AMDGPU: Allow vectorization of round intrinsic

There seems to be a small benefit to the legalized sequence for v2f16
round with packed instructions, so allow vectorizing it by reducing
the cost.

An unintended side effect is vectorization of f32 round also
happens. The current FMA logic seems off to me, and isn't checking for
packed instructions.

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

@@ -476,11 +476,24 @@ int GCNTTIImpl::getArithmeticInstrCost(unsigned Opcode, Type *Ty,
                                        Opd1PropInfo, Opd2PropInfo);
 }
 
+// Return true if there's a potential benefit from using v2f16 instructions for
+// an intrinsic, even if it requires nontrivial legalization.
+static bool intrinsicHasPackedVectorBenefit(Intrinsic::ID ID) {
+  switch (ID) {
+  case Intrinsic::fma: // TODO: fmuladd
+  // There's a small benefit to using vector ops in the legalized code.
+  case Intrinsic::round:
+    return true;
+  default:
+    return false;
+  }
+}
+
 template <typename T>
 int GCNTTIImpl::getIntrinsicInstrCost(Intrinsic::ID ID, Type *RetTy,
                                       ArrayRef<T *> Args, FastMathFlags FMF,
                                       unsigned VF, const Instruction *I) {
-  if (ID != Intrinsic::fma)
+  if (!intrinsicHasPackedVectorBenefit(ID))
     return BaseT::getIntrinsicInstrCost(ID, RetTy, Args, FMF, VF, I);
 
   EVT OrigTy = TLI->getValueType(DL, RetTy);
@@ -502,8 +515,14 @@ int GCNTTIImpl::getIntrinsicInstrCost(Intrinsic::ID ID, Type *RetTy,
   if (ST->has16BitInsts() && SLT == MVT::f16)
     NElts = (NElts + 1) / 2;
 
-  return LT.first * NElts * (ST->hasFastFMAF32() ? getHalfRateInstrCost()
-                                                 : getQuarterRateInstrCost());
+  // TODO: Get more refined intrinsic costs?
+  unsigned InstRate = getQuarterRateInstrCost();
+  if (ID == Intrinsic::fma) {
+    InstRate = ST->hasFastFMAF32() ? getHalfRateInstrCost()
+                                   : getQuarterRateInstrCost();
+  }
+
+  return LT.first * NElts * InstRate;
 }
 
 int GCNTTIImpl::getIntrinsicInstrCost(Intrinsic::ID ID, Type *RetTy,

llvm/test/Transforms/SLPVectorizer/AMDGPU/round.ll

@@ -0,0 +1,38 @@
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -mcpu=hawaii -slp-vectorizer %s | FileCheck -check-prefixes=GCN,GFX7 %s
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -mcpu=fiji -slp-vectorizer %s | FileCheck -check-prefixes=GCN,GFX8 %s
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -slp-vectorizer %s | FileCheck -check-prefixes=GCN,GFX8 %s
+
+; GCN-LABEL: @round_v2f16(
+; GFX7: call half @llvm.round.f16(
+; GFX7: call half @llvm.round.f16(
+
+; GFX8: call <2 x half> @llvm.round.v2f16(
+define <2 x half> @round_v2f16(<2 x half> %arg) {
+bb:
+  %tmp = extractelement <2 x half> %arg, i64 0
+  %tmp1 = tail call half @llvm.round.half(half %tmp)
+  %tmp2 = insertelement <2 x half> undef, half %tmp1, i64 0
+  %tmp3 = extractelement <2 x half> %arg, i64 1
+  %tmp4 = tail call half @llvm.round.half(half %tmp3)
+  %tmp5 = insertelement <2 x half> %tmp2, half %tmp4, i64 1
+  ret <2 x half> %tmp5
+}
+
+; TODO: Should probably not really be vectorizing this
+; GCN-LABEL: @round_v2f32(
+; GCN: call <2 x float> @llvm.round.v2f32
+define <2 x float> @round_v2f32(<2 x float> %arg) {
+bb:
+  %tmp = extractelement <2 x float> %arg, i64 0
+  %tmp1 = tail call float @llvm.round.f32(float %tmp)
+  %tmp2 = insertelement <2 x float> undef, float %tmp1, i64 0
+  %tmp3 = extractelement <2 x float> %arg, i64 1
+  %tmp4 = tail call float @llvm.round.f32(float %tmp3)
+  %tmp5 = insertelement <2 x float> %tmp2, float %tmp4, i64 1
+  ret <2 x float> %tmp5
+}
+
+declare half @llvm.round.half(half) #0
+declare float @llvm.round.f32(float) #0
+
+attributes #0 = { nounwind readnone speculatable willreturn }