[IR] Make Value::getType() work better with invalid IR.

[lattner]

Summary

The asmprinter would crash when dumping IR objects that had their
operands dropped. With this change, we now get this output, which
makes op->dump() style debugging more useful.

%5 = "firrtl.eq"(<<NULL>>, <<NULL>>) : (<<NULL TYPE>>, <<NULL TYPE>>) -> !firrtl.uint<1>

Previously the asmprinter would crash getting the types of the null operands.

[lattner]

thanks for the quick review, landed in 87c032f7b44

[rriddle]

mlir/lib/IR/Value.cpp

35

Can you instead fix asmprinter? It would avoid weird null checks slipping into these types, which awkwardly breaks the expectation of being pointer-like.

[lattner]

mlir/lib/IR/Value.cpp

35

Yes, I can definitely do that if you'd prefer that! It will require deabstracting some stuff, but can definitely do that.

[lattner]

I'm preparing a patch to deabstract this, but it is ugly. The other middle-ground approach is to change ValueTypeIterator to do the null check.

[lattner]

Alternate approach is here: https://reviews.llvm.org/D93908

mlir/lib/IR/Value.cpp

Value::Value(Operation *op, unsigned resultNo) {
 
   // If we can't pack the result directly, grab the use list from the parent op.
   unsigned trailingNo = resultNo - OpResult::getMaxInlineResults();
   ownerAndKind = {op->getTrailingResult(trailingNo), Kind::TrailingOpResult};
 }
 
 /// Return the type of this value.
 Type Value::getType() const {
+  // Support a null Value so the asmprinter doesn't crash on invalid IR (e.g.
+  // operations that have dropAllReferences() called on them).
+  if (!*this)
+    return Type();
+
   if (BlockArgument arg = dyn_cast<BlockArgument>())
     return arg.getType();
 
   // If this is an operation result, query the parent operation.
   OpResult result = cast<OpResult>();
   Operation *owner = result.getOwner();
   if (owner->hasSingleResult)
     return owner->resultType;