Hi,
On Wed, Nov 28 2018, Richard Sandiford wrote:
>>> The pass gives a 7.6% improvement on Cortex-A72 for 554.roms_r at -O3
>>> and a 2.4% improvement for 465.tonto. I haven't found any SPEC tests
>>> that regress.
>>>
>>> Sizewise, there's a 10% increase in .text for both 554.roms_r and
>>> 465.tonto. That's obviously a lot, but in tonto's case it's because
>>> the whole program is written using assumed-shape arrays and pointers,
>>> so a large number of functions really do benefit from versioning.
>>> roms likewise makes heavy use of assumed-shape arrays, and that
>>> improvement in performance IMO justifies the code growth.
>>
>> Ouch. I know that at least with LTO IPA-CP can do "quite" some
>> propagation of constant strides. Not sure if we're aggressive
>> enough in actually doing the cloning for all cases we figure out
>> strides though. But my question is how we can avoid doing the
>> versioning for loops in the copy that did not have the IPA-CPed
>> stride of one? Ideally we'd be able to mark individual references
>> as {definitely,likely,unlikely,not}-unit-stride?
>
> This is a more elaborate version of what I was trying to do with
> the IFN I'd mentioned on IRC a few weeks back. It would be a bit
> like a long-living ASSERT_EXPR that provides hints about the value
> of the SSA name. Another alternative would be to annotate the
> SSA name itself, but then we'd easily lose the information during
> optimisation.
>
> But will IPA-CP conditionally use a clone for a stride of 1 for
> calls that pass a variable stride that might or might not be 1?
> E.g. if it clones foo as foo.1 for calls in which a stride parameter
> is 1 at compile time, does it also make foo call foo.1 when the stride
> parameter is 1 at runtime? If not, that sounds like a missed optimisation.
> If so, prune_conditions should stop us from versioning.
IPA-CP only creates clones for compile-time constants, it does not add
any run-time check to the callers. It is an interesting idea that it
could add these for some cases when it decides to do the cloning
anyway.
Martin
