On Thu, Mar 28, 2013 at 2:51 AM, William "Chops" Westfield
<wes...@mac.com>wrote:
> On Mar 27, 2013, at 6:21 PM, Peter Bigot wrote:
>
> > For a while there I thought it might be finding the minimum pwm value
> and eliminating the iterations up to that value from the loop--which it's
> entitled to do given what was coded.
>
> That's a discouraging thought. How would I prevent this from happening?
>
It doesn't happen, so it's not an issue. The way to stop it from happening
is the same way that's always used: don't use number of loop iterations as
a timing control mechanism.
Making the pwm_delay loop counter volatile resulted in the avr compiler
> (which is actually based on 4.6.2) generating the more obvious code, but
> the msp430 code stayed "obscure."
>
> Binary searching through the optimization options…
> -fno-ivopts turns this off for the avr compiler, but not for msp430.
> -fno-tree-loop-optimize turns this off for both avr and msp430.
>
> the only somewhat readable description I found of just what
> -ftree-loop-optimize is likely to do is here:
> http://gcc.gnu.org/onlinedocs/gccint/Tree-SSA-passes.html , and it
> implies that it does rather a lot of things that are generally a good idea.
> Although in this case it definitely ends up producing slower and bigger
> code.
>
> Ah, well. Thanks for your help; I mostly wanted confirmation that I
> wasn't using some known buggy version of the compiler before trying to
> delve too deeply into what was going on.
>
> Does this warrant a bug for the resulting poorer-than-possible
> optimization?
>
No. Failure to be optimal is not wrong.
>
> Thanks
> Bill W
>
>
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