[ConstantRange] Handle `Intrinsic::ctlz`

Introduce ConstantRange support for ctlz intrinsic, including
exhaustive testing. Among other things, LVI may now be able to
propagate information about cltz constant ranges lattice values.

Differential Revision: https://reviews.llvm.org/D142234

1 files changed, 46 insertions, 0 deletions

diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 0dbccaa..ab94053 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -945,6 +945,7 @@ bool ConstantRange::isIntrinsicSupported(Intrinsic::ID IntrinsicID) {
   case Intrinsic::smin:
   case Intrinsic::smax:
   case Intrinsic::abs:
+  case Intrinsic::ctlz:
     return true;
   default:
     return false;
@@ -976,6 +977,12 @@ ConstantRange ConstantRange::intrinsic(Intrinsic::ID IntrinsicID,
     assert(IntMinIsPoison->getBitWidth() == 1 && "Must be boolean");
     return Ops[0].abs(IntMinIsPoison->getBoolValue());
   }
+  case Intrinsic::ctlz: {
+    const APInt *ZeroIsPoison = Ops[1].getSingleElement();
+    assert(ZeroIsPoison && "Must be known (immarg)");
+    assert(ZeroIsPoison->getBitWidth() == 1 && "Must be boolean");
+    return Ops[0].ctlz(ZeroIsPoison->getBoolValue());
+  }
   default:
     assert(!isIntrinsicSupported(IntrinsicID) && "Shouldn't be supported");
     llvm_unreachable("Unsupported intrinsic");
@@ -1667,6 +1674,45 @@ ConstantRange ConstantRange::abs(bool IntMinIsPoison) const {
                                     APIntOps::umax(-SMin, SMax) + 1);
 }
 
+ConstantRange ConstantRange::ctlz(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 ConstantRange(
+          APInt(getBitWidth(), (getUpper() - 1).countLeadingZeros()),
+          APInt(getBitWidth(), (getLower() + 1).countLeadingZeros() + 1));
+    } else if ((getUpper() - 1).isZero()) {
+      // Compute the resulting range by excluding zero from Upper.
+      return ConstantRange(
+          Zero, APInt(getBitWidth(), getLower().countLeadingZeros() + 1));
+    } else {
+      return ConstantRange(Zero, APInt(getBitWidth(), getBitWidth()));
+    }
+  }
+
+  // Zero is either safe or not in the range. The output range is composed by
+  // the result of countLeadingZero of the two extremes.
+  return getNonEmpty(
+      APInt(getBitWidth(), getUnsignedMax().countLeadingZeros()),
+      APInt(getBitWidth(), getUnsignedMin().countLeadingZeros() + 1));
+}
+
 ConstantRange::OverflowResult ConstantRange::unsignedAddMayOverflow(
     const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())