On Sunday, 11 December 2016 at 18:08:04 UTC, safety0ff wrote:
On Sunday, 11 December 2016 at 17:20:24 UTC, Stefan Koch wrote:
That means you have to compute the mangled name which is crazy
expensive.
And you can't cache the parent part of mangle because it all
freshly generated by the template.
How often would the mangle be needed regardless later on in
compilation?
I don't know too much about dmd internals.
It seemed that Martin Nowak had a viable proof of concept, but
the bugzilla discussion is quite terse.
It seems like I fail to express the problem properly so let me
try again.
AliasSeq's that are append to look like this
: AliasSeq!(AliasSeq!(AliasSeq!(...)))
An aliasSeq that is "appended to" n times will produce (n^2)
with ((n-1)^2) sub instances in them all the way till n is 0.
When you want to compare their parameter times you need to go
through all of them and recursively call findTemplateInstance.
I don't see the n^2 instances, I'm likely missing an
implementation detail.
However, I understand the quadratic nature of comparing:
AliasSeq!(AliasSeq!(AliasSeq!(...)))
to:
AliasSeq!(AliasSeq!(...))
I also don't see why you'd need to do this comparison often
(assuming good hash functions are used.)
The comment "Hash collisions will happen!" seems to
overestimate the current implementation IMO.
All in all, it would probably be best to have some degenerate
code so that the issue can be investigated on a common footing.
Just use this little program to simulate the process.
It assumes you always hit the l1 cache and that there are no hash
collisions.
int templateTime(uint n, uint k = 300)
{
uint result = k;
foreach(_;0 .. n-1)
result += templateTime(n/2, k);
return result;
}
void main(string[] args)
{
import std.stdio;
import std.conv : to;
uint n = to!uint(args[1]);
writeln("AliasSeq with ", n, "Elements will take around ",
templateTime(n), " us to intanciate" );
}
