[llvm] [ConstantRange] Handle `Intrinsic::cttz` (PR #67917)

[Yingwei Zheng via llvm-commits]

================
@@ -1735,6 +1746,122 @@ ConstantRange ConstantRange::ctlz(bool ZeroIsPoison) const {
   return getNonEmpty(APInt(getBitWidth(), getUnsignedMax().countl_zero()),
                      APInt(getBitWidth(), getUnsignedMin().countl_zero() + 1));
 }
+static ConstantRange getUnsignedCountTrailingZerosRange(const APInt &Lower,
+                                                        const APInt &Upper) {
+  assert(Lower.ule(Upper));
+  unsigned BitWidth = Lower.getBitWidth();
+  if (Lower == Upper)
+    return ConstantRange::getEmpty(BitWidth);
+  if (Lower + 1 == Upper)
+    return ConstantRange(APInt(BitWidth, Lower.countr_zero()));
+  if (Lower.isZero())
+    return ConstantRange(APInt::getZero(BitWidth),
+                         APInt(BitWidth, BitWidth + 1));
+
+  // Calculate longest common prefix.
+  unsigned LCPLength = (Lower ^ (Upper - 1)).countl_zero();
+  // If Lower is {LCP, 000...}, the maximum is Lower.countr_zero().
+  // Otherwise, the maximum is BitWidth - LCPLength - 1 ({LCP, 100...}).
+  return ConstantRange(
+      APInt::getZero(BitWidth),
+      APInt(BitWidth, std::max(BitWidth - LCPLength, Lower.countr_zero() + 1)));
+}
+
+ConstantRange ConstantRange::cttz(bool ZeroIsPoison) const {
+  if (isEmptySet())
+    return getEmpty();
+
+  APInt Zero = APInt::getZero(getBitWidth());
+
+  if (ZeroIsPoison && contains(Zero)) {
+    // ZeroIsPoison is set, and zero is contained. We discern three cases, in
+    // which a zero can appear:
+    // 1) Lower is zero, handling cases of kind [0, 1), [0, 2), etc.
+    // 2) Upper is zero, wrapped set, handling cases of kind [3, 0], etc.
+    // 3) Zero contained in a wrapped set, e.g., [3, 2), [3, 1), etc.
+
+    if (getLower().isZero()) {
+      if ((getUpper() - 1).isZero()) {
+        // We have in input interval of kind [0, 1). In this case we cannot
+        // really help but return empty-set.
+        return getEmpty();
+      }
+
+      // Compute the resulting range by excluding zero from Lower.
+      return getUnsignedCountTrailingZerosRange(getLower() + 1, getUpper());
+    } else if ((getUpper() - 1).isZero()) {
+      // Compute the resulting range by excluding zero from Upper.
+      return ConstantRange(
+          Zero, APInt(getBitWidth(),
+                      (getUnsignedMax() - getLower() + 1).logBase2() + 1));
----------------
dtcxzyw wrote:

It calculates the range of `cttz` for the input range `[Lower, UnsignedMax]`.
See also https://godbolt.org/z/jMhcs5375

https://github.com/llvm/llvm-project/pull/67917