libc++: First cut at <dynarray>

[Hal Finkel]

----- Original Message -----
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> On Thu, Sep 12, 2013 at 9:04 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:
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> > The best approach that I have found is:
> > * Implement a general heap-to-stack optimization for LLVM
> 
> I'm completely on-board with this.
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> 
> It's not trivial to implement though, and even if we implement it
> (and I do want one!)...

Me too; why is it not simple:

 - For all calls to malloc, for which we see calls to free along all control-flow paths
 - Replace the malloc with alloca, remove the free (and add lifetime intrinsics immediately after the alloca and where the free used to be)

What am I missing?

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> > * 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, 

Only because you made it do so; we could enabling inlining just as easily. When you originally did this, we did not have stack coloring, right?

>increases register pressure,

This is true, but it is not clear that it would be a significant problem. Function calls have overhead too, obviously, so I think that for small functions it would probably be okay, and for larger functions you'd probably want to maintain the prohibition.

> etc...

> I just don't see a real demand for it.

This is somewhat of a chicken and egg problem.

 -Hal

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory