On Mon, Nov 11, 2013 at 11:08:14AM +0100, Ondřej Bílka wrote:
> On Sun, Nov 10, 2013 at 04:27:00PM +0100, Marc Glisse wrote:
> > I am posting this patch to get some feedback on the approach. The
> > goal is to replace malloc+free with a stack allocation (a decl
> > actually) when the size is a small constant.
> >
> Why constraint yourself to small sizes. Stack allocation benefits is
> speed and less memory comsumption due lack of fragmentation.
Because you can hardly predict what the program will have as stack
requirements in functions you call? In leaf functions sure, you only care
not to create too large allocations that would go over the stack size,
but if you call other functions, you usually can't know (at least without
sufficient IPA analysis, but that is really hard because it is too early)
how much stack will it really need (both fixed requirements for non-VLA
vars on the stack, spill space, function call arguments and other overhead
and VLAs and also these malloc turned into stack allocation).
So, if you say have a malloc with corresponding free shortly afterwards
but some call in between and decide that it is fine to change it into stack
allocation when it is half the size of the remaining stack space, but then
two frames down there will be some non-VLA var that needs 3/4 of the old
remaining stack space, you've turned a correct program into a broken one.
Jakub
