[m-rev.] for review: Cache module file names when writing dependency files.

[Julien Fischer]

On Thu, 2 Feb 2023, Peter Wang wrote:

> When writing out dependency files, the compiler would call
> module_name_to_file_name often for the same pairs of module name and
> file extension. Caching the results of those calls saves on
> constructing temporary strings.
>
> This change reduces the time to make dependencies in the compiler
> directory from 2.40 seconds to 2.15 seconds on my machine.
>
> compiler/write_deps_file.m:
>    Add a predicate make_module_file_name that caches calls to
>    module_name_to_file_name.
>
>    Thread the cache map through the predicates in this module.
>    Replace calls to module_name_to_file_name with calls to
>    make_module_file_name when appropriate.
>
> diff --git a/compiler/write_deps_file.m b/compiler/write_deps_file.m
> index 30885a5c0..65f6acd68 100644
> --- a/compiler/write_deps_file.m
> +++ b/compiler/write_deps_file.m


> +make_module_file_name(Globals, From, Ext, ModuleName, FileName,
> +        !Cache, !IO) :-
> +    (
> +        Ext = ext_src,
> +        % No point caching these.
> +        module_name_to_file_name(Globals, From, do_not_create_dirs,
> +            Ext, ModuleName, FileName, !IO)
> +    ;
> +        Ext = ext_other(OtherExt),
> +        ModuleNameExt = module_name_and_ext(ModuleName, OtherExt),
> +        ( if map.search(!.Cache, ModuleNameExt, FileName0) then
> +            FileName = FileName0
> +        else
> +            module_name_to_file_name(Globals, From, do_not_create_dirs,
> +                Ext, ModuleName, FileName, !IO),
> +            map.det_insert(ModuleNameExt, FileName, !Cache)
> +        )
> +    ).

I suggest adding a comment somewhere around saying that the rationale for this
caching is that it saves on temporary string construction.

Otherwise, that's fine.

Julien.