[WebAssembly] Optimize BUILD_VECTOR lowering for size

Summary:
Implements custom lowering logic that finds the optimal value for the
initial splat of the vector and either uses it or uses v128.const if
it is available and if it would produce smaller code. This logic
replaces large TableGen ISEL patterns that would lower all non-splat
BUILD_VECTORs into a splat followed by a fixed number of replace_lane
instructions. This CL fixes PR39685.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D56633

llvm-svn: 352592

1 files changed, 110 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
index 1a8b598c074..7cb7f2750ff 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -131,6 +131,13 @@ WebAssemblyTargetLowering::WebAssemblyTargetLowering(
       for (auto T : {MVT::v16i8, MVT::v8i16})
         setOperationAction(Op, T, Legal);
 
+    // Custom lower BUILD_VECTORs to minimize number of replace_lanes
+    for (auto T : {MVT::v16i8, MVT::v8i16, MVT::v4i32, MVT::v4f32})
+      setOperationAction(ISD::BUILD_VECTOR, T, Custom);
+    if (Subtarget->hasUnimplementedSIMD128())
+      for (auto T : {MVT::v2i64, MVT::v2f64})
+        setOperationAction(ISD::BUILD_VECTOR, T, Custom);
+
     // We have custom shuffle lowering to expose the shuffle mask
     for (auto T : {MVT::v16i8, MVT::v8i16, MVT::v4i32, MVT::v4f32})
       setOperationAction(ISD::VECTOR_SHUFFLE, T, Custom);
@@ -886,6 +893,8 @@ SDValue WebAssemblyTargetLowering::LowerOperation(SDValue Op,
     return LowerINTRINSIC_VOID(Op, DAG);
   case ISD::SIGN_EXTEND_INREG:
     return LowerSIGN_EXTEND_INREG(Op, DAG);
+  case ISD::BUILD_VECTOR:
+    return LowerBUILD_VECTOR(Op, DAG);
   case ISD::VECTOR_SHUFFLE:
     return LowerVECTOR_SHUFFLE(Op, DAG);
   case ISD::SHL:
@@ -1103,6 +1112,107 @@ WebAssemblyTargetLowering::LowerSIGN_EXTEND_INREG(SDValue Op,
   return SDValue();
 }
 
+SDValue WebAssemblyTargetLowering::LowerBUILD_VECTOR(SDValue Op,
+                                                     SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+  const EVT VecT = Op.getValueType();
+  const EVT LaneT = Op.getOperand(0).getValueType();
+  const size_t Lanes = Op.getNumOperands();
+  auto IsConstant = [](const SDValue &V) {
+    return V.getOpcode() == ISD::Constant || V.getOpcode() == ISD::ConstantFP;
+  };
+
+  // Find the most common operand, which is approximately the best to splat
+  using Entry = std::pair<SDValue, size_t>;
+  SmallVector<Entry, 16> ValueCounts;
+  size_t NumConst = 0, NumDynamic = 0;
+  for (const SDValue &Lane : Op->op_values()) {
+    if (Lane.isUndef()) {
+      continue;
+    } else if (IsConstant(Lane)) {
+      NumConst++;
+    } else {
+      NumDynamic++;
+    }
+    auto CountIt = std::find_if(ValueCounts.begin(), ValueCounts.end(),
+                                [&Lane](Entry A) { return A.first == Lane; });
+    if (CountIt == ValueCounts.end()) {
+      ValueCounts.emplace_back(Lane, 1);
+    } else {
+      CountIt->second++;
+    }
+  }
+  auto CommonIt =
+      std::max_element(ValueCounts.begin(), ValueCounts.end(),
+                       [](Entry A, Entry B) { return A.second < B.second; });
+  assert(CommonIt != ValueCounts.end() && "Unexpected all-undef build_vector");
+  SDValue SplatValue = CommonIt->first;
+  size_t NumCommon = CommonIt->second;
+
+  // If v128.const is available, consider using it instead of a splat
+  if (Subtarget->hasUnimplementedSIMD128()) {
+    // {i32,i64,f32,f64}.const opcode, and value
+    const size_t ConstBytes = 1 + std::max(size_t(4), 16 / Lanes);
+    // SIMD prefix and opcode
+    const size_t SplatBytes = 2;
+    const size_t SplatConstBytes = SplatBytes + ConstBytes;
+    // SIMD prefix, opcode, and lane index
+    const size_t ReplaceBytes = 3;
+    const size_t ReplaceConstBytes = ReplaceBytes + ConstBytes;
+    // SIMD prefix, v128.const opcode, and 128-bit value
+    const size_t VecConstBytes = 18;
+    // Initial v128.const and a replace_lane for each non-const operand
+    const size_t ConstInitBytes = VecConstBytes + NumDynamic * ReplaceBytes;
+    // Initial splat and all necessary replace_lanes
+    const size_t SplatInitBytes =
+        IsConstant(SplatValue)
+            // Initial constant splat
+            ? (SplatConstBytes +
+               // Constant replace_lanes
+               (NumConst - NumCommon) * ReplaceConstBytes +
+               // Dynamic replace_lanes
+               (NumDynamic * ReplaceBytes))
+            // Initial dynamic splat
+            : (SplatBytes +
+               // Constant replace_lanes
+               (NumConst * ReplaceConstBytes) +
+               // Dynamic replace_lanes
+               (NumDynamic - NumCommon) * ReplaceBytes);
+    if (ConstInitBytes < SplatInitBytes) {
+      // Create build_vector that will lower to initial v128.const
+      SmallVector<SDValue, 16> ConstLanes;
+      for (const SDValue &Lane : Op->op_values()) {
+        if (IsConstant(Lane)) {
+          ConstLanes.push_back(Lane);
+        } else if (LaneT.isFloatingPoint()) {
+          ConstLanes.push_back(DAG.getConstantFP(0, DL, LaneT));
+        } else {
+          ConstLanes.push_back(DAG.getConstant(0, DL, LaneT));
+        }
+      }
+      SDValue Result = DAG.getBuildVector(VecT, DL, ConstLanes);
+      // Add replace_lane instructions for non-const lanes
+      for (size_t I = 0; I < Lanes; ++I) {
+        const SDValue &Lane = Op->getOperand(I);
+        if (!Lane.isUndef() && !IsConstant(Lane))
+          Result = DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, VecT, Result, Lane,
+                               DAG.getConstant(I, DL, MVT::i32));
+      }
+      return Result;
+    }
+  }
+  // Use a splat for the initial vector
+  SDValue Result = DAG.getSplatBuildVector(VecT, DL, SplatValue);
+  // Add replace_lane instructions for other values
+  for (size_t I = 0; I < Lanes; ++I) {
+    const SDValue &Lane = Op->getOperand(I);
+    if (Lane != SplatValue)
+      Result = DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, VecT, Result, Lane,
+                           DAG.getConstant(I, DL, MVT::i32));
+  }
+  return Result;
+}
+
 SDValue
 WebAssemblyTargetLowering::LowerVECTOR_SHUFFLE(SDValue Op,
                                                SelectionDAG &DAG) const {