[LV] Optimize for size when vectorizing loops with tiny trip count

It may be detrimental to vectorize loops with very small trip count, as various
costs of the vectorized loop body as well as enclosing overheads including
runtime tests and scalar iterations may outweigh the gains of vectorizing. The
current cost model measures the cost of the vectorized loop body only, expecting
it will amortize other costs, and loops with known or expected very small trip
counts are not vectorized at all. This patch allows loops with very small trip
counts to be vectorized, but under OptForSize constraints, which ensure the cost
of the loop body is dominant, having no runtime guards nor scalar iterations.

Patch inspired by D32451.

Differential Revision: https://reviews.llvm.org/D34373

llvm-svn: 306803

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