[mlir][affine] Add unit tests for isProjectedPermutation

The only way to test `isProjectedPermutation` is through unit tests. The
concept of "projected permutations" is tricky to document and these
tests are a good source documentation of the expected/intended
behavoiur. Hence these additional unit tests.

Author: banach-space

mlir/unittests/IR/AffineMapTest.cpp

@@ -21,3 +21,44 @@ TEST(AffineMapTest, inferMapFromAffineExprs) {
   map.replace(replacements);
   EXPECT_EQ(map, map);
 }
+
+TEST(AffineMapTest, isProjectedPermutation) {
+  MLIRContext ctx;
+  OpBuilder b(&ctx);
+
+  // 1. Empty map - a projected permutation.
+  AffineMap map1 = b.getEmptyAffineMap();
+  EXPECT_TRUE(map1.isProjectedPermutation());
+
+  // 2. Contains a symbol - not a projected permutation.
+  AffineMap map2 = AffineMap::get(0, 1, &ctx);
+  EXPECT_FALSE(map2.isProjectedPermutation());
+
+  // 3. The result map is {0} - since zero results are _allowed_, this _is_ a
+  // projected permutation.
+  auto zero = b.getAffineConstantExpr(0);
+  AffineMap map3 = AffineMap::get(1, 0, {zero}, &ctx);
+  EXPECT_TRUE(map3.isProjectedPermutation(/*allowZeroInResults=*/true));
+
+  // 4. The result map is {0} - since zero results are _not allowed_, this _is
+  // not_ a projected permutation.
+  AffineMap map4 = AffineMap::get(1, 0, {zero}, &ctx);
+  EXPECT_FALSE(map4.isProjectedPermutation(/*allowZeroInResults=*/false));
+
+  // 5. The number of results > inputs, not a projected permutation.
+  AffineMap map5 = AffineMap::get(1, 0, {zero, zero}, &ctx);
+  EXPECT_FALSE(map5.isProjectedPermutation(/*allowZeroInResults=*/true));
+
+  // 6. A constant result that's not a {0} - not a projected permutation.
+  auto one = b.getAffineConstantExpr(1);
+  AffineMap map6 = AffineMap::get(1, 0, {one}, &ctx);
+  EXPECT_FALSE(map6.isProjectedPermutation(/*allowZeroInResults=*/true));
+
+  // 7. Not a dim expression - not a projected permutation.
+  auto d0 = b.getAffineDimExpr(0);
+  auto d1 = b.getAffineDimExpr(1);
+
+  auto sum = d0 + d1;
+  AffineMap map7 = AffineMap::get(2, 0, {sum}, &ctx);
+  EXPECT_FALSE(map7.isProjectedPermutation());
+}