[llvm-commits] [llvm] r168074 - /llvm/trunk/lib/CodeGen/StackColoring.cpp

[Ulrich Weigand]

Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote on 16.11.2012 00:54:00:
> On Nov 15, 2012, at 11:33 AM, Ulrich Weigand
> <Ulrich.Weigand at de.ibm.com> wrote:
> >   // Sort the slots according to their size. Place unused slots at the
end.
> > -  std::sort(SortedSlots.begin(), SortedSlots.end(), SlotSizeSorter
(MFI));
> > +  // Use stable sort to guarantee deterministic code generation.
> > +  std::stable_sort(SortedSlots.begin(), SortedSlots.end(),
> > +                   SlotSizeSorter(MFI));
>
> Hi Ulrich,
>
> Both stable_sort() and sort() expand into more than 100K of code
> which is why LLVM provides array_pod_sort() as a wrapper for the
> libc qsort() function.
>
> In this case, it is very easy to provide a total ordering so you
> don't need a stable sort.

Hi Jakob,

the difficulty I see with using array_pod_sort or something else
based on qsort is that those just use a plain function as compare
routine, while std::sort uses a function object ... which is used
here to hold the "MFI" parameter needed to implement the comparison.

I don't immediately see how to do this with a plain function
(except by resorting to global state or nested functions, which
I'm not sure is better ...).

Anything I'm overlooking here?  (I'm not a particular C++ expert ...)

I certainly agree that the ordering can easily be made total;
I could implement this by changing SlotSizeSorter so we can continue
to use std::sort instead of std::stable_sort.  Would this be
preferable?

Bye,
Ulrich