Simplify constant handling in Painless

[jdconrad]

This change makes constant handling simpler by doing the following:

1. Removes the ir tree node RegexNode entirely in favor or using more generic nodes such as ConstantNode and MemberFieldLoadNode generated by the user tree node ERegex. This requires a clinit method added to the ClassNode, but allows for additional removals. This clinit method can also be used for potential optimization passes.
2. Removes the Constant class as with the changes to regex nodes it's no longer necessary.
3. Removes the Globals class from the ir tree write phase. With the previous removals, Globals is no longer necessary.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[stu-elastic]

Why is this necessary?

[jdconrad]

After speaking with you (@stu-elastic) and @rjernst I've decided to change this a bit. I've removed noop in favor of doPop which will pop extraneous values off the stack when necessary such as a method call with side effects, but an ignored return value somecall();.

I also removed the need for StatementExpressionNode to handle return values. Instead, SExpression will just directly translate to a ReturnNode if a return is required. This moves the responsibility to where is makes sense instead of having redundant returns in multiple nodes.

[jdconrad]

I continued to iterate on this after more discussion with @rjernst . I removed the doPop and will instead use an expression's type to determine if a pop is required or not (popping the void type is the same as a noop in this case).

[stu-elastic]

There's now MemberFieldLoadNode and MemberFieldStoreNode where Store has the following new code:

        if (isStatic == false) {
            methodWriter.loadThis();
        }

        getChildNode().write(classWriter, methodWriter, scopeTable);

Why the new code?

[jdconrad]

The new code is to handle putting the value on the stack that will be written to the field. So store requires an expression to generate a value to store where as load does not.

[stu-elastic]

Where is this being written to clinit?

[stu-elastic]

Oh perhaps in classNode.addFieldNode(fieldNode);?

[jdconrad]

classNode.addFieldNode creates a declaration for a member field.

These lines actually add the regex constant generation and assignment to the clinit method:

BlockNode blockNode = classNode.getClinitNode().getBlockNode();
blockNode.addStatementNode(statementExpressionNode);

[rjernst]

minimal javadoc please

[jdconrad]

Added.

[rjernst]

Could we add this when creating the clinit block, and have an inner block that we expose as the one that statements can be added to? This way we don't manipulate the IR tree when calling write.

[jdconrad]

I changed this to do no modifications of the ir tree from write. I just do the method writer for clinit and the return statement in the ClassNode. I'll consider better ways to do this, but for now this makes the most sense because it's the simplest change.

[jdconrad]

@rjernst @stu-elastic Thanks for the initial rounds of review.

[jdconrad]

@rjernst I think this is ready for another look. I believe I addressed all your comments and continued iterating on the best way to handle StatementExpressionNode - see Stu's initial comments on this for my follow ups.

[rjernst]

LGTM

[rjernst]

LGTM

[jdconrad]

@rjernst Thanks for the second review.