[m-dev.] compiler not building with --intermodule-optimization

[Zoltan Somogyi]

On Thu, 8 Mar 2018 06:18:56 -0500 (EST), Julien Fischer <jfischer at opturion.com> wrote:
> The compiler is not currently building when --intermodule-optimization is 
> enabled, due to:
> 
>     clause_to_proc.m:056: In module `check_hlds.clause_to_proc':
>     clause_to_proc.m:056:   warning: module `hlds.vartypes' has a
>     clause_to_proc.m:056:   `:- import_module' declaration, but is not used
>     clause_to_proc.m:056:   anywhere in the module.
> 
> The fix is trivial; however, why is the warning only being triggered
> when --intermodule-optimization is enabled?

I have a strong hunch that the reason is that the code that
generates that warning does not know about the expansion
of equivalence types.

Clause_to_proc.m calls two predicates whose arguments
include the type "vartypes". Since vartypes.m defines this type,
it is not unused, and without intermodule optimization,
the compiler sees that.

*With* intermodule optimization, the compiler expands out
the type "vartypes" to its definition, which is
"map(prog_var, mer_type)". This expansion removes
from the HLDS any trace of anything defined in vartypes.m,
and this leads to the warning.

I think the type expansion passes (both equiv_type.m and
equiv_type_hlds.m) both have some code to record
the stuff they use from other modules, but either they
don't record the right stuff, or unused_modules.m
does not pay attention to it.

The fact that the compiler can happily compile clause_to_proc.m
without knowing the definition of vartypes shows that
unused_imports.m takes, um, a "generous view" of what
it considers used. This is a second, unrelated problem,
but harder to fix, because it requires unused_imports.m
to know exactly what occurrences of item names (e.g. type names)
require the compiler to know the definition of the
named item, and which do not impose such a requirement.

Fixing either problem should make the above warning
disappear, but it would be nicer to fix both.

Zoltan.