New attempt: subword-based DCE, PR42575

[Eric Botcazou]

> Or something else may happen entirely.  Can you enumerate all the ways dead
> code can happen for non-subregs and fix each in a way that does not involve
> running DCE? I'm sure you'll agree that attempting this would be absurd.

All certainly not, indeed, the significant ones would be good enough as a 
first approach.  If, in the process of doing that, there is some evidence 
that there are too many of them, then, yes, adding a custom pass is probably 
the best approach.

> As for effort - for reference, below is the lower-subreg patch I
> experimented with (and intend to finish, eventually) after you suggested
> your second patch.  You can judge for yourself whether that constitutes
> trying very hard.

Indeed, now I see it, thanks for posting the patch.

> Well, we can _never_ know what it would take to enhance existing code,
> unless you manage to enumerate every way dead code can appear in the
> RTL.  Your approach can never fix the problem entirely, since it would
> only add one band-aid for every instance of the problem we find.

No band-aids, just small improvements.  This is the way I think production 
compilers should evolve, unless there is clear evidence that some "quantum 
leap" is required.  PR42575 wasn't such a clear evidence to me.

> I don't disagree that we should be doing these kinds of fixes if we find
> they make the compiler better (again, see the patch below), but it can
> never fix the _class_ of problems.

If there is a class of problems.  So you're thinking that eliminating dead 
word-sized subregs of multi-word registers is a class of problems on its own, 
whereas I'm thinking that it's the intersection of other classes of problems.
That would indeed sort of explain the total disagreement.

-- 
Eric Botcazou