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RE: Best way to compute cost of a sequence of gimple stmt
- From: "Thomas Preud'homme" <thomas dot preudhomme at arm dot com>
- To: "'Richard Biener'" <richard dot guenther at gmail dot com>
- Cc: "GCC Development" <gcc at gcc dot gnu dot org>
- Date: Wed, 11 Jun 2014 16:27:11 +0800
- Subject: RE: Best way to compute cost of a sequence of gimple stmt
- Authentication-results: sourceware.org; auth=none
- References: <003901cf8486$8e71a3d0$ab54eb70$ at arm dot com> <CAFiYyc1Rvt9UtGUtqUzxq4ww7F96WV-354adZX5EknTArdbhCg at mail dot gmail dot com> <000301cf8538$63e07170$2ba15450$ at arm dot com> <CAFiYyc311_A0caPQQNy_7e_UuSfmXvQqrkznzkHrEGG1V8FqtA at mail dot gmail dot com>
> From: Richard Biener [mailto:email@example.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 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