Re: Instruction scheduling with gcc on alpha

Robert.Harley@inria.fr (Robert Harley) wrote:


>Claus Denk (denk@obelix.cica.es) writes:


>>I am just looking at the machine code created by gcc. I am interested
>>in simple floating vector operations, as for example:


>> for (i = 0; i< n; i++)
>> dy[i] = da*dx[i];


>>For pipelined architectures like the alpha, loop unrolling is
>>essential. [...]


>It is indeed, but neither gcc nor other compilers will do much with
>this because dx and dy might overlap. If you are sure they don't, you
>can qualify dx as 'const', use some 'no-aliasing' flag etc.


Or use Fortran (i.e., g77).


[ This is not really directed at Claus, because I _know_ he uses
Fortran, but the number of times one has to explain why Fortran code
can be compiled to faster code than C *just because of this single
language feature* is amazing.


Without specifying that dy and dx are not allowed to overlap, one
couldn't even write a vectorizing compiler ]


--
Toon Moene (mailto:toon@moene.indiv.nluug.nl)
Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
Phone: +31 346 214290; Fax: +31 346 214286
g77 Support: mailto:fortran@gnu.ai.mit.edu; NWP: http://www.knmi.nl/hirlam
--

Return to the comp.compilers page.
Search the comp.compilers archives again.