[llvm] [ConstantRange] Handle `Intrinsic::ctpop` (PR #68310)

[Nikita Popov via llvm-commits]

================
@@ -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(getLower(), getUpper());
----------------
nikic wrote:

```suggestion
    return getUnsignedPopCountRange(Lower, Upper);
```
etc. No need to go through the getters.

https://github.com/llvm/llvm-project/pull/68310