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[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
This commit is contained in:
Arjun P 2021-11-26 16:29:55 +05:30
parent e714394ab8
commit f074bbb04a
2 changed files with 24 additions and 1 deletions
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2
mlir/lib/Analysis/Presburger/Simplex.cpp
View File

@ -240,7 +240,7 @@ void Simplex::pivot(unsigned pivotRow, unsigned pivotCol) {
}
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.

23
mlir/unittests/Analysis/Presburger/SimplexTest.cpp
View File

@ -373,6 +373,29 @@ TEST(SimplexTest, isMarkedRedundantTiledLoopNestConstraints) {
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.