeHints::FK_Enabled)
       DEBUG(dbgs() << " But vectorizing was explicitly forced.\n");
     else {
       DEBUG(dbgs() << "\n");
-      ORE->emit(createMissedAnalysis(Hints.vectorizeAnalysisPassName(),
-                                     "NotBeneficial", L)
-                << "vectorization is not beneficial "
-                   "and is not explicitly forced");
-      return false;
+      // Loops with a very small trip count are considered for vectorization
+      // under OptForSize, thereby making sure the cost of their loop body is
+      // dominant, free of runtime guards and scalar iteration overheads.
+      OptForSize = true;
     }
   }
 
-  PredicatedScalarEvolution PSE(*SE, *L);
-
-  // Check if it is legal to vectorize the loop.
-  LoopVectorizationRequirements Requirements(*ORE);
-  LoopVectorizationLegality LVL(L, PSE, DT, TLI, AA, F, TTI, GetLAA, LI, ORE,
-                                &Requirements, &Hints);
-  if (!LVL.canVectorize()) {
-    DEBUG(dbgs() << "LV: Not vectorizing: Cannot prove legality.\n");
-    emitMissedWarning(F, L, Hints, ORE);
-    return false;
-  }
-
-  // Check the function attributes to find out if this function should be
-  // optimized for size.
-  bool OptForSize =
-      Hints.getForce() != LoopVectorizeHints::FK_Enabled && F->optForSize();
-
   // Check the function attributes to see if implicit floats are allowed.
   // FIXME: This check doesn't seem possibly correct -- what if the loop is
   // an integer loop and the vector instructions selected are purely integer