[MLIR] Printing a null Value.

[sfuniak]

Summary

This diff adds support to printing a Value when it is null. We encounter this situation when debugging the PDL bytcode execution (where a null Value is perfectly valid). Currently, the AsmPrinter crashes (with an assert in a cast) when it encounters such Value.

We follow the same format used in other printed entities (e.g., null attribute).

[mehdi_amini]

In SSANameState we have:

if (!value) {
  stream << "<<NULL>>";
  return;
}

Can you update it there to also print NULL VALUE to keeps the same message everywhere?

[bondhugula]

Useful improvement - thanks!

[sfuniak]

Fix printing of null value in SSANameState::printValueID.

[rriddle]

mlir/lib/IR/AsmPrinter.cpp

2829

I would check against impl directly. !*this looks dangerously close to !this

[sfuniak]

Check against impl directly

[sfuniak]

Rebased.

mlir/lib/IR/AsmPrinter.cpp

SSANameState::SSANameState(
   // Manually remove all the scopes.
   while (usedNames.getCurScope() != nullptr)
     usedNames.getCurScope()->~UsedNamesScopeTy();
 }
 
 void SSANameState::printValueID(Value value, bool printResultNo,
                                 raw_ostream &stream) const {
   if (!value) {
-    stream << "<<NULL>>";
+    stream << "<<NULL VALUE>>";
     return;
   }
 
   Optional<int> resultNo;
   auto lookupValue = value;
 
   // If this is an operation result, collect the head lookup value of the result
   // group and the result number of 'result' within that group.
void AffineMap::print(raw_ostream &os) const {
   AsmPrinter::Impl(os).printAffineMap(*this);
 }
 
 void IntegerSet::print(raw_ostream &os) const {
   AsmPrinter::Impl(os).printIntegerSet(*this);
 }
 
 void Value::print(raw_ostream &os) {
+  if (!impl) {
+    os << "<<NULL VALUE>>";
+    return;
+  }
+
   if (auto *op = getDefiningOp())
     return op->print(os);
   // TODO: Improve BlockArgument print'ing.
   BlockArgument arg = this->cast<BlockArgument>();
   os << "<block argument> of type '" << arg.getType()
      << "' at index: " << arg.getArgNumber();
 }
 void Value::print(raw_ostream &os, AsmState &state) {
+  if (!impl) {
+    os << "<<NULL VALUE>>";
+    return;
+  }
+
   if (auto *op = getDefiningOp())
     return op->print(os, state);
 
   // TODO: Improve BlockArgument print'ing.
   BlockArgument arg = this->cast<BlockArgument>();
   os << "<block argument> of type '" << arg.getType()
      << "' at index: " << arg.getArgNumber();
 }