When parsing, treat an IntegerSet with no constraints as a degenerate true case. Also update the spec to note that affine constraints are optional.

PiperOrigin-RevId: 232158673

mlir/g3doc/LangRef.md

@@ -460,7 +460,7 @@ name.
 integer-set-id ::= `#` suffix-id
 
 integer-set-inline
-   ::= dim-and-symbol-id-lists `:` affine-constraint-conjunction
+   ::= dim-and-symbol-id-lists `:` '(' affine-constraint-conjunction? ')'
 
 // Declarations of integer sets are at the top of the file.
 integer-set-decl ::= integer-set-id `=` integer-set-inline

mlir/lib/Parser/Parser.cpp

@@ -3279,10 +3279,9 @@ AffineExpr AffineParser::parseAffineConstraint(bool *isEq) {
 /// Parse the constraints that are part of an integer set definition.
 ///  integer-set-inline
 ///                ::= dim-and-symbol-id-lists `:`
-///                affine-constraint-conjunction
-///  affine-constraint-conjunction ::= /*empty*/
-///                                 | affine-constraint (`,`
-///                                 affine-constraint)*
+///                '(' affine-constraint-conjunction? ')'
+///  affine-constraint-conjunction ::= affine-constraint (`,`
+///                                       affine-constraint)*
 ///
 IntegerSet AffineParser::parseIntegerSetConstraints(unsigned numDims,
                                                     unsigned numSymbols) {
@@ -3303,17 +3302,15 @@ IntegerSet AffineParser::parseIntegerSetConstraints(unsigned numDims,
     return res;
   };
 
-  // Parse a list of affine constraints (comma-separated) .
-  // Grammar: affine-constraint-conjunct ::= `(` affine-constraint (`,`
-  // affine-constraint)* `)
-  auto constraintListLoc = getToken().getLoc();
+  // Parse a list of affine constraints (comma-separated).
   if (parseCommaSeparatedListUntil(Token::r_paren, parseElt, true))
     return IntegerSet();
 
-  // Check that at least one constraint was parsed.
+  // If no constraints were parsed, then treat this as a degenerate 'true' case.
   if (constraints.empty()) {
-    emitError(constraintListLoc, "expected a valid affine constraint");
-    return IntegerSet();
+    /* 0 == 0 */
+    auto zero = getAffineConstantExpr(0, getContext());
+    return builder.getIntegerSet(numDims, numSymbols, zero, true);
   }
 
   // Parsed a valid integer set.

mlir/test/IR/invalid.mlir

@@ -802,10 +802,6 @@ func @invalid_tensor_literal() {
 
 // -----
 
-#set_without_constraints = (i, j) : () // expected-error {{expected a valid affine constraint}}
-
-// -----
-
 func @invalid_affine_structure() {
   %c0 = constant 0 : index
   %idx = affine_apply (d0, d1) (%c0, %c0) // expected-error {{expected '->' or ':'}}

mlir/test/IR/parser.mlir

@@ -42,6 +42,8 @@
 // CHECK-DAG: #set{{[0-9]+}} = (d0) : (d0 - 1 == 0)
 #set2 = (d0) : (d0 - 1 == 0)
 
+// CHECK-DAG: [[SET_TRUE:#set[0-9]+]] = () : (0 == 0)
+
 // CHECK-DAG: #set{{[0-9]+}} = (d0)[s0] : (d0 - 2 >= 0, -d0 + 4 >= 0)
 
 // CHECK: func @foo(i32, i64) -> f32
@@ -749,6 +751,14 @@ func @type_alias() -> !i32_type_alias {
   return %0 : i32
 }
 
+// CHECK-LABEL: func @no_integer_set_constraints(
+func @no_integer_set_constraints() {
+  // CHECK: if [[SET_TRUE]]() {
+  if () : () () {
+  }
+  return
+}
+
 // CHECK-LABEL: func @verbose_if(
 func @verbose_if(%N: index) {
   %c = constant 200 : index