[MLIR] Simplex::pivot: also update the redundant rows when pivoting
Previously, the pivot function would only update the non-redundant rows when
pivoting. This is incorrect because in some cases, when rolling back past a
`detectRedundant` call, the basis being used could be different from that which
was used at the time of returning from the `detectRedundant` call. Therefore,
it is important to update the redundant rows as well during pivots. This could
also be triggered by pivots that occur when testing successive constraints for
being redundant in `detectRedundant` after some initial constraints are marked redundant.
Reviewed By: Groverkss
Differential Revision: https://reviews.llvm.org/D114614
GitOrigin-RevId: f074bbb04a617f366093a860e5b0976d80129ff4
diff --git a/lib/Analysis/Presburger/Simplex.cpp b/lib/Analysis/Presburger/Simplex.cpp
index a56213c..b040601 100644
--- a/lib/Analysis/Presburger/Simplex.cpp
+++ b/lib/Analysis/Presburger/Simplex.cpp
@@ -240,7 +240,7 @@
}
normalizeRow(pivotRow);
- for (unsigned row = nRedundant; row < nRow; ++row) {
+ for (unsigned row = 0; row < nRow; ++row) {
if (row == pivotRow)
continue;
if (tableau(row, pivotCol) == 0) // Nothing to do.
diff --git a/unittests/Analysis/Presburger/SimplexTest.cpp b/unittests/Analysis/Presburger/SimplexTest.cpp
index 2f1ce81..d1d7254 100644
--- a/unittests/Analysis/Presburger/SimplexTest.cpp
+++ b/unittests/Analysis/Presburger/SimplexTest.cpp
@@ -373,6 +373,29 @@
EXPECT_FALSE(simplex.isMarkedRedundant(5));
}
+TEST(Simplextest, pivotRedundantRegressionTest) {
+ Simplex simplex(2);
+ simplex.addInequality({-1, 0, -1}); // x <= -1.
+ unsigned snapshot = simplex.getSnapshot();
+
+ simplex.addInequality({-1, 0, -2}); // x <= -2.
+ simplex.addInequality({-3, 0, -6});
+
+ // This first marks x <= -1 as redundant. Then it performs some more pivots
+ // to check if the other constraints are redundant. Pivot must update the
+ // non-redundant rows as well, otherwise these pivots result in an incorrect
+ // tableau state. In particular, after the rollback below, some rows that are
+ // NOT marked redundant will have an incorrect state.
+ simplex.detectRedundant();
+
+ // After the rollback, the only remaining constraint is x <= -1.
+ // The maximum value of x should be -1.
+ simplex.rollback(snapshot);
+ Optional<Fraction> maxX =
+ simplex.computeOptimum(Simplex::Direction::Up, {1, 0, 0});
+ EXPECT_TRUE(maxX.hasValue() && *maxX == Fraction(-1, 1));
+}
+
TEST(SimplexTest, addInequality_already_redundant) {
Simplex simplex(1);
simplex.addInequality({1, -1}); // x >= 1.
