Robert Jacques wrote:
On Tue, 22 Sep 2009 12:09:23 -0400, Jeremie Pelletier
<[email protected]> wrote:
#ponce wrote:
In practice it's about an 8X speed difference!
On AMD K8, it's only 2 vs 5 ops, and on K10 it's 2 vs 3 ops.
On i7, movups on aligned data is the same speed as movaps. It's
still slower if it's an unaligned access.
It all depends on how important you think performance on Core2 and
earlier Intel processors is.
I wasn't aware of that, and here I was wondering why my SSE code was
slower than the FPU in certain places on my core2 quad, I now recall
using a lot of movups instructions, thanks for the tip.
Indeed SSE is known to be overkill when dealing with unaligned data.
In C++ writing SSE code is so painful you either have to use
intrisics, or use libraries like Eigen (a SIMD vectorization library
based on expression templates, which can generate SSE, AVX or FPU
code). But using such a library is often way too intrusive, and
alignement is not in standard C++.
D does already understand arrays operations like Eigen do, in order
to increase cacheability. It would be great if it could statically
detect 16-byte aligned data and perform SSE when possible (though
there must be many others things to do :) ).
The D memory manager already aligns data on 16 bytes boundaries. The
only case I can think of right now is when data is in a struct or class:
struct {
float[4] vec; // aligned!
int a;
float[4] vec; // unaligned!
}
Yes, although classes have hidden vars, which are runtime dependent,
changing the offset. Structs may be embedded in other things (therefore
offset). And then there's the whole slicing from an array issue.
Ah yes, you are right. Then I guess it really is up to the programmer to
know if the data is aligned or not and select different code paths from
it. Adding checks at runtime just adds to the overhead we're trying to
save by using SSE in the first place.
It would be great if we could declare aliases to asm instructions and
use template functions with a (bool aligned = true) and set a movps
alias to either movaps or movups depending on the value of aligned.
