[llvm] r175382 - Add support for updating the LiveIntervals of registers used by 'exotic'

[Chandler Carruth]

On Sat, Feb 16, 2013 at 6:40 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

> >> I think it is too confusing to use reverse_iterators in an API. Can't
> you keep that detail internal to the function? Better yet, just use -- to
> move backwards.
> >
> > I originally wrote it that way, but it felt awkward since we are
> iterating backwards over an interval (A, B] not forward over an interval
> [A, B). I guess you are just suggesting that I do the conversion from
> left-open to right-open intervals in the function itself.
>
> Yes, but even then I usually just use:
>
> for (iterator I = End; I != Begin;) {
>   --I;
>   use(I);
> }
>

This prevents ever using an algorithm on the range, or (eventually)
range-based for loops. I think that is the primary benefit of writing the
above in terms of reverse iterators. It confused me the first time I saw
it, but now I find the reverse iterator pattern quite friendly.

We could also help it by providing:

iterator RBegin, REnd;
tie(RBegin, REnd) = reverse_iterator_pair(Begin, End);


>
> It's less confusing IMHO.
>
> Note that dereferencing a MachineBasicBlock::reverse_iterator causes it to
> scan the whole bundle it is pointing to:
>
>   reference operator*() const {__t = current; return *--__t;}


Regardless of the above, this seems bad. Couldn't this be factored into the
increment step (much as I assume the decrement step in forward iterators
does?)
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