[m-rev.] for review: fix equivalence type performance problems

[Simon Taylor]

On 18-Dec-2003, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> On 17-Dec-2003, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> > On 12-Dec-2003, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > > On 12-Dec-2003, Simon Taylor <stayl at cs.mu.OZ.AU> wrote:
> > > > compiler/mercury_compile.m:
> > > > 	Re-enable equiv_type_hlds.m.
> > > > 
> > > > tests/hard_coded/Mmakefile:
> > > > 	Re-enable testing of equiv_type_hlds.m.
> > > 
> > > I would like to see some performance figures for how much
> > > it costs to run this extra pass before we re-enable it.
> > 
> > With the extra change below, the equiv_type_hlds pass adds about
> > 5% to compile time, much of that in equiv_type.replace_in_type.
> 
> The extra changes below are fine.
> 
> I'd still rather avoid the extra 5% if we can...
> for the --high-level-data grades we can't, but is this pass
> really needed for the hlc and LLDS grades?

I'd say yes, because otherwise `:- pragma foreign_proc' and
`:- pragma export' will behave differently in different grades;
the foreign types of the arguments will be different.
 
> In fact, it's not just avoiding the extra compilation time;
> enabling this pass also weakens our separate compilation guarantees.
> For --high-level-data grades, that's undesirable but unavoidable.
> For --no-high-level-data grades, I'd like to avoid it if we can.

I don't think it's avoidable.

Simon.
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