[SLP] Vectorize all-constant entries.

[mzolotukhin]

Summary

Currently SLP gives up when a tree contains a set of constants, and that can be easily fixed.

[nadav]

LGTM!

[mzolotukhin]

The patch was committed in r240144.

lib/Transforms/Vectorize/SLPVectorizer.cpp

 bool BoUpSLP::isFullyVectorizableTinyTree() {
   DEBUG(dbgs() << "SLP: Check whether the tree with height " <<
         VectorizableTree.size() << " is fully vectorizable .\n");
 
   // We only handle trees of height 2.
   if (VectorizableTree.size() != 2)
     return false;
 
-  // Handle splat stores.
-  if (!VectorizableTree[0].NeedToGather && isSplat(VectorizableTree[1].Scalars))
+  // Handle splat and all-constants stores.
+  if (!VectorizableTree[0].NeedToGather &&
+      (allConstant(VectorizableTree[1].Scalars) ||
+       isSplat(VectorizableTree[1].Scalars)))
     return true;
 
   // Gathering cost would be too much for tiny trees.
   if (VectorizableTree[0].NeedToGather || VectorizableTree[1].NeedToGather)
     return false;
 
   return true;
 }

test/Transforms/SLPVectorizer/X86/tiny-tree.ll

define void @store_splat(float*, float) {
   %4 = getelementptr inbounds float, float* %0, i64 1
   store float %1, float* %4, align 4
   %5 = getelementptr inbounds float, float* %0, i64 2
   store float %1, float* %5, align 4
   %6 = getelementptr inbounds float, float* %0, i64 3
   store float %1, float* %6, align 4
   ret void
 }
+
+
+; CHECK-LABEL: store_const
+; CHECK: store <4 x i32>
+define void @store_const(i32* %a) {
+entry:
+  %ptr0 = getelementptr inbounds i32, i32* %a, i64 0
+  store i32 10, i32* %ptr0, align 4
+  %ptr1 = getelementptr inbounds i32, i32* %a, i64 1
+  store i32 30, i32* %ptr1, align 4
+  %ptr2 = getelementptr inbounds i32, i32* %a, i64 2
+  store i32 20, i32* %ptr2, align 4
+  %ptr3 = getelementptr inbounds i32, i32* %a, i64 3
+  store i32 40, i32* %ptr3, align 4
+  ret void
+}