[MLIR][Presburger] Fix subtract processing extra inequalities

This patch fixes a bug in PresburgeRelation::subtract that made it process the
inequality at index 0, multiple times. This was caused by allocating memory
instead of reserving memory in llvm::SmallVector.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D127228

mlir/lib/Analysis/Presburger/PresburgerRelation.cpp

@@ -300,7 +300,8 @@ static PresburgerRelation getSetDifference(IntegerRelation b,
         canIgnoreIneq[j] = simplex.isMarkedRedundant(offset + j);
       simplex.rollback(snapshotBeforeIntersect);
 
-      SmallVector<unsigned, 8> ineqsToProcess(totalNewSimplexInequalities);
+      SmallVector<unsigned, 8> ineqsToProcess;
+      ineqsToProcess.reserve(totalNewSimplexInequalities);
       for (unsigned i = 0; i < totalNewSimplexInequalities; ++i)
         if (!canIgnoreIneq[i])
           ineqsToProcess.push_back(i);

mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp

@@ -746,3 +746,21 @@ TEST(SetTest, computeVolume) {
                                         /*trueVolume=*/{},
                                         /*resultBound=*/{});
 }
+
+TEST(SetTest, subtractOutputSizeRegression) {
+  PresburgerSet set1 =
+      parsePresburgerSetFromPolyStrings(1, {"(i) : (i >= 0, 10 - i >= 0)"});
+  PresburgerSet set2 =
+      parsePresburgerSetFromPolyStrings(1, {"(i) : (i - 5 >= 0)"});
+
+  PresburgerSet set3 =
+      parsePresburgerSetFromPolyStrings(1, {"(i) : (i >= 0, 4 - i >= 0)"});
+
+  PresburgerSet result = set1.subtract(set2);
+
+  EXPECT_TRUE(result.isEqual(set3));
+
+  // Previously, the subtraction result was producing an extra empty set, which
+  // is correct, but bad for output size.
+  EXPECT_EQ(result.getNumDisjuncts(), 1u);
+}