use ComputeNumSignBits() instead and remove zext handling

Author: Aethezz

llvm/lib/Analysis/ValueTracking.cpp

@@ -430,40 +430,13 @@ static void computeKnownBitsMul(const Value *Op0, const Value *Op1, bool NSW,
   // possible square. This allows InstCombine and other passes to fold (x * x) &
   // (1 << N) to 0 when N is out of range.
   const Value *A = nullptr;
-  // Only handle the case where both operands are the same extension of the same
-  // value.
-  if ((match(Op0, m_SExt(m_Value(A))) && match(Op1, m_SExt(m_Specific(A)))) ||
-      (match(Op0, m_ZExt(m_Value(A))) && match(Op1, m_ZExt(m_Specific(A))))) {
-    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 both signed and unsigned, the maximum absolute value is max(|min|,
-      // |max|)
-      APInt minVal(FromBits, 0), maxVal(FromBits, 0);
-      bool isSigned = isa<SExtInst>(Op0);
-      if (isSigned) {
-        minVal = APInt::getSignedMinValue(FromBits);
-        maxVal = APInt::getSignedMaxValue(FromBits);
-      } else {
-        minVal = APInt::getMinValue(FromBits);
-        maxVal = APInt::getMaxValue(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;
-      }
-    }
+
+  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);
   }
 }