[clang] [llvm] [ConstantRange] Estimate tighter lower (upper) bounds for masked binary and (or) (PR #120352)

[Fangrui Song via llvm-commits]

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@@ -1520,15 +1520,72 @@ ConstantRange ConstantRange::binaryNot() const {
   return ConstantRange(APInt::getAllOnes(getBitWidth())).sub(*this);
 }
 
+/// Estimate the 'bit-masked AND' operation's lower bound.
+///
+/// E.g., given two ranges as follows (single quotes are separators and
+/// have no meaning here),
+///
+///   LHS = [10'00101'1,  ; LLo
+///          10'10000'0]  ; LHi
+///   RHS = [10'11111'0,  ; RLo
+///          10'11111'1]  ; RHi
+///
+/// we know that the higher 2 bits of the result is always 10; and we also
+/// notice that RHS[1:6] are always 1, so the result[1:6] cannot be less than
+/// LHS[1:6] (i.e., 00101). Thus, the lower bound is 10'00101'0.
+///
+/// The algorithm is as follows,
+/// 1. we first calculate a mask to find the higher common bits by
+///       Mask = ~((LLo ^ LHi) | (LLo ^ LHi) | (LLo ^ RLo));
----------------
MaskRay wrote:

`Mask = ~((LLo ^ LHi) | (RLo ^ RHi) | (LLo ^ RLo))`

https://github.com/llvm/llvm-project/pull/120352