[MLIR][Presburger] subtract: improve redundant constraint detection

When constraints in the two operands make each other redundant, prefer constraints of the second because this affects the number of sets in the output at each level; reducing these can help prevent exponential blowup.

This is accomplished by adding extra overloads to Simplex::detectRedundant that only scan a subrange of the constraints for redundancy.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D127237

Author: Superty

mlir/include/mlir/Analysis/Presburger/Simplex.h

@@ -705,7 +705,28 @@ class Simplex : public SimplexBase {
   /// the set of solutions does not change if these constraints are removed.
   /// Marks these constraints as redundant. Whether a specific constraint has
   /// been marked redundant can be queried using isMarkedRedundant.
-  void detectRedundant();
+  ///
+  /// The first overload only tries to find redundant constraints with indices
+  /// in the range [offset, offset + count), by scanning constraints from left
+  /// to right in this range. If `count` is not provided, all constraints
+  /// starting at `offset` are scanned, and if neither are provided, all
+  /// constraints are scanned, starting from 0 and going to the last constraint.
+  ///
+  /// As an example, in the set (x) : (x >= 0, x >= 0, x >= 0), calling
+  /// `detectRedundant` with no parameters will result in the first two
+  /// constraints being marked redundant. All copies cannot be marked redundant
+  /// because removing all the constraints changes the set. The first two are
+  /// the ones marked redundant because we scan from left to right. Thus, when
+  /// there is some preference among the constraints as to which should be
+  /// marked redundant with priority when there are multiple possibilities, this
+  /// could be accomplished by succesive calls to detectRedundant(offset,
+  /// count).
+  void detectRedundant(unsigned offset, unsigned count);
+  void detectRedundant(unsigned offset) {
+    assert(offset <= con.size() && "invalid offset!");
+    detectRedundant(offset, con.size() - offset);
+  }
+  void detectRedundant() { detectRedundant(0, con.size()); }
 
   /// Returns a (min, max) pair denoting the minimum and maximum integer values
   /// of the given expression. If no integer value exists, both results will be

mlir/lib/Analysis/Presburger/PresburgerRelation.cpp

@@ -291,11 +291,23 @@ static PresburgerRelation getSetDifference(IntegerRelation b,
         continue;
       }
 
-      simplex.detectRedundant();
-
       // Equalities are added to simplex as a pair of inequalities.
       unsigned totalNewSimplexInequalities =
           2 * sI.getNumEqualities() + sI.getNumInequalities();
+      // Look for redundant constraints among the constraints of sI. We don't
+      // care about redundant constraints in `b` at this point.
+      //
+      // When there are two copies of a constraint in `simplex`, i.e., among the
+      // constraints of `b` and `sI`, only one of them can be marked redundant.
+      // (Assuming no other constraint makes these redundant.)
+      //
+      // In a case where there is one copy in `b` and one in `sI`, we want the
+      // one in `sI` to be marked, not the one in `b`. Therefore, it's not
+      // enough to ignore the constraints of `b` when checking which
+      // constraints `detectRedundant` has marked redundant; we explicitly tell
+      // `detectRedundant` to only mark constraints from `sI` as being
+      // redundant.
+      simplex.detectRedundant(offset, totalNewSimplexInequalities);
       for (unsigned j = 0; j < totalNewSimplexInequalities; j++)
         canIgnoreIneq[j] = simplex.isMarkedRedundant(offset + j);
       simplex.rollback(snapshotBeforeIntersect);

mlir/lib/Analysis/Presburger/Simplex.cpp

@@ -1387,7 +1387,8 @@ void Simplex::markRowRedundant(Unknown &u) {
 }
 
 /// Find a subset of constraints that is redundant and mark them redundant.
-void Simplex::detectRedundant() {
+void Simplex::detectRedundant(unsigned offset, unsigned count) {
+  assert(offset + count <= con.size() && "invalid range!");
   // It is not meaningful to talk about redundancy for empty sets.
   if (empty)
     return;
@@ -1401,7 +1402,8 @@ void Simplex::detectRedundant() {
   // two identical constraints both being marked redundant since each is
   // redundant given the other one. In this example, only the first of the
   // constraints that is processed will get marked redundant, as it should be.
-  for (Unknown &u : con) {
+  for (unsigned i = 0; i < count; ++i) {
+    Unknown &u = con[offset + i];
     if (u.orientation == Orientation::Column) {
       unsigned column = u.pos;
       Optional<unsigned> pivotRow = findPivotRow({}, Direction::Down, column);

mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp

@@ -763,4 +763,10 @@ TEST(SetTest, subtractOutputSizeRegression) {
   // Previously, the subtraction result was producing an extra empty set, which
   // is correct, but bad for output size.
   EXPECT_EQ(result.getNumDisjuncts(), 1u);
+
+  PresburgerSet subtractSelf = set1.subtract(set1);
+  EXPECT_TRUE(subtractSelf.isIntegerEmpty());
+  // Previously, the subtraction result was producing several unnecessary empty
+  // sets, which is correct, but bad for output size.
+  EXPECT_EQ(subtractSelf.getNumDisjuncts(), 0u);
 }