1) fixed constant folding for mod operation in CanonicalSimplify 2) a…

…dded a unit test (apache#2487)

src/arithmetic/canonical.cc

@@ -363,7 +363,7 @@ class Canonical::Internal : public IRMutator {
       return Binary_(op, e, a.value, b.value);
     }
     if (is_const(a.value) && is_const(b.value)) {
-      return ComputeExpr<Mul>(a.value, b.value);
+      return ComputeExpr<Mod>(a.value, b.value);
     } else if (is_const(b.value)) {
       return SumModConst(a.AsSum(), b.value);
     } else {

tests/cpp/ir_simplify_test.cc

@@ -27,6 +27,16 @@ TEST(IRSIMPLIFY, Mul) {
   CHECK(is_zero(es));
 }
 
+TEST(IRSIMPLIFY, Mod) {
+  auto x = tvm::Integer(10);
+  auto y = tvm::Integer(12);
+  // Mod::make is used instead of % to avoid constant folding during
+  // calling operator%(x,y). Mod::make doesn't try constant folding,
+  // and therefore, the constant folding will be attempted in CanonicalSimplify
+  auto mod = tvm::ir::CanonicalSimplify(tvm::ir::Mod::make(x, y));
+  auto es = tvm::ir::CanonicalSimplify(mod - x);
+  CHECK(is_zero(es));
+}
 int main(int argc, char ** argv) {
   testing::InitGoogleTest(&argc, argv);
   testing::FLAGS_gtest_death_test_style = "threadsafe";