Loop invariant motion from cold block

Pat Haugen pthaugen@linux.vnet.ibm.com
Thu Nov 6 20:36:00 GMT 2014


On 11/06/2014 01:00 PM, Richard Biener wrote:
> Shouldn't we never hoist anything from a bb with lower execution frequency
> to a bb with higher one?  It seems LIM simply assumes that inside a loop
> is always higher frequency than outside of it.
>
> So - why artificially have that factor of 0.1 instead of simply checking for
> less than?
Yes, that would seem to be the best general approach. My initial hack 
was done wrt to example given and apparently I just kept my tunnel 
vision goggles on.
> Btw, we also hoist through multiple loop nests so even an unlikely
> branch in the innermost loop may execute more times than the outermost
> loop entry.   So I believe the change has to be made somewhere else,
> like in invariantness_dom_walker::before_dom_children.
>
> Btw, for your example with calling a noreturn function in the loop exit
> destination the block containing the k/5 shouldn't be in the loop at all.
>
> I wonder why they are not using __attribute__((noreturn)) in which case
> they'd even get better profiles can can get rid of the __builtin_expect?
Good point, and probably the approach that should really be taken in 
that code.

Thanks,
Pat



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