[maemo-developers] Performance of floating point instructions

[From: Siarhei Siamashka]

On Wednesday 10 March 2010, Laurent GUERBY wrote:
> On Wed, 2010-03-10 at 21:54 +0200, Siarhei Siamashka wrote:
> > I wonder why the compiler does not use real NEON instructions with
> > -ffast-math option, it should be quite useful even for scalar code.
> >
> > something like:
> >
> > vld1.32  {d0[0]}, [r0]
> > vadd.f32 d0, d0, d0
> > vst1.32  {d0[0]}, [r0]
> >
> > instead of:
> >
> > flds     s0, [r0]
> > fadds    s0, s0, s0
> > fsts     s0, [r0]
> >
> > for:
> >
> > *float_ptr = *float_ptr + *float_ptr;
> >
> > At least NEON is pipelined and should be a lot faster on more complex
> > code examples where it can actually benefit from pipelining. On x86, SSE2
> > is used quite nicely for floating point math.
>
> Hi,
>
> Please open a report on http://gcc.gnu.org/bugzilla with your test
> sources and command line, at least GCC developpers will notice there's
> interest :).

This sounds reasonable :)

> GCC comes with some builtins for neon, they're defined in arm_neon.h
> see below.

This does not sound like a good idea. If the code has to be modified and
changed into something nonportable, there are way better options than
intrinsics.

Regarding the use of NEON instructions via C++ operator overloading. A test
program is attached.

# gcc -O3 -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -ffast-math
      -o neon_float neon_float.cpp

=== ieee754 floats ===

real    0m3.396s
user    0m3.391s
sys     0m0.000s

=== runfast floats ===

real    0m2.285s
user    0m2.273s
sys     0m0.008s

=== NEON C++ wrapper ===

real    0m1.312s
user    0m1.313s
sys     0m0.000s

But the quality of generated code is quite bad. That's also something to be
reported to gcc bugzilla :)

-- 
Best regards,
Siarhei Siamashka
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