[MLIR] Simplex::normalizeRow: early exit when gcd is one

[bondhugula]

mlir/lib/Analysis/Presburger/Simplex.cpp

133

What's the denominator here? Doesn't seem to be in context.

[arjunp]

Clarified where the denominator is.

mlir/lib/Analysis/Presburger/Simplex.cpp

133

The first element of every row is the denominator. This is also documented in the class header, but I mentioned it here as well for clarity. Thanks for the quick review.

[mehdi_amini]

mlir/lib/Analysis/Presburger/Simplex.cpp

134

Nit: in general instead of two lines of comments, I like to "document with assert":

assert(gcd && "first element of each row is the denominator and should be non-zero");

mlir/lib/Analysis/Presburger/Simplex.cpp

unsigned SimplexBase::addRow(ArrayRef<int64_t> coeffs) {
   return con.size() - 1;
 }
 
 /// Normalize the row by removing factors that are common between the
 /// denominator and all the numerator coefficients.
 void SimplexBase::normalizeRow(unsigned row) {
   int64_t gcd = 0;
   for (unsigned col = 0; col < nCol; ++col) {
-    if (gcd == 1)
-      break;
     gcd = llvm::greatestCommonDivisor(gcd, std::abs(tableau(row, col)));
+    // If the gcd becomes 1 then the row is already normalized.
+    if (gcd == 1)
+      return;
   }
+
+  // Note that the gcd can never become zero since the first element of the row,
+  // the denominator, is non-zero.
   for (unsigned col = 0; col < nCol; ++col)
     tableau(row, col) /= gcd;
 }
 
 namespace {
 bool signMatchesDirection(int64_t elem, Direction direction) {
   assert(elem != 0 && "elem should not be 0");
   return direction == Direction::Up ? elem > 0 : elem < 0;