Hi Andrew,
thanks for reviewing I'll work on your comments. Just replying to the
high level questions.
Andrew Pinski <pins...@gmail.com> writes:
> On Wed, Jul 22, 2020 at 3:10 AM Andrea Corallo <andrea.cora...@arm.com> wrote:
>>
>> Hi all,
>>
>> this second patch implements the AArch64 specific back-end pass
>> 'branch-dilution' controllable by the followings command line options:
>>
>> -mbranch-dilution
>>
>> --param=aarch64-branch-dilution-granularity={num}
>>
>> --param=aarch64-branch-dilution-max-branches={num}
>>
>> Some cores known to be able to benefit from this pass have been given
>> default tuning values for their granularity and max-branches. Each
>> affected core has a very specific granule size and associated max-branch
>> limit. This is a microarchitecture specific optimization. Typical
>> usage should be -mbranch-dilution with a specified -mcpu. Cores with a
>> granularity tuned to 0 will be ignored. Options are provided for
>> experimentation.
>
> Can you give a simple example of what this patch does?
Sure, this pass simply moves a sliding window over the insns trying to
make sure that we never have more then 'max_branch' branches for every
'granule_size' insns.
If too many branches are detected nops are added where considered less
armful to correct that.
There are obviously many scenarios where the compiler can generate a
branch dense pieces of code but say we have the equivalent of:
====
.L389:
bl foo
b .L43
.L388:
bl foo
b .L42
.L387:
bl foo
b .L41
.L386:
bl foo
b .L40
====
Assuming granule size 4 and max branches 2 this will be transformed in
the equivalent of:
====
.L389:
bl foo
b .L43
nop
nop
.L388:
bl foo
b .L42
nop
nop
.L387:
bl foo
b .L41
nop
nop
.L386:
bl foo
b .L40
nop
nop
====
> Also your testcases seem too sensitive to other optimizations which
> could happen. E.g. the call to "branch (i)" could be pulled out of
> the switch statement. Or even the "*i += N;" could be moved to one
> Basic block and the switch becomes just one if statement.
>
>> Observed performance improvements on Neoverse N1 SPEC CPU 2006 where
>> up to ~+3% (xalancbmk) and ~+1.5% (sjeng). Average code size increase
>> for all the testsuite proved to be ~0.4%.
>
> Also does this improve any non-SPEC benchmarks or has it only been
> benchmarked with SPEC?
So far I tried it only on SPEC 2006. The transformation is not
benchmark specific tho, other code may benefit from it.
Thanks
Andrea
