[PATCH 5/6][AArch64] Enable -fprefetch-loop-arrays at -O3 for cores that benefit from prefetching.

[Maxim Kuvyrkov]

> On Jan 30, 2017, at 5:50 PM, Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org> wrote:
> 
>> On Jan 30, 2017, at 3:23 PM, Kyrill Tkachov <kyrylo.tkachov@foss.arm.com> wrote:
>> 
>> Hi Maxim,
>> 
>> On 30/01/17 12:06, Maxim Kuvyrkov wrote:
>>> This patch enables prefetching at -O3 for aarch64 cores that set "simultaneous prefetches" parameter above 0.  There are currently no such settings, so this patch doesn't change default code generation.
>>> 
>>> I'm now working on improvements to -fprefetch-loop-arrays pass to make it suitable for -O2. I'll post this work in the next month.
>>> 
>>> Bootstrapped and regtested on x86_64-linux-gnu and aarch64-linux-gnu.
>>> 
>> 
>> Are you aiming to get this in for GCC 8?
>> I have one small comment on this patch:
>> 
>> +  /* Enable sw prefetching at -O3 for CPUS that have prefetch, and we
>> +     have deemed it beneficial (signified by setting
>> +     prefetch.num_slots to 1 or more).  */
>> +  if (flag_prefetch_loop_arrays < 0
>> +      && HAVE_prefetch
>> 
>> HAVE_prefetch will always be true on aarch64.
>> I imagine midend code that had logic like this would need this check, but aarch64-specific code shouldn't need it.
> 
> Agree, I'll remove HAVE_prefetch.
> 
> This pattern was copied from other backends, and HAVE_prefetch is most likely a historical artifact.

Andrew raised a good point in the review of his patch that it is a bad idea to use one of prefetching parameters (simultaneous_prefetches) as indicator for whether to enable prefetching pass by default.  Indeed there are cases when we want to set simultaneous_prefetch according to HW documentation (or experimental results), but not enable prefetching pass by default.

This update to the patch addresses it.  The patch adds a new explicit field to prefetch tuning structure "default_opt_level" that sets optimization level from which prefetching should be enabled by default.  The current value is to enable prefetching at -O3; additionally, this parameter will come handy for enabling prefetching at -O2 [when it is ready].

--
Maxim Kuvyrkov
www.linaro.org


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