Re: Automatically transform or expand do loop in a subroutine

Richard Maine wrote:
highegg <highegg@xxxxxxxxx> wrote:

On Jan 31, 7:06 am, nos...@xxxxxxxxxxxxx (Richard Maine) wrote:
yaqi <yaqiw...@xxxxxxxxx> wrote:
I want to save computing time as much as I can.
...

do i=1,d
c = c + i
end do
The "obvious" way to save a lot of computing time here is to replace the
loop with

c = c + d*i

certainly not. c = c + d*(d+1)/2, more likely :)

Um. Er. Right.

c = c + ishft(d*(d+l),-1) will be faster, with most compilers, in the
case l >= 0. Even in analogous cases, where there is information
implying that the expression need be valid only for positive operands,
compilers don't take advantage of it.
Perhaps i and l aren't good choices for newsgroup posts. It was
difficult enough to remove obvious mistakes from my response.
.

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