temporary revert back to previous changes and remove zext handling

Author: Aethezz

llvm/lib/Analysis/ValueTracking.cpp

@@ -426,13 +426,31 @@ static void computeKnownBitsMul(const Value *Op0, const Value *Op1, bool NSW,
 
   // Check if both operands are the same sign-extension of a single value.
   const Value *A = nullptr;
-
   if (match(Op0, m_SExt(m_Value(A))) && match(Op1, m_SExt(m_Specific(A)))) {
     // Product of (sext x) * (sext x) is always non-negative.
-    // So we know the sign bit itself is zero.
-    unsigned SignBits = ComputeNumSignBits(Op0, Q, Depth);
-    if (SignBits > 1)
-      Known.Zero.setHighBits(SignBits - 1);
+    // Compute the maximum possible square and fold all out-of-range bits.
+    Type *FromTy = A->getType();
+    Type *ToTy = Op0->getType();
+    if (FromTy->isIntegerTy() && ToTy->isIntegerTy() &&
+        FromTy->getScalarSizeInBits() < ToTy->getScalarSizeInBits()) {
+      unsigned FromBits = FromTy->getScalarSizeInBits();
+      unsigned ToBits = ToTy->getScalarSizeInBits();
+      // For signed, the maximum absolute value is max(|min|, |max|)
+      APInt minVal = APInt::getSignedMinValue(FromBits);
+      APInt maxVal = APInt::getSignedMaxValue(FromBits);
+      APInt absMin = minVal.abs();
+      APInt absMax = maxVal.abs();
+      APInt maxAbs = absMin.ugt(absMax) ? absMin : absMax;
+      APInt maxSquare = maxAbs.zext(ToBits);
+      maxSquare = maxSquare * maxSquare;
+      // All bits above the highest set bit in maxSquare are known zero.
+      unsigned MaxBit = maxSquare.isZero() ? 0 : maxSquare.logBase2();
+      if (MaxBit + 1 < ToBits) {
+        APInt KnownZeroMask =
+            APInt::getHighBitsSet(ToBits, ToBits - (MaxBit + 1));
+        Known.Zero |= KnownZeroMask;
+      }
+    }
   }
 }