[SLP] Preserve IR flags when vectorizing horizontal reductions.

Summary:
The SLP vectorizer should propagate IR-level optimization hints/flags
(nsw, nuw, exact, fast-math) when converting scalar horizontal
reductions instructions into vectors, just like for other vectorized
instructions.
It doe not include IR propagation for extra arguments, we need to handle
original scalar operations for extra args to propagate correct flags.

Reviewers: mkuper, mzolotukhin, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30418

llvm-svn: 296614

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -4459,11 +4459,12 @@ public:
 
       // Emit a reduction.
       Value *ReducedSubTree =
-          emitReduction(VectorizedRoot, Builder, ReduxWidth);
+          emitReduction(VectorizedRoot, Builder, ReduxWidth, ReductionOps);
       if (VectorizedTree) {
         Builder.SetCurrentDebugLocation(Loc);
         VectorizedTree = Builder.CreateBinOp(ReductionOpcode, VectorizedTree,
                                              ReducedSubTree, "bin.rdx");
+        propagateIRFlags(VectorizedTree, ReductionOps);
       } else
         VectorizedTree = ReducedSubTree;
       i += ReduxWidth;
@@ -4477,6 +4478,7 @@ public:
         Builder.SetCurrentDebugLocation(I->getDebugLoc());
         VectorizedTree =
             Builder.CreateBinOp(ReductionOpcode, VectorizedTree, I);
+        propagateIRFlags(VectorizedTree, ReductionOps);
       }
       for (auto &Pair : ExternallyUsedValues) {
         assert(!Pair.second.empty() &&
@@ -4527,7 +4529,7 @@ private:
 
   /// \brief Emit a horizontal reduction of the vectorized value.
   Value *emitReduction(Value *VectorizedValue, IRBuilder<> &Builder,
-                       unsigned ReduxWidth) {
+                       unsigned ReduxWidth, ArrayRef<Value *> RedOps) {
     assert(VectorizedValue && "Need to have a vectorized tree node");
     assert(isPowerOf2_32(ReduxWidth) &&
            "We only handle power-of-two reductions for now");
@@ -4554,6 +4556,7 @@ private:
           TmpVec, UndefValue::get(TmpVec->getType()), UpperHalf, "rdx.shuf");
         TmpVec = Builder.CreateBinOp(ReductionOpcode, TmpVec, Shuf, "bin.rdx");
       }
+      propagateIRFlags(TmpVec, RedOps);
     }
 
     // The result is in the first element of the vector.