Re: Instruction scheduling with gcc on alpha

Claus Denk wrote:
>
> I have been posting this to gcc.help, but no answer so far. Maybe this
> is the right group for this question ?
>
> 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. [snip]


It's probably quite important to get the branch at the end of the loop
to be the right flavour. Confitional branches with a negative
displacement (it says here) are predicted to be taken. Given
something like this:


loop:
<assignments>
r0 <- n-i
BGT loop


the pipeline will do the right thing (assuming I've got the test the
right way around :-). However, there's that extract bit of arithmetic
involved so C code like this


for (i = n-1; i >= 0; i--)
dy[i] = da*dx[i];


should be faster since the test instruction need do no extra arithmetic.
It would be interesting to see what this change to the C does to the
execution time.


--
John Haxby
jch@pwd.hp.com http://www.ice.hp.com/
--

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