4.11.2 TC-B FAQ

The bounds checking extension relies on the use of casts (see TC-B Samples), see See Language Extensions in Tiger Compiler Reference Manual. However, a simplistic implementation of casts introduces ambiguities in the grammar that even a GLR parser cannot resolve dynamically.

Consider the following example, where foo is an l-value :

_cast(foo, string)

This piece of code can be parsed in two different ways:

1. exp -> cast-exp -> exp -> lvalue (foo)
2. exp -> lvalue -> cast-lvalue -> lvalue (foo)

As the cast must preserve the l-value nature of foo, it must itself produce an l-value. Hence we want the latter interpretation. This is a true ambiguity, not a local ambiguity that GLR can resolve simply by “waiting for enough look-ahead”.

To help it take the right decision, you can favor the right path by assigning dynamic priorities to relevant rules, using Bison’s %dprec keyword. See Bison’s manual (see Flex & Bison) for more information on this feature.