rtl semantics

[Russell Shaw]

Hi,
I just read all the gcc internals manual.
I have trouble with the contextual polymorphic
nature of rtl objects.

In h8300.md, there is:

   (define_attr "cpu" "h8300,h8300h"
     (const (symbol_ref "cpu_type")))

yet in gccint.info, const should have a machine mode: (const:M EXP)

Do rtl keywords have a variable meaning, depending on the context
where they're used? Is there other languages similar to rtl i could
learn from? (scheme?)