On Tuesday, 3 November 2015 at 23:29:45 UTC, TheFlyingFiddle
wrote:
Is there a built in way to do this in dmd?
Basically I want to do this:
auto decode(T)(...)
{
while(...)
{
T t = T.init; //I want this aligned to 64 bytes.
}
}
Currently I am using:
align(64) struct Aligner(T)
{
T value;
}
auto decode(T)(...)
{
Aligner!T t = void;
while(...)
{
t.value = T.init;
}
}
But is there a less hacky way? From the documentation of align
it seems i cannot use that for this kind of stuff. Also I don't
want to have to use align(64) on my T struct type since for my
usecase I am decoding arrays of T.
The reason that I want to do this in the first place is that if
the variable is aligned i get about a 2.5x speedup (i don't
really know why... found it by accident)
Note that there are two different alignments:
to control padding between instances on the stack
(arrays)
to control padding between members of a struct
align(64) //arrays
struct foo
{
align(16) short baz; //between members
align (1) float quux;
}
your 2.5x speedup is due to aligned vs. unaligned loads and
stores which for SIMD type stuff has a really big effect.
Basically misaligned stuff is really slow. IIRC there was a
(blog/paper?) of someone on a uC spending a vast amount of time
in ONE misaligned integer assignment causing traps and getting
the kernel involved. Not quite as bad on x86 but still with doing.
As to a less jacky solution I'm not sure there is one.
