[m-rev.] for review: reduce link time in debug grades

[Julien Fischer]

On Tue, 20 Oct 2009, Julien Fischer wrote:

> On Mon, 19 Oct 2009, Zoltan Somogyi wrote:
>
>> I have asked Julien to measure how long linking takes on Snow Leopard 
>> before
>> this diff, so we can see whether this diff improves link times as intended,
>> and so, by how much.
>
> As it turns out linking is only one of the problems on Snow Leopard.
> Compilation in debugging grades, e.g. none.gc.debug, on Mac OS 10.6, can
> be summarised in one word: ouch!
>
> Compiling the .c files into .o files in none.gc.debug is taking forever.
>
> Doing: time mmake from the top-level of a freshly checked out workspace
> on eris in grade none.gc.debug took
>
>    real    227m23.884s
>    user    67m13.706s
>    sys     38m3.656s
>
> to get _only_ as far as compiling io.c from the standard library, i.e. this 
> step:
>
>    ../scripts/mgnuc --grade none.gc.debug --no-ansi -- -c io.c -o io.o
>
> I killed it after this point - nearly all of the time is being spent
> compiling C to object code; compiling Mercury to C contributes very
> little to the overall time.

With this change compilation of the entire standard library (static and
shared versions) on eris in none.gc.debug now takes just over three hours.

Julien.
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