[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(Lower.ule(Upper) && "Unexpected wrapped set.");
+  unsigned BitWidth = Lower.getBitWidth();
+  if (Lower == Upper)
+    return ConstantRange::getEmpty(BitWidth);
+  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));
+}
+
+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 (!isUpperWrapped()) {
+    return getUnsignedPopCountRange(getLower(), getUpper());
+  }
+  ConstantRange CR1 = ConstantRange(
+      APInt(BitWidth,
+            BitWidth - (getUnsignedMax() - getLower() + 1).logBase2()),
----------------
nikic wrote:

getUnsignedMax() will always be AllOnesValues here. And AllOnesValue+1 is zero. So I think this is equivalent to:
```suggestion
            BitWidth - (-getLower()).logBase2()),
```
However, I don't quite understand the logic behind this bit of code. Could you maybe add a comment?

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