GCC does a nice enough job compiling the more readable form (though
admittedly, hadd is possibly more readable than what's there for
dot[fd], hadd is supposedly slower than the shuffles and adds, and qfvis
seems to support that).
Having to refigure out what values are going into the vectors got old
very fast. The comments don't help with verifying the values, but at
least I can tell at a glance where 2(xy - wz) goes and thus determine
the "orientation" of the matrix.
gcc and clang have rather different swizzle builtins, but both do a nice
job of optimizing the intuitive initializer swizzle (I think gcc 8(?)
didn't do such a good job thus my use of __builtin_shuffle).
In some cases, gcc-11 does a good enough job translating normal looking
C expressions so use just those, but other times need to dig around for
an appropriate intrinsic.
Also, now need to disable psapi warnings when compiling for anything
less than avx.
Always setting w to 0 made it impossible to use the resulting 4d vectors
in division-based operations as they would result in divide-by-zero and
thus an unavoidable exception (CPUs don't like integer div-by-zero).
I'll probably add similar for float and double, but they're not as
critical as they'll just give inf or nan. This also increases my doubts
about the value of keeping 3d vector operations.
It turns out gcc optimizes the obvious code nicely. It doesn't do so
well for cmul, but I decided to use obvious code anyway (the instruction
counts were the same, so maybe it doesn't get better for a single pair
of operands).
For int, long, float and double. I've been meaning to add them for a
while, and they're part of the new Ruamoko instructions set (which is
progressing nicely).
This failed with errors such as:
from ./include/QF/simd/vec4d.h:32,
from libs/util/simd.c:37:
./include/QF/simd/vec4d.h: In function ‘qmuld’:
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/include/avx2intrin.h:1049:1: error: inlining failed in call to ‘always_inline’ ‘_mm256_permute4x64_pd’: target specific option mismatch
1049 | _mm256_permute4x64_pd (__m256d __X, const int __M)
GCC does a fairly nice job of producing code for vector types when the
hardware doesn't support SIMD, but it seems to break certain math
optimization rules due to excess precision (?). Still, it works well
enough for the core engine, but may not be well suited to the tools.
However, so far, only qfvis uses vector types (and it's not tested yet),
and tools should probably be used on suitable machines anyway (not
forces, of course).
This seems to be pretty close to as fast as it gets (might be able to do
better with some shuffles of the negation constants instead of loading
separate constants).
Care needs to be taken to ensure the right function is used with the
right arguments, but with these, the need to use qconj(d|f) for a
one-off inverse rotation is removed.
They take advantage of gcc's vector_size attribute and so only cross,
dot, qmul, qvmul and qrot (create rotation quaternion from two vectors)
are needed at this stage as basic (per-component) math is supported
natively by gcc.
The provided functions work on horizontal (array-of-structs) data, ie a
vec4d_t or vec4f_t represents a single vector, or traditional vector
layout. Vertical layout (struct-of-arrays) does not need any special
functions as the regular math can be used to operate on four vectors at
a time.
Functions are provided for loading a vec4 from a vec3 (4th element set
to 0) and storing a vec4 into a vec3 (discarding the 4th element).
With this, QF will require AVX2 support (needed for vec4d_t). Without
support for doubles, SSE is possible, but may not be worthwhile for
horizontal data.
Fused-multiply-add is NOT used because it alters the results between
unoptimized and optimized code, resulting in -mfma really meaning
-mfast-math-anyway. I really do not want to have to debug issues that
occur only in optimized code.
Bill Currie
D G Turner