Some aspect of GCC memory consumption increased by recent patch

[Nathan Sidwell]

Ben Elliston wrote:
> Jan Hubicka <jh@suse.cz> writes:
> 
> 
>>up to 3% memory consumption increase looks pretty serious even
>>tought we save some memory on generate-3.4.ii.  Is there some
>>understanding why it happens and/or some hope that it will get
>>solved?
> 
> 
> I spent a bit of time thinking about this today.
> 
> Nathan, is it possible that the pre-allocation strategy for the
> type-safe vectors is causing a higher than usual peak in memory
> consumption?  I note that vec.h says "Reallocation causes an
> exponential increase in vector size."

yeah, I talked with Honza on IRC about this.  I don't think the exponential
increase is the problem.  There are two other issues.

1) The default length is 4 slots.  As most blocks only have 1 or 2 edges,
that is probably wrong.  I'd guess 2 would be a better default.  It's not
obvious to me as to whether _most_ blocks have 1 exit, or 2 exits.  I suspect
that defaulting to 1 is too conservative.

2) The length and alloc members are 32bits, they need not be.  I think
I should make them default to 'unsigned short', thereby removing 1 word
from a vector.  This extra size is someting that's causing Geoff Keating not
to want to use them for an attribute mechanism rewrite.  For vectors
which could get really long, we'd have yet another definition macro (bleah!).

It'd have to be something like DEF_LONG_VEC_{GC,MALLOC}_{P,O}.  Do you all
think the LONG bit should be part of use too (i.e. allow LONG_VEC(thing) and
VEC(thing)).  It seems to me that that would be the right thing.

IIRC, the old structure contained three words per edge (next, prev and edge),
thus a vector defaulting to 4 slots (6words), would only be a saving if the
average number of edges per block was >= 2.

Can you experiment with reducing the default allocation size? It's contained
in vec.c.

BTW, is forwarding the memory usage info from the point of call of the vector
api functions the right thing? (Rather than merely indicate the vector API
function itself?)

nathan

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