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

[Yingwei Zheng 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] can no 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));
+///       Mask = clear all non-leading-ones bits in Mask;
+///    in the example, the Mask is set to 11'00000'0;
+/// 2. calculate a new mask by setting all common leading bits to 1 in RHS, and
+///    keeping the longest leading ones (i.e., 11'11111'0 in the example).
+/// 3. return (LLo & new mask) as the lower bound;
+/// 4. repeat the step 2 and 3 with LHS and RHS swapped, and update the lower
+///    bound with the larger one.
+static APInt estimateBitMaskedAndLowerBound(const ConstantRange &LHS,
+                                            const ConstantRange &RHS) {
+  auto BitWidth = LHS.getBitWidth();
+  // If either is full set or unsigned wrapped, then the range must contain '0'
+  // which leads the lower bound to 0.
+  if ((LHS.isFullSet() || RHS.isFullSet()) ||
+      (LHS.isWrappedSet() || RHS.isWrappedSet()))
+    return APInt::getZero(BitWidth);
+
+  auto LLo = LHS.getLower();
+  auto LHi = LHS.getUpper() - 1;
+  auto RLo = RHS.getLower();
+  auto RHi = RHS.getUpper() - 1;
+
+  // Calculate the mask for the higher common bits.
+  auto Mask = ~((LLo ^ LHi) | (RLo ^ RHi) | (LLo ^ RLo));
+  unsigned LeadingOnes = Mask.countLeadingOnes();
+  Mask.clearLowBits(BitWidth - LeadingOnes);
+
+  auto estimateBound = [BitWidth, &Mask](APInt ALo, const APInt &BLo,
+                                         const APInt &BHi) {
+    unsigned LeadingOnes = ((BLo & BHi) | Mask).countLeadingOnes();
+    unsigned StartBit = BitWidth - LeadingOnes;
+    ALo.clearLowBits(StartBit);
----------------
dtcxzyw wrote:

Use `ALo.clearHighBits(LeadingOnes)`?


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