On Sat, 23 Jun 2012, Dag Lem wrote:
Using gcc 4.7.0, I am trying to use vector extensions and AVX builtins
to sum 8 complex numbers, where the real and imaginary parts are
stored in separate YMM registers, and store the result in m64. In
other words, I'd like to achieve the following:
v2sf* z = ...;
v8sf re = ...;
v8sf im = ...;
*z = { sum(re[0..7]), sum(im[0..7]) };
My first attempt at this was:
v8sf a = __builtin_ia32_haddps256(re, im); // iirr iirr
v4sf b = __builtin_ia32_vextractf128_ps256(a, 1);
v4sf c = (v4sf)a + b; // iirr
v4sf d = __builtin_ia32_haddps(c, c); // irir
*z = (v2sf)d; // ir
However, gcc does not seem to allow casting between vectors of
different lengths (why?!).
Ugly.
Hence, my second attempt is as follows:
v8sf a = __builtin_ia32_haddps256(re, im); // iirr iirr
v4sf b = __builtin_ia32_vextractf128_ps256(a, 1);
v4sf c = *((v4sf*)&a) + b; // iirr
v4sf d = __builtin_ia32_haddps(c, c); // irir
__builtin_ia32_storelps(z, d); // ir
The problem with this is that in the calculation of "c = a + b", gcc
generates an intermediate instruction (vmovdqa) to store the contents
of the YMM register holding "a" to memory, instead of directly
accessing the low 128 bits via the corresponding XMM register.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53101
I have also looked into using inline assembly to avoid the overhead,
however as far as I can tell it is not possible to use YMM registers
as parameters.
Yes it is, you may want to re-read the doc remembering that the YMM
registers are the XMM registers (you just use them more fully).
Is there any way to cast from a YMM to an XMM register without
incurring any performance penalty?
ISTR that _mm256_extractf128_ps(*,0) is properly optimized to nothing when
possible (I may misremember).
--
Marc Glisse
