[llvm-dev] [RFC] Aggreate load/store, proposed plan

[deadal nix via llvm-dev]

It is pretty clear people need this. Let's get this moving.

I'll try to sum up the point that have been made and I'll try to address
them carefully.

1/ There is no good solution for large aggregates.
That is true. However, I don't think this is a reason to not address
smaller aggregates, as they appear to be needed. Realistically, the
proportion of aggregates that are very large is small, and there is no
expectation that such a thing would map nicely to the hardware anyway (the
hardware won't have enough registers to load it all anyway). I do think
this is reasonable to expect a reasonable handling of relatively small
aggregates like fat pointers while accepting that larges ones will be
inefficient.

This limitation is not unique to the current discussion, as SROA suffer
from the same limitation.
It is possible to disable to transformation for aggregates that are too
large if this is too big of a concern. It should maybe also be done for
SROA.

2/ Slicing the aggregate break the semantic of atomic/volatile.
That is true. It means slicing the aggregate should not be done for
atomic/volatile. It doesn't mean this should not be done for regular ones
as it is reasonable to handle atomic/volatile differently. After all, they
have different semantic.

3/ Not slicing can create scalar that aren't supported by the target. This
is undesirable.
Indeed. But as always, the important question is compared to what ?

The hardware has no notion of aggregate, so an aggregate or a large scalar
ends up both requiring legalization. Doing the transformation is still
beneficial :
 - Some aggregates will generate valid scalars. For such aggregate, this is
100% win.
 - For aggregate that won't, the situation is still better as various
optimization passes will be able to handle the load in a sensible manner.
 - The transformation never make the situation worse than it is to begin
with.

On previous discussion, Hal Finkel seemed to think that the scalar solution
is preferable to the slicing one.

Is that a fair assessment of the situation ? Considering all of this, I
think the right path forward is :
 - Go for the scalar solution in the general case.
 - If that is a problem, the slicing approach can be used for non
atomic/volatile.
 - If necessary, disable the transformation for very large aggregates (and
consider doing so for SROA as well).

Do we have a plan ?
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