SIMD with blackfin-gcc

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I'm developing a bare-metal firmware on BF537(Surveyor SVS board). I don't have
the time to learn assembly for blackfin, fortunately I saw SIMD i spossible with
plain C. So

1- Does the compiler automatically use SIMD features if I do some loop unrolling
like:
instead of

for(i=0;i<N;i++)
   c[i]=a[i]+b[i];

this:
for(i=0;i<N;i+=2)
{
   c[i]=a[i]+b[i];
   c[i+1]=a[i+1]+b[i+1];
}

2- I saw some gcc built-in constructs like "__attribute__ ((mode(V2HI)))" to
pack data for SIMD operations. I think this is two short integers packed. Is
there a list of such attributes? Do I really need to use them?

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Op Tue, 21 Sep 2010 13:16:29 +0200 schreef Atilla  
<atilla@stack+rest+is+junk+junk.nl>:
> I'm developing a bare-metal firmware on BF537(Surveyor SVS board). I  
> don't have
> the time to learn assembly for blackfin, fortunately I saw SIMD i  
> spossible with plain C. So
>
> 1- Does the compiler

Which compiler?

> automatically use SIMD features if I do some loop unrolling like:
> instead of
>
> for(i=0;i<N;i++)
>    c[i]=a[i]+b[i];
>
> this:
> for(i=0;i<N;i+=2)
> {
>    c[i]=a[i]+b[i];
>    c[i+1]=a[i+1]+b[i+1];
> }

Most compilers can perform loop unrolling themselves.

> 2- I saw some gcc built-in constructs like "__attribute__  
> ((mode(V2HI)))" to pack data for SIMD operations.
> I think this is two short integers packed. Is
> there a list of such attributes?

http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html
6.37.1 Blackfin Variable Attributes

Your attribute is not on the list, so that attribute is either not a GCC  
attribute and/or your compiler is not standard GCC.

> Do I really need to use them?

It depends what they really do.



-- 
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(remove the obvious prefix to reply by mail)

Atilla wrote:

> I'm developing a bare-metal firmware on BF537(Surveyor SVS board). I don't have
>  the time to learn assembly for blackfin,

Too bad. You will have to learn assembler for BlackFin even if you don't 
have time for it.

> fortunately I saw SIMD i spossible with plain C. So
> 
> 1- Does the compiler automatically use SIMD features

No. Unless you use built-ins. Even if you do so, it does miserable job.



Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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On 09/21/2010 02:50 PM, Boudewijn Dijkstra wrote:

> Which compiler?

$ bfin-elf-gcc --version
bfin-elf-gcc (GCC) 4.1.2 (ADI svn)

$ bfin-elf-gcc -v
Using built-in specs.
Target: bfin-elf
Configured with:
/usr/src/packages/BUILD/blackfin-toolchain-09r1/../gcc-4.1/configure
- --build=i686-pc-linux-gnu --host=i686-pc-linux-gnu --target=bfin-elf
- --prefix=/opt/uClinux/bfin-elf --with-newlib --disable-libstdcxx-pch
- --enable-languages=c,c++ --disable-symvers --disable-libssp --disable-libffi
- --enable-clocale=generic --enable-version-specific-runtime-libs
- --enable-__cxa_atexit
- --with-bugurl=URL:http://blackfin.uclinux.org/gf/project/toolchain/tracker
- --with-pkgversion=ADI-09r1-10
Thread model: single
gcc version 4.1.2 (ADI svn)

> Most compilers can perform loop unrolling themselves.

thanks

> http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html
> 6.37.1 Blackfin Variable Attributes
> 
> Your attribute is not on the list, so that attribute is either not a GCC
> attribute and/or your compiler is not standard GCC.

It is standard gcc for blackfin. For some reason that list does not include
packing attributes. See http://ds9a.nl/gcc-simd/example.html for x86 packing.
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