SLP Vectorier: Don't vectorize really short chains because they are a…

…lready handled by the SelectionDAG store-vectorizer, which does a better job in deciding when to vectorize.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187267 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -898,8 +898,12 @@ int BoUpSLP::getTreeCost() {
   DEBUG(dbgs() << "SLP: Calculating cost for tree of size " <<
         VectorizableTree.size() << ".\n");
 
-  if (!VectorizableTree.size()) {
-    assert(!ExternalUses.size() && "We should not have any external users");
+  // Don't vectorize tiny trees. Small load/store chains or consecutive stores
+  // of constants will be vectoried in SelectionDAG in MergeConsecutiveStores.
+  if (VectorizableTree.size() < 3) {
+    if (!VectorizableTree.size()) {
+      assert(!ExternalUses.size() && "We should not have any external users");
+    }
     return 0;
   }
 

test/Transforms/SLPVectorizer/X86/reduction2.ll

@@ -16,11 +16,13 @@ define double @foo(double* nocapture %D) {
   %3 = getelementptr inbounds double* %D, i32 %2
   %4 = load double* %3, align 4
   %A4 = fmul double %4, %4
+  %A42 = fmul double %A4, %A4
   %5 = or i32 %2, 1
   %6 = getelementptr inbounds double* %D, i32 %5
   %7 = load double* %6, align 4
   %A7 = fmul double %7, %7
-  %8 = fadd double %A4, %A7
+  %A72 = fmul double %A7, %A7
+  %8 = fadd double %A42, %A72
   %9 = fadd double %sum.01, %8
   %10 = add nsw i32 %i.02, 1
   %exitcond = icmp eq i32 %10, 100