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RE: Best way to compute cost of a sequence of gimple stmt


> From: Richard Biener [mailto:richard.guenther@gmail.com]
> Sent: Wednesday, June 11, 2014 4:09 PM

> >
> > Oh I see. Doing it there would mean instead of two independent
> > operations you'd do the best combination possible, is that right?
> 
> Yes (but probably it's not worth the trouble).

I understood that.

> 
> > I'm tempted to use a simple heuristic such as comparing the
> > number of loads before and after, adding one if the load is
> > unaligned. So in the above example, supposing that there is
> > some computation done around x[0] before the return line,
> > we'd have 2 loads before Vs 2 x is unaligned and we would
> > cancel the optimization. If x is aligned the optimization would
> > proceed.
> >
> > Do you thing this approach is also too much trouble or would
> > not work?
> 
> I'm not sure.  For noop-loads I'd keep them unconditionally, even if
> unaligned.  I'd disable unaligned-load + bswap for now.  People
> interested and sitting on such a target should do the measurements
> and decide if it's worth the trouble (is arm affected?).

Yes it is.

> 
> But I see that the code currently does not limit itself to single-use
> chains and thus may end up keeping the whole original code life
> by unrelated uses.  So a good thing would be to impose proper
> restrictions here.  For example, in find_bswap_or_nop_1 do
> 
>   if (TREE_CODE (rhs1) != SSA_NAME
>      || !has_single_use (rhs1))

But then the example in gcc.dg/optimize-bswapdi-2.c would not
work for instance. Same for swap32_b in gcc.dg/optimize-bswapsi-1.c

To make it work you'd need to check that there is no use outside the
sets of statements that form the bitwise OR operation you are
considering.

Best regards,

Thomas



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