[ValueTracking][X86] Compute KnownBits for phadd/phsub (#92429)

Add KnownBits computations to ValueTracking and X86 DAG lowering.
    
These instructions add/subtract adjacent vector elements in their operands. Example: phadd [X1, X2] [Y1, Y2] = [X1 + X2, Y1 + Y2]. This means that, in this example, we can compute the KnownBits of the operation by computing the KnownBits of [X1, X2] + [X1, X2] and [Y1, Y2] + [Y1, Y2] and intersecting the results. This approach also generalizes to all x86 vector types.
    
There are also the operations phadd.sw and phsub.sw, which perform saturating addition/subtraction. Use sadd_sat and ssub_sat to compute the KnownBits of these operations.
    
Also adjust the existing test case pr53247.ll because it can be transformed to a constant using the new KnownBits computation.
    
Fixes #82516.

Author: mskamp

llvm/include/llvm/Analysis/VectorUtils.h

@@ -255,6 +255,23 @@ void processShuffleMasks(
     function_ref<void(ArrayRef<int>, unsigned, unsigned)> SingleInputAction,
     function_ref<void(ArrayRef<int>, unsigned, unsigned)> ManyInputsAction);
 
+/// Compute the demanded elements mask of horizontal binary operations. A
+/// horizontal operation combines two adjacent elements in a vector operand.
+/// This function returns a mask for the elements that correspond to the first
+/// operand of this horizontal combination. For example, for two vectors
+/// [X1, X2, X3, X4] and [Y1, Y2, Y3, Y4], the resulting mask can include the
+/// elements X1, X3, Y1, and Y3. To get the other operands, simply shift the
+/// result of this function to the left by 1.
+///
+/// \param VectorBitWidth the total bit width of the vector
+/// \param DemandedElts   the demanded elements mask for the operation
+/// \param DemandedLHS    the demanded elements mask for the left operand
+/// \param DemandedRHS    the demanded elements mask for the right operand
+void getHorizDemandedEltsForFirstOperand(unsigned VectorBitWidth,
+                                         const APInt &DemandedElts,
+                                         APInt &DemandedLHS,
+                                         APInt &DemandedRHS);
+
 /// Compute a map of integer instructions to their minimum legal type
 /// size.
 ///

llvm/lib/Analysis/ValueTracking.cpp

@@ -959,6 +959,32 @@ getKnownBitsFromAndXorOr(const Operator *I, const APInt &DemandedElts,
   return KnownOut;
 }
 
+static KnownBits computeKnownBitsForHorizontalOperation(
+    const Operator *I, const APInt &DemandedElts, unsigned Depth,
+    const SimplifyQuery &Q,
+    const function_ref<KnownBits(const KnownBits &, const KnownBits &)>
+        KnownBitsFunc) {
+  APInt DemandedEltsLHS, DemandedEltsRHS;
+  getHorizDemandedEltsForFirstOperand(Q.DL.getTypeSizeInBits(I->getType()),
+                                      DemandedElts, DemandedEltsLHS,
+                                      DemandedEltsRHS);
+
+  const auto ComputeForSingleOpFunc =
+      [Depth, &Q, KnownBitsFunc](const Value *Op, APInt &DemandedEltsOp) {
+        return KnownBitsFunc(
+            computeKnownBits(Op, DemandedEltsOp, Depth + 1, Q),
+            computeKnownBits(Op, DemandedEltsOp << 1, Depth + 1, Q));
+      };
+
+  if (DemandedEltsRHS.isZero())
+    return ComputeForSingleOpFunc(I->getOperand(0), DemandedEltsLHS);
+  if (DemandedEltsLHS.isZero())
+    return ComputeForSingleOpFunc(I->getOperand(1), DemandedEltsRHS);
+
+  return ComputeForSingleOpFunc(I->getOperand(0), DemandedEltsLHS)
+      .intersectWith(ComputeForSingleOpFunc(I->getOperand(1), DemandedEltsRHS));
+}
+
 // Public so this can be used in `SimplifyDemandedUseBits`.
 KnownBits llvm::analyzeKnownBitsFromAndXorOr(const Operator *I,
                                              const KnownBits &KnownLHS,
@@ -1756,6 +1782,44 @@ static void computeKnownBitsFromOperator(const Operator *I,
       case Intrinsic::x86_sse42_crc32_64_64:
         Known.Zero.setBitsFrom(32);
         break;
+      case Intrinsic::x86_ssse3_phadd_d_128:
+      case Intrinsic::x86_ssse3_phadd_w_128:
+      case Intrinsic::x86_avx2_phadd_d:
+      case Intrinsic::x86_avx2_phadd_w: {
+        Known = computeKnownBitsForHorizontalOperation(
+            I, DemandedElts, Depth, Q,
+            [](const KnownBits &KnownLHS, const KnownBits &KnownRHS) {
+              return KnownBits::computeForAddSub(/*Add=*/true, /*NSW=*/false,
+                                                 /*NUW=*/false, KnownLHS,
+                                                 KnownRHS);
+            });
+        break;
+      }
+      case Intrinsic::x86_ssse3_phadd_sw_128:
+      case Intrinsic::x86_avx2_phadd_sw: {
+        Known = computeKnownBitsForHorizontalOperation(I, DemandedElts, Depth,
+                                                       Q, KnownBits::sadd_sat);
+        break;
+      }
+      case Intrinsic::x86_ssse3_phsub_d_128:
+      case Intrinsic::x86_ssse3_phsub_w_128:
+      case Intrinsic::x86_avx2_phsub_d:
+      case Intrinsic::x86_avx2_phsub_w: {
+        Known = computeKnownBitsForHorizontalOperation(
+            I, DemandedElts, Depth, Q,
+            [](const KnownBits &KnownLHS, const KnownBits &KnownRHS) {
+              return KnownBits::computeForAddSub(/*Add=*/false, /*NSW=*/false,
+                                                 /*NUW=*/false, KnownLHS,
+                                                 KnownRHS);
+            });
+        break;
+      }
+      case Intrinsic::x86_ssse3_phsub_sw_128:
+      case Intrinsic::x86_avx2_phsub_sw: {
+        Known = computeKnownBitsForHorizontalOperation(I, DemandedElts, Depth,
+                                                       Q, KnownBits::ssub_sat);
+        break;
+      }
       case Intrinsic::riscv_vsetvli:
       case Intrinsic::riscv_vsetvlimax: {
         bool HasAVL = II->getIntrinsicID() == Intrinsic::riscv_vsetvli;

llvm/lib/Analysis/VectorUtils.cpp

@@ -567,6 +567,34 @@ void llvm::processShuffleMasks(
   }
 }
 
+void llvm::getHorizDemandedEltsForFirstOperand(unsigned VectorBitWidth,
+                                               const APInt &DemandedElts,
+                                               APInt &DemandedLHS,
+                                               APInt &DemandedRHS) {
+  assert(VectorBitWidth >= 128 && "Vectors smaller than 128 bit not supported");
+  int NumLanes = VectorBitWidth / 128;
+  int NumElts = DemandedElts.getBitWidth();
+  int NumEltsPerLane = NumElts / NumLanes;
+  int HalfEltsPerLane = NumEltsPerLane / 2;
+
+  DemandedLHS = APInt::getZero(NumElts);
+  DemandedRHS = APInt::getZero(NumElts);
+
+  // Map DemandedElts to the horizontal operands.
+  for (int Idx = 0; Idx != NumElts; ++Idx) {
+    if (!DemandedElts[Idx])
+      continue;
+    int LaneIdx = (Idx / NumEltsPerLane) * NumEltsPerLane;
+    int LocalIdx = Idx % NumEltsPerLane;
+    if (LocalIdx < HalfEltsPerLane) {
+      DemandedLHS.setBit(LaneIdx + 2 * LocalIdx);
+    } else {
+      LocalIdx -= HalfEltsPerLane;
+      DemandedRHS.setBit(LaneIdx + 2 * LocalIdx);
+    }
+  }
+}
+
 MapVector<Instruction *, uint64_t>
 llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
                                const TargetTransformInfo *TTI) {

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -5204,29 +5204,10 @@ static void getPackDemandedElts(EVT VT, const APInt &DemandedElts,
 // Split the demanded elts of a HADD/HSUB node between its operands.
 static void getHorizDemandedElts(EVT VT, const APInt &DemandedElts,
                                  APInt &DemandedLHS, APInt &DemandedRHS) {
-  int NumLanes = VT.getSizeInBits() / 128;
-  int NumElts = DemandedElts.getBitWidth();
-  int NumEltsPerLane = NumElts / NumLanes;
-  int HalfEltsPerLane = NumEltsPerLane / 2;
-
-  DemandedLHS = APInt::getZero(NumElts);
-  DemandedRHS = APInt::getZero(NumElts);
-
-  // Map DemandedElts to the horizontal operands.
-  for (int Idx = 0; Idx != NumElts; ++Idx) {
-    if (!DemandedElts[Idx])
-      continue;
-    int LaneIdx = (Idx / NumEltsPerLane) * NumEltsPerLane;
-    int LocalIdx = Idx % NumEltsPerLane;
-    if (LocalIdx < HalfEltsPerLane) {
-      DemandedLHS.setBit(LaneIdx + 2 * LocalIdx + 0);
-      DemandedLHS.setBit(LaneIdx + 2 * LocalIdx + 1);
-    } else {
-      LocalIdx -= HalfEltsPerLane;
-      DemandedRHS.setBit(LaneIdx + 2 * LocalIdx + 0);
-      DemandedRHS.setBit(LaneIdx + 2 * LocalIdx + 1);
-    }
-  }
+  getHorizDemandedEltsForFirstOperand(VT.getSizeInBits(), DemandedElts,
+                                      DemandedLHS, DemandedRHS);
+  DemandedLHS |= DemandedLHS << 1;
+  DemandedRHS |= DemandedRHS << 1;
 }
 
 /// Calculates the shuffle mask corresponding to the target-specific opcode.
@@ -37174,6 +37155,32 @@ static void computeKnownBitsForPMADDUBSW(SDValue LHS, SDValue RHS,
   Known = KnownBits::sadd_sat(Lo, Hi);
 }
 
+static KnownBits computeKnownBitsForHorizontalOperation(
+    const SDValue Op, const APInt &DemandedElts, unsigned Depth,
+    const SelectionDAG &DAG,
+    const function_ref<KnownBits(const KnownBits &, const KnownBits &)>
+        KnownBitsFunc) {
+  APInt DemandedEltsLHS, DemandedEltsRHS;
+  getHorizDemandedEltsForFirstOperand(Op.getValueType().getSizeInBits(),
+                                      DemandedElts, DemandedEltsLHS,
+                                      DemandedEltsRHS);
+
+  const auto ComputeForSingleOpFunc =
+      [&DAG, Depth, KnownBitsFunc](SDValue Op, APInt &DemandedEltsOp) {
+        return KnownBitsFunc(
+            DAG.computeKnownBits(Op, DemandedEltsOp, Depth + 1),
+            DAG.computeKnownBits(Op, DemandedEltsOp << 1, Depth + 1));
+      };
+
+  if (DemandedEltsRHS.isZero())
+    return ComputeForSingleOpFunc(Op.getOperand(0), DemandedEltsLHS);
+  if (DemandedEltsLHS.isZero())
+    return ComputeForSingleOpFunc(Op.getOperand(1), DemandedEltsRHS);
+
+  return ComputeForSingleOpFunc(Op.getOperand(0), DemandedEltsLHS)
+      .intersectWith(ComputeForSingleOpFunc(Op.getOperand(1), DemandedEltsRHS));
+}
+
 void X86TargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
                                                       KnownBits &Known,
                                                       const APInt &DemandedElts,
@@ -37503,6 +37510,17 @@ void X86TargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     }
     break;
   }
+  case X86ISD::HADD:
+  case X86ISD::HSUB: {
+    Known = computeKnownBitsForHorizontalOperation(
+        Op, DemandedElts, Depth, DAG,
+        [Opc](const KnownBits &KnownLHS, const KnownBits &KnownRHS) {
+          return KnownBits::computeForAddSub(
+              /*Add=*/Opc == X86ISD::HADD, /*NSW=*/false, /*NUW=*/false,
+              KnownLHS, KnownRHS);
+        });
+    break;
+  }
   case ISD::INTRINSIC_WO_CHAIN: {
     switch (Op->getConstantOperandVal(0)) {
     case Intrinsic::x86_sse2_pmadd_wd: