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Re: [RFC] Migrate pointers to members to the middle end
On 8/9/07, Mark Mitchell <mark@codesourcery.com> wrote:
> Daniel Berlin wrote:
>
> >> This is the source of my current woes, as this may involve virtual
> >> function resolution, which can't be done with the information
> >> currently available to the middle end.
>
> Ollie, IIRC, you posted an example where, together with your front-end
> lowering patch (i.e., with the strategy that Daniel is advocating), we
> don't do some optimization that we could in theory do. Have you worked
> out why not? Perhaps that would shed some light on whether or not it's
> tractable to recover this information from our current IR.
The example I posted before indicated that the check to determine
whether or not the member function is virtual wasn't optimized out.
It didn't know that (fn_ptr & 1) == 0. That should be easy to fix by
incorporating pointer alignment requirements in fold_binary().
Offhand, I don't remember what happened with the various other cases,
but my testing at the time wasn't particularly thorough. The feedback
I've gotten so far seems overwhelmingly negative, so I think the next
step is to revisit the lowering approach, exercise the hell out of it,
and see what, if any, limitations pop up.
Daniel mentioned something about a pre-existing type-based
devirtualizer. I'd be interested to see how it works. From what I've
been able to gleam from the GCC code, it seems that such an approach
would need to hook back into the front end, which is a showstopper
from an LTO perspective. It's entirely possible that I'm missing
something, though.
Ollie