diff --git a/mlir/include/mlir/Analysis/Presburger/Simplex.h b/mlir/include/mlir/Analysis/Presburger/Simplex.h
--- a/mlir/include/mlir/Analysis/Presburger/Simplex.h
+++ b/mlir/include/mlir/Analysis/Presburger/Simplex.h
@@ -440,10 +440,9 @@
void appendSymbol();
/// Try to move the specified row to column orientation while preserving the
- /// lexicopositivity of the basis transform. The row must have a negative
- /// sample value. If this is not possible, return failure. This only occurs
- /// when the constraints have no solution; the tableau will be marked empty in
- /// such a case.
+ /// lexicopositivity of the basis transform. The row must have a non-positive
+ /// sample value. If this is not possible, return failure. This occurs when
+ /// the constraints have no solution or the sample value is zero.
LogicalResult moveRowUnknownToColumn(unsigned row);
/// Given a row that has a non-integer sample value, add an inequality to cut
diff --git a/mlir/lib/Analysis/Presburger/Simplex.cpp b/mlir/lib/Analysis/Presburger/Simplex.cpp
--- a/mlir/lib/Analysis/Presburger/Simplex.cpp
+++ b/mlir/lib/Analysis/Presburger/Simplex.cpp
@@ -212,8 +212,10 @@
/// add these to the set of ignored columns and continue to the next row. If we
/// run out of rows, then A*y is zero and we are done.
MaybeOptimum> LexSimplex::findRationalLexMin() {
- if (restoreRationalConsistency().failed())
+ if (restoreRationalConsistency().failed()) {
+ markEmpty();
return OptimumKind::Empty;
+ }
return getRationalSample();
}
@@ -679,16 +681,16 @@
}
// Move the row unknown to column orientation while preserving lexicopositivity
-// of the basis transform. The sample value of the row must be negative.
+// of the basis transform. The sample value of the row must be non-positive.
//
// We only consider pivots where the pivot element is positive. Suppose no such
// pivot exists, i.e., some violated row has no positive coefficient for any
// basis unknown. The row can be represented as (s + c_1*u_1 + ... + c_n*u_n)/d,
// where d is the denominator, s is the sample value and the c_i are the basis
-// coefficients. Since any feasible assignment of the basis satisfies u_i >= 0
-// for all i, and we have s < 0 as well as c_i < 0 for all i, any feasible
-// assignment would violate this row and therefore the constraints have no
-// solution.
+// coefficients. If s != 0, then since any feasible assignment of the basis
+// satisfies u_i >= 0 for all i, and we have s < 0 as well as c_i < 0 for all i,
+// any feasible assignment would violate this row and therefore the constraints
+// have no solution.
//
// We can preserve lexicopositivity by picking the pivot column with positive
// pivot element that makes the lexicographically smallest change to the sample
@@ -726,10 +728,10 @@
// B'.col(k) = B.col(k) - B(i,k) * B.col(j) / B(i,j) for k != j
// and similarly, s' = s - s_i * B.col(j) / B(i,j).
//
-// Since the row is violated, we have s_i < 0, so the change in sample value
-// when pivoting with column a is lexicographically smaller than that when
-// pivoting with column b iff B.col(a) / B(i, a) is lexicographically smaller
-// than B.col(b) / B(i, b).
+// If s_i == 0, then the sample value remains unchanged. Otherwise, if s_i < 0,
+// the change in sample value when pivoting with column a is lexicographically
+// smaller than that when pivoting with column b iff B.col(a) / B(i, a) is
+// lexicographically smaller than B.col(b) / B(i, b).
//
// Since B(i, j) > 0, column j remains lexicopositive.
//
@@ -749,10 +751,8 @@
!maybeColumn ? col : getLexMinPivotColumn(row, *maybeColumn, col);
}
- if (!maybeColumn) {
- markEmpty();
+ if (!maybeColumn)
return failure();
- }
pivot(row, *maybeColumn);
return success();