llvm dev - Jun 2021 - TBAA

I would think that in a language like C, the role of the optimizer should
be to do optimizations that are difficult or impossible or time consuming
for the programmer.
My concern is that TBAA has a trade-off in terms of the complexity of the
language for the programmer so I'd like to know if the tradeoff is really
worth it.
BTW: the programmer based hoisting does not depend on the type of bound
being different from the types off arr - so it is more robust.


vy


On Fri, Jun 18, 2021 at 10:33 AM Richard Kenner <kenner at adacore.com>
wrote:
> > The programmer can get this with a simple change, no?
>
> Sure.  The programmer can do all sorts of optimizations themselves, in
> fact almost all of them.  But the purpose of an optimizer is to do these
> things so that the programmer doesn't have to do that.
>
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> I would think that in a language like C, the role of the optimizer
> should be to do optimizations that are difficult or impossible or
> time consuming for the programmer.
That's a philosphical question.  Keep in mind that very few
programmers nowadays, even in C, have a good idea of the underlying
machine model (or even the fact that there *is* such a model), so they
don't really even know what to do.  Also, think of macro expansion and
such.  And maintenance costs over time if you add all of the
"clutter".  I would argue against that view.
> My concern is that TBAA has a trade-off in terms of the complexity of the
> language for the programmer
I don't understand.  TBAA implements the rules of the programming language
with respect to aliasing.  It doesn't change the language in any way, so 
I don't see how it can increase the complexity for the programmer.

Perhaps you're arguing that anti-aliasing rules in languages aren't
worth the complexity, but that's a different issue and one that language
design groups look at, not compiler writers.