Error report in multipass compilers

I am currently writing a multipass compiler where the parser builds
an Abstract Syntax Tree which is later traversed in several passes.
I would like to keep track of the linenumbers so I can refer to a
specific line when an error is detected during the tree traversal.


There are two approaches I can think of:


1. Store the linenumbers in the leaf nodes only.
When an error occur, go first right then all the
way down the left spine to find a leaf.
This doesn't work in all cases; for instance


                      stmt ::= RETURN ';' { ... }


                      doesn't have a leaf node and no line number can be
                      associated with that construct.


                2. Store the linenumbers in all nodes.
                      The problem here is how to synthesize a linenumber from
                      two subtrees in an non-ad-hoc way. Example


expr ::= expr '+' expr { ... }


The linenumber stored in $$ should probably be the linenumber
where the '+' appeared. But some productions have right-hand
sides without any terminals and so $$'s linenumber must be
synthesized from one of the nonterminals. Empty subtrees must
of course be avoided so this method requires careful thinking.


Is there some clean, obvious way that I have missed?


--Kjell (kjell@saturn.ucsc.edu)
[At parse time, I'd set the line number in a node from the current line number
kept by the lexer. Crude, but as likely as anything else to be right. -John]
--

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