[MLIR] Simplex::pivot: also update the redundant rows when pivoting

[arjunp]

Summary

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.

[grosser]

mlir/unittests/Analysis/Presburger/SimplexTest.cpp

14

This header is unused except in the commented code.

398

I guess you do not want to add this commented code?

[arjunp]

Remove commented out test and header. Improve test documentation.

[arjunp]

mlir/unittests/Analysis/Presburger/SimplexTest.cpp

398

Right. Thanks for pointing it out.

[grosser]

There is a typo in the commit message: deteectRedundant

[arjunp]

Fixed the commit message.

[Groverkss]

LGTM! Thank you for the fix.

[arjunp]

Try to rebase. I don't understand why the windows build fails on something in flang, and consistently as well.

[arjunp]

I accidentally replaced this with a different patch, fixed now.

[grosser]

Maybe the windows build is just broken. If this is clearly unrelated, I would not hold this patch back.

[arjunp]

It seems to have worked now.

mlir/lib/Analysis/Presburger/Simplex.cpp

if (tableau(pivotRow, 0) < 0) {
     for (unsigned col = 1; col < nCol; ++col) {
       if (col == pivotCol)
         continue;
       tableau(pivotRow, col) = -tableau(pivotRow, col);
     }
   }
   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.
       continue;
     tableau(row, 0) *= tableau(pivotRow, 0);
     for (unsigned j = 1; j < nCol; ++j) {
       if (j == pivotCol)
         continue;

mlir/unittests/Analysis/Presburger/SimplexTest.cpp

 //===- SimplexTest.cpp - Tests for Simplex --------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "mlir/Analysis/Presburger/Simplex.h"
 
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 namespace mlir {
 
 /// Take a snapshot, add constraints making the set empty, and rollback.
 /// The set should not be empty after rolling back.
 TEST(SimplexTest, emptyRollback) {
   Simplex simplex(2);
   // (u - v) >= 0
   simplex.addInequality({1, -1, 0});
   EXPECT_FALSE(simplex.isEmpty());
TEST(SimplexTest, isMarkedRedundantTiledLoopNestConstraints) {
   EXPECT_FALSE(simplex.isMarkedRedundant(0));
   EXPECT_TRUE(simplex.isMarkedRedundant(1));
   EXPECT_TRUE(simplex.isMarkedRedundant(2));
   EXPECT_FALSE(simplex.isMarkedRedundant(3));
   EXPECT_FALSE(simplex.isMarkedRedundant(4));
   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.
   simplex.addInequality({1, 0});  // x >= 0.
   simplex.detectRedundant();
   ASSERT_FALSE(simplex.isEmpty());
   EXPECT_FALSE(simplex.isMarkedRedundant(0));
   EXPECT_TRUE(simplex.isMarkedRedundant(1));