libc++: First cut at <dynarray>

[Chandler Carruth]

On Thu, Sep 12, 2013 at 9:04 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> > The best approach that I have found is:
> > * Implement a general heap-to-stack optimization for LLVM
>
> I'm completely on-board with this.
>

It's not trivial to implement though, and even if we implement it (and I do
want one!)...


>
> > * Add metadata on a call that LLVM recognizes as an allocation
> > function (eg, _Znwm or malloc) that says "try to promote this to the
> > stack, even if the allocation size is not constant"
>
> This seems like a simple addition to the first option, allowing the user
> to override the cost model, right? I imagine that we might want to keep the
> malloc calls if the cost of the code in between the malloc and free is high
> (on the assumption that it is better not to unnecessarily increase register
> pressure by introducing dynamic allocations). This metadata would just tell
> the optimizer to do the transformation whenever legal. Is that the idea?


But I'm really unsure when (if ever) we will *actually* want to move to the
stack.... It blocks inlining, increases register pressure, etc... I just
don't see a real demand for it.
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