Change semantics of fadd/fmul vector reductions.

This patch changes how LLVM handles the accumulator/start value
in the reduction, by never ignoring it regardless of the presence of
fast-math flags on callsites. This change introduces the following
new intrinsics to replace the existing ones:

  llvm.experimental.vector.reduce.fadd -> llvm.experimental.vector.reduce.v2.fadd
  llvm.experimental.vector.reduce.fmul -> llvm.experimental.vector.reduce.v2.fmul

and adds functionality to auto-upgrade existing LLVM IR and bitcode.

Reviewers: RKSimon, greened, dmgreen, nikic, simoll, aemerson

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D60261

llvm-svn: 363035

1 files changed, 4 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 03d84c39b66..29ae77c385d 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -801,13 +801,9 @@ Value *llvm::createSimpleTargetReduction(
     ArrayRef<Value *> RedOps) {
   assert(isa<VectorType>(Src->getType()) && "Type must be a vector");
 
-  Value *ScalarUdf = UndefValue::get(Src->getType()->getVectorElementType());
   std::function<Value *()> BuildFunc;
   using RD = RecurrenceDescriptor;
   RD::MinMaxRecurrenceKind MinMaxKind = RD::MRK_Invalid;
-  // TODO: Support creating ordered reductions.
-  FastMathFlags FMFFast;
-  FMFFast.setFast();
 
   switch (Opcode) {
   case Instruction::Add:
@@ -827,15 +823,15 @@ Value *llvm::createSimpleTargetReduction(
     break;
   case Instruction::FAdd:
     BuildFunc = [&]() {
-      auto Rdx = Builder.CreateFAddReduce(ScalarUdf, Src);
-      cast<CallInst>(Rdx)->setFastMathFlags(FMFFast);
+      auto Rdx = Builder.CreateFAddReduce(
+          Constant::getNullValue(Src->getType()->getVectorElementType()), Src);
       return Rdx;
     };
     break;
   case Instruction::FMul:
     BuildFunc = [&]() {
-      auto Rdx = Builder.CreateFMulReduce(ScalarUdf, Src);
-      cast<CallInst>(Rdx)->setFastMathFlags(FMFFast);
+      Type *Ty = Src->getType()->getVectorElementType();
+      auto Rdx = Builder.CreateFMulReduce(ConstantFP::get(Ty, 1.0), Src);
       return Rdx;
     };
     break;