[ConstantRange] Handle Intrinsic::ctpop (#68310)

This patch adds support for `Intrinsic::ctpop` in ConstantRange. It
calculates the range in O(1) with the LCP-based method.

Migrated from https://reviews.llvm.org/D153505.

llvm/include/llvm/IR/ConstantRange.h

@@ -530,6 +530,9 @@ class [[nodiscard]] ConstantRange {
   /// ignoring a possible zero value contained in the input range.
   ConstantRange ctlz(bool ZeroIsPoison = false) const;
 
+  /// Calculate ctpop range.
+  ConstantRange ctpop() const;
+
   /// Represents whether an operation on the given constant range is known to
   /// always or never overflow.
   enum class OverflowResult {

llvm/lib/IR/ConstantRange.cpp

@@ -949,6 +949,7 @@ bool ConstantRange::isIntrinsicSupported(Intrinsic::ID IntrinsicID) {
   case Intrinsic::smax:
   case Intrinsic::abs:
   case Intrinsic::ctlz:
+  case Intrinsic::ctpop:
     return true;
   default:
     return false;
@@ -986,6 +987,8 @@ ConstantRange ConstantRange::intrinsic(Intrinsic::ID IntrinsicID,
     assert(ZeroIsPoison->getBitWidth() == 1 && "Must be boolean");
     return Ops[0].ctlz(ZeroIsPoison->getBoolValue());
   }
+  case Intrinsic::ctpop:
+    return Ops[0].ctpop();
   default:
     assert(!isIntrinsicSupported(IntrinsicID) && "Shouldn't be supported");
     llvm_unreachable("Unsupported intrinsic");
@@ -1736,6 +1739,52 @@ ConstantRange ConstantRange::ctlz(bool ZeroIsPoison) const {
                      APInt(getBitWidth(), getUnsignedMin().countl_zero() + 1));
 }
 
+static ConstantRange getUnsignedPopCountRange(const APInt &Lower,
+                                              const APInt &Upper) {
+  assert(!ConstantRange(Lower, Upper).isWrappedSet() &&
+         "Unexpected wrapped set.");
+  assert(Lower != Upper && "Unexpected empty set.");
+  unsigned BitWidth = Lower.getBitWidth();
+  if (Lower + 1 == Upper)
+    return ConstantRange(APInt(BitWidth, Lower.popcount()));
+
+  APInt Max = Upper - 1;
+  // Calculate longest common prefix.
+  unsigned LCPLength = (Lower ^ Max).countl_zero();
+  unsigned LCPPopCount = Lower.getHiBits(LCPLength).popcount();
+  // If Lower is {LCP, 000...}, the minimum is the popcount of LCP.
+  // Otherwise, the minimum is the popcount of LCP + 1.
+  unsigned MinBits =
+      LCPPopCount + (Lower.countr_zero() < BitWidth - LCPLength ? 1 : 0);
+  // If Max is {LCP, 111...}, the maximum is the popcount of LCP + (BitWidth -
+  // length of LCP).
+  // Otherwise, the minimum is the popcount of LCP + (BitWidth -
+  // length of LCP - 1).
+  unsigned MaxBits = LCPPopCount + (BitWidth - LCPLength) -
+                     (Max.countr_one() < BitWidth - LCPLength ? 1 : 0);
+  return ConstantRange(APInt(BitWidth, MinBits), APInt(BitWidth, MaxBits + 1));
+}
+
+ConstantRange ConstantRange::ctpop() const {
+  if (isEmptySet())
+    return getEmpty();
+
+  unsigned BitWidth = getBitWidth();
+  APInt Zero = APInt::getZero(BitWidth);
+  if (isFullSet())
+    return getNonEmpty(Zero, APInt(BitWidth, BitWidth + 1));
+  if (!isWrappedSet())
+    return getUnsignedPopCountRange(Lower, Upper);
+  // The range is wrapped. We decompose it into two ranges, [0, Upper) and
+  // [Lower, 0).
+  // Handle [Lower, 0) == [Lower, Max]
+  ConstantRange CR1 = ConstantRange(APInt(BitWidth, Lower.countl_one()),
+                                    APInt(BitWidth, BitWidth + 1));
+  // Handle [0, Upper)
+  ConstantRange CR2 = getUnsignedPopCountRange(Zero, Upper);
+  return CR1.unionWith(CR2);
+}
+
 ConstantRange::OverflowResult ConstantRange::unsignedAddMayOverflow(
     const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())

llvm/test/Transforms/CorrelatedValuePropagation/range.ll

@@ -1085,8 +1085,7 @@ define i1 @ctpop_fold(i16 %x) {
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
 ; CHECK-NEXT:    [[CTPOP:%.*]] = call i16 @llvm.ctpop.i16(i16 [[X]])
-; CHECK-NEXT:    [[RES:%.*]] = icmp ule i16 [[CTPOP]], 8
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       else:
 ; CHECK-NEXT:    ret i1 true
 ;

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -2438,6 +2438,12 @@ TEST_F(ConstantRangeTest, Ctlz) {
       });
 }
 
+TEST_F(ConstantRangeTest, Ctpop) {
+  TestUnaryOpExhaustive(
+      [](const ConstantRange &CR) { return CR.ctpop(); },
+      [](const APInt &N) { return APInt(N.getBitWidth(), N.popcount()); });
+}
+
 TEST_F(ConstantRangeTest, castOps) {
   ConstantRange A(APInt(16, 66), APInt(16, 128));
   ConstantRange FpToI8 = A.castOp(Instruction::FPToSI, 8);