[mlir] Add a pre-check before checking constraints of an optional attribute.

[hanchung]

Summary

When an attribute is optional and if it's missing, a mlir::Attribute() is
created to signal the missing state. In this case, it is a nullptr. This patch
adds a pre-check before checking constraints because there are bunch of
tblgen_attr.isa<...> in a condition template. Otherwise, it fails with an
assertion because it is not valid to apply isa method on a nullptr.

[antiagainst]

mlir/tools/mlir-tblgen/RewriterGen.cpp

385

What happens if I have a matcher like IntValueIsFive but an op instance does not have the attribute? I would expect the match to fail but per the implementation here, it will succeed because of the check is not performed?

[hanchung]

Had a quick chat with @antiagainst , this is too aggressive. A proper fix might be using isa_and_nonnull for optional attribute.

With Lei's suggestion, this no longer blocks me. I'm going to close the patch. Thank you, Lei. :)

mlir/tools/mlir-tblgen/RewriterGen.cpp

if (!matcher.isUnspecified()) {
     if (!matcher.isAttrMatcher()) {
       PrintFatalError(
           loc, formatv("the {1}-th argument of op '{0}' should be an attribute",
                        op.getOperationName(), argIndex + 1));
     }
 
     // If a constraint is specified, we need to generate C++ statements to
     // check the constraint.
+    // Only check the constraint of an optional attribute if it has value.
+    if (attr.isOptional()) {
+      os.indent(indent) << "if (tblgen_attr) {\n";
+      indent += 2;
+    }
     emitMatchCheck(
         depth,
         tgfmt(matcher.getConditionTemplate(), &fmtCtx.withSelf("tblgen_attr")),
         formatv("\"op '{0}' attribute '{1}' failed to satisfy constraint: "
                 "{2}\"",
                 op.getOperationName(), namedAttr->name,
                 matcher.getAsConstraint().getDescription()));
+    if (attr.isOptional()) {
+      os.indent(indent) << "}\n";
+      indent -= 2;
+    }
   }
 
   // Capture the value
   auto name = tree.getArgName(argIndex);
   // `$_` is a special symbol to ignore op argument matching.
   if (!name.empty() && name != "_") {
     os.indent(indent) << formatv("{0} = tblgen_attr;\n", name);
   }