[maemo-developers] Performance of floating point instructions

[From: Laurent GUERBY]

On Thu, 2010-03-11 at 00:32 +0200, Siarhei Siamashka wrote:
> On Wednesday 10 March 2010, Laurent GUERBY wrote:
> > 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.

I've no idea if this comes from a standard but ARM seems to imply
arm_neon.h is supposed to be supported by various toolchains:

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dht0002a/ch01s04s02.html
<<
GCC and RVCT support the same NEON intrinsic syntax, making C or C++
code portable between the toolchains. To add support for NEON
intrinsics, include the header file arm_neon.h. Example 1.3 implements
the same functionality as the assembler examples, using intrinsics in C
code instead of assembler instructions.
>>

(nice test :)

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

Seems that in some limited cases GCC is making progress on neon:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43001

I'm building current SVN g++ for arm to see what it does on your code
(GCC 4.4.1 get it to run in 1.5s on an 800 MHz efika MX box).

Sincerely,

Laurent