more comments

Author: Hardcode84

mlir/include/mlir/Interfaces/InferIntRangeInterface.h

@@ -103,12 +103,14 @@ class ConstantIntRanges {
   /// value.
   std::optional<APInt> getConstantValue() const;
 
-  /// Returns true if signed range is poisoned, i.e. no valid signed value
-  /// can be represented.
+  /// Returns true if signed range is poisoned, poisoned ranges are propagated
+  /// through the DAG and will cause the immediate UB if reached the
+  /// side-effecting operation.
   bool isSignedPoison() const;
 
-  /// Returns true if unsigned range is poisoned, i.e. no valid unsigned value
-  /// can be represented.
+  /// Returns true if unsigned range is poisoned, poisoned ranges are propagated
+  /// through the DAG and will cause the immediate UB if reached the
+  /// side-effecting operation.
   bool isUnsignedPoison() const;
 
   friend raw_ostream &operator<<(raw_ostream &os,

mlir/lib/Interfaces/InferIntRangeInterface.cpp

@@ -112,6 +112,13 @@ ConstantIntRanges::rangeUnion(const ConstantIntRanges &other) const {
   APInt sminUnion;
   APInt smaxUnion;
 
+  // Union of poisoned range with any other range is the other range.
+  // Union is used when we need to merge ranges from multiple indepdenent
+  // sources, e.g. in `arith.select` or CFG merge. "Observing" a poisoned
+  // value (using it in side-effecting operation) will cause the immediate UB.
+  // Well-formed programs should never observe the immediate UB so we assume
+  // result is either unused or only used in circumstances when it received the
+  // non-poisoned argument.
   if (isUnsignedPoison()) {
     uminUnion = other.umin();
     umaxUnion = other.umax();
@@ -151,6 +158,7 @@ ConstantIntRanges::intersection(const ConstantIntRanges &other) const {
   APInt sminIntersect;
   APInt smaxIntersect;
 
+  // Intersection of poisoned range with any other range is poisoned.
   if (isUnsignedPoison()) {
     uminIntersect = umin();
     umaxIntersect = umax();