Date: Mon, 14 Jul 2014 22:42:24 +0700 From: Alexey Dokuchaev <danfe@nsu.ru> To: powerpc@freebsd.org Subject: How to convert SSEish _mm_set1_ps() into AltiVec correctly? Message-ID: <20140714154224.GA28612@regency.nsu.ru>
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Hi there,
I'm a bit confused about how to convert _mm_set1_ps() [1] SSE function into
its AltiVec equivalent. To start with, I need to set all four floats of a
vector to the same value. So far, I've come up with two versions that work
with GCC or Clang, but I want to have a code that works with any compiler,
and is technically correct (works not just by accident).
On PowerPC, there are two altivec.h files provided by GCC 4.2 and Clang:
/usr/include/clang/3.4.1/altivec.h
/usr/include/gcc/4.2/altivec.h
The problem is that they are substantially different (read: offer different
APIs). For Clang, I can simply write something like this:
union {
float f1, f2, f3, f4;
vector float f;
} foo;
foo.f = vec_splats(42.f);
// all f1, f2, f3, f4 are 42.f now
But this does not work with GCC: it simply does not offer vec_splats(float);
however, I can do this:
float init = 42.f;
foo.f = vec_ld(0, &init);
And it will set all four components to 42.f. Yet this is technically wrong,
as apparently I'm supposed to pass an entire array of floats, e.g. if built
with Clang all floats are "nan". Lets change the code to this:
float init[4] = { 42.f };
foo.f = vec_ld(0, init);
This works with both compilers, but I'm not sure if it is correct. Can any
of our AltiVec experts give me some hint here? Thanks.
./danfe
[1] http://msdn.microsoft.com/en-us/library/vstudio/2x1se8ha%28v=vs.100%29.aspx
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