Re: : impossible alignment requirements?
On Nov 13, 5:03 pm, levick...@sezampro.yu wrote:
Problem lies with vector types both on Mac and on PC and Playstation 3
also comes to mind.
*** C++ standard is terribly outdated ***
Not having an operator new which returns _aligned_ memory seriously
hampers SIMD use in C++.
There's nothing in the C++ standard which prevents the implementation
from returning memory aligned for SIMD use. If the implementation
chooses not to do that, complain to your compiler vendor.
What should one do with this code?
class foo
{
public:
__m128 vec;
...
foo(void)
{
vec = _mm_setzero_ps();
}
};
foo *p = new [](10);
I don't know. At the trivial level, I presume you meant:
foo *p = new foo[10];
At the deeper level, you have to see what your compiler vendor
provides.
C++ regulatory body is obviously sitting with their collective thumbs
up their arses instead of listening to developers and their needs.
The C++ standards committee is composed of volunteers who PAY to take
part. They /do/ listen to developers, but different developers have
different needs. Maybe a coherent proposal a few years ago would have
made it to C++0X, but I fear it is too late now.
Not only that, compiler vendors are not able to provide workarounds
for such issues because that would break the standard which btw is
already broken.
Why would it break the standard? As I said above, there's nothing to
stop new returning 16-byte aligned memory.
Let me touch on stack alignment when you mentioned it:
There is a compiler option to force stack alignment on every
function entry to a given value. Problem is in the standard again,
if you force say 16-byte alignment:
void somefunc(void)
{
char buf[32]; // sure, this will be aligned to 16 bytes
...
for (int i = 0; i < 32; i++) {
char foo[32]; // but this won't!
__declspec(align(16)) char bar[32]; // unless you add explicit
alignment directive
}
I /completely/ fail to see why that is a problem with the standard.
There is no reason that your compiler vendor can't associate the
property __dclspec(align(16)) with all auto declarations. (I suspect
they don't because they think that on average the additional space
used will have a more negative impact on performance than the
performance benefit of being able to use SIMD).
}
Then what is the point of having forced alignment if you are not
consistent? For every performance conscious developer those
things just suck.
Talk to your compiler vendor.
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