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Re: [PATCH][AArch64] PR79262: Adjust vector cost
Hi James,
> We have 7 unique target tuning structures in the AArch64 backend, of which
> only one has a 2x ratio between scalar_int_cost and vec_to_scalar_cost. Other
> ratios are 1, 3, 8, 3, 4, 6.
I wouldn't read too much in the exact value here - the costs are simply relative to
other values for the same tuning, ie. cores that use 4 or 6 also tend to use larger
values for the other entries.
> What makes this choice correct? What makes it more correct than what we
> have now? On which of the 28 entries in config/aarch64/aarch64-cores.def does
> performance improve?
It's correct since it's the minimum value that stops vectorization for this particular
bug without increasing costs too much and accidentally blocking useful loops from
being vectorized (as confirmed by benchmarking). Given that all other vector cost
tunings already block vectorization for this case, this is clearly what is required
for best performance. So this improves performance for all 28 entries.
> Please validate the performance effect of this patch, which changes default
> code generation for everyone, on more than one testcase in a bug report.
I did validate the performance like I mentioned in the patch. Since it has been a
while, I can easily rerun the benchmarks using latest trunk to verify it's still a gain.
> Are the Spec benchmarks sufficiently representative to change the generic
> vectorisation costs?
SPEC is sufficiently large and complex to show there is a gain without regressions.
Do you have reason to believe other benchmarks might not see the same gain?
Wilco