[mlir][polynomial] Add and verify constraints of coefficientModulus for ringAttr (llvm#111016)

Currently the semantic of coefficientModulus is unclear and a lowering
of it faces uncertainty, for example,
google/heir#995 (comment)

Also, it lacks a verifier which should conform to the definition in the
document.

This PR tries to further define the semantic of coefficientModulus and
adds a verifier for it.

Cc @j2kun for review and suggestions.

Author: ZenithalHourlyRate

mlir/include/mlir/Dialect/Polynomial/IR/PolynomialAttributes.td

@@ -161,13 +161,25 @@ def Polynomial_RingAttr : Polynomial_Attr<"Ring", "ring"> {
 
     The coefficient and polynomial modulus parameters are optional, and the
     coefficient modulus is only allowed if the coefficient type is integral.
+
+    The coefficient modulus, if specified, should be positive and not larger
+    than `2 ** width(coefficientType)`.
+
+    If the coefficient modulus is not specified, the handling of coefficients
+    overflows is determined by subsequent lowering passes, which may choose to
+    wrap around or widen the overflow at their discretion.
+
+    Note that coefficient modulus is contained in `i64` by default, which is signed.
+    To specify a 64 bit number without intepreting it as a negative number, its container
+    type should be manually specified like `coefficientModulus=18446744073709551615:i128`.
   }];
 
   let parameters = (ins
     "Type": $coefficientType,
     OptionalParameter<"::mlir::IntegerAttr">: $coefficientModulus,
     OptionalParameter<"::mlir::polynomial::IntPolynomialAttr">: $polynomialModulus
   );
+  let genVerifyDecl = 1;
   let assemblyFormat = "`<` struct(params) `>`";
   let builders = [
     AttrBuilderWithInferredContext<

mlir/lib/Dialect/Polynomial/IR/PolynomialAttributes.cpp

@@ -203,5 +203,34 @@ Attribute FloatPolynomialAttr::parse(AsmParser &parser, Type type) {
   return FloatPolynomialAttr::get(parser.getContext(), result.value());
 }
 
+LogicalResult
+RingAttr::verify(function_ref<mlir::InFlightDiagnostic()> emitError,
+                 Type coefficientType, IntegerAttr coefficientModulus,
+                 IntPolynomialAttr polynomialModulus) {
+  if (coefficientModulus) {
+    auto coeffIntType = llvm::dyn_cast<IntegerType>(coefficientType);
+    if (!coeffIntType) {
+      return emitError() << "coefficientModulus specified but coefficientType "
+                            "is not integral";
+    }
+    APInt coeffModValue = coefficientModulus.getValue();
+    if (coeffModValue == 0) {
+      return emitError() << "coefficientModulus should not be 0";
+    }
+    if (coeffModValue.slt(0)) {
+      return emitError() << "coefficientModulus should be positive";
+    }
+    auto coeffModWidth = (coeffModValue - 1).getActiveBits();
+    auto coeffWidth = coeffIntType.getWidth();
+    if (coeffModWidth > coeffWidth) {
+      return emitError() << "coefficientModulus needs bit width of "
+                         << coeffModWidth
+                         << " but coefficientType can only contain "
+                         << coeffWidth << " bits";
+    }
+  }
+  return success();
+}
+
 } // namespace polynomial
 } // namespace mlir

mlir/test/Dialect/Polynomial/attributes.mlir

@@ -37,3 +37,37 @@
 // expected-error@below {{failed to parse Polynomial_RingAttr parameter 'coefficientModulus' which is to be a `::mlir::IntegerAttr`}}
 // expected-error@below {{expected attribute value}}
 #ring1 = #polynomial.ring<coefficientType=i32, coefficientModulus=x, polynomialModulus=#my_poly>
+
+// -----
+
+// expected-error@below {{coefficientModulus specified but coefficientType is not integral}}
+#ring1 = #polynomial.ring<coefficientType=f32, coefficientModulus=17>
+
+// -----
+
+// expected-error@below {{coefficientModulus should not be 0}}
+#ring1 = #polynomial.ring<coefficientType=i32, coefficientModulus=0>
+
+// -----
+
+// expected-error@below {{coefficientModulus should be positive}}
+#ring1 = #polynomial.ring<coefficientType=i32, coefficientModulus=-1>
+
+// -----
+
+// expected-error@below {{coefficientModulus needs bit width of 33 but coefficientType can only contain 32 bits}}
+#ring1 = #polynomial.ring<coefficientType=i32, coefficientModulus=4294967297>
+
+// -----
+
+#ring1 = #polynomial.ring<coefficientType=i32, coefficientModulus=4294967296>
+
+// -----
+
+// expected-error@below {{coefficientModulus should be positive}}
+#ring1 = #polynomial.ring<coefficientType=i64, coefficientModulus=18446744073709551615>
+
+// -----
+
+// unfortunately, coefficientModulus of 64bit should be contained in larger type
+#ring1 = #polynomial.ring<coefficientType=i64, coefficientModulus=18446744073709551615 : i128>