[PATCH] [AArch64] Implement Clang CLI interface proposal about "-march".

[Eric Christopher]

On Tue, Jul 8, 2014 at 11:51 PM, Kevin Qin <kevinqindev at gmail.com> wrote:
> HI Eric,
>
> Thanks for your feedback. Below is my comments.
>
>
> 2014-07-09 2:25 GMT+08:00 Eric Christopher <echristo at gmail.com>:
>
>> >> > 4. Implement support of "-mtune". Usage is: "-march=CPU_NAME". For
>> >> > instance, "-march=cortex-a57". This option will ONLY get
>> >> > micro-architecture
>> >> > level feature enabled specifying to target CPU, like "zcm" and "zcz"
>> >> > for
>> >> > cyclone. Any architecture features WON'T be modified.
>> >>
>> >> That's not what -mtune is. According to GCC's manual: "Tune to
>> >> cpu-type everything applicable about the generated code, except for
>> >> the ABI and the set of available instructions."
>> >>
>> >> The difference between -mcup and -mtune is that the former selects ABI
>> >> and ISAs supported by the CPU, while the former doesn't. This is
>> >> particularly important if you want to run the code on a newer CPU but
>> >> doesn't want to break older ones, so you can't use instructions that
>> >> the old ones don't have, but you can optimise for the pipeline and
>> >> branch decisions of the newer CPU, as long as it just slows down the
>> >> older ones.
>> >
>> > I didn't explain it clearly. Your point is totally what I did in this
>> > patch.
>> > I emphasize " ONLY get micro-architecture level feature enabled" is want
>> > to
>> > say ISA won't be changed by this option. This option is to select target
>> > CPU
>> > to optimize for, including enabling micro-architecture level feature,
>> > choosing MI scheduler and triggering any optimizations specific for
>> > target.
>> >>
>> >>
>> >>
>> >> > 5. Change usage of "-mcpu" to "-mcpu=CPU_NAME+[no]feature", which is
>> >> > an
>> >> > alias to "-march={feature of CPU_NAME}+[no]feature" and
>> >> > "-mtune=CPU_NAME"
>> >> > together. An warning is added to discourage use of this option.
>> >>
>> >> I find this one redundant with -march and don't think we should add
>> >> deprecated features. -mcpu is the flag you want for the behaviour
>> >> you've done -mtune above. AFAIK, we don't have the infrastructure to
>> >> implement -mtune yet. Also, the driver is a bit bonkers when going
>> >> from CPU to Arch from a different arch than the host without using
>> >> -target (which is the point with -march, I guess).
>> >>
>> >> I don't think -mcpu should be used on its own, only in conjunction
>> >> with -target or -march.
>> >
>> > In my patch, the difference between "-mcpu" and "-mtune" is that,
>> > "-mcpu"
>> > will enable all ISAs which target CPU supports, while "-mtune" won't do
>> > this. And "-mcpu" can accept extra feature modifiers to make a change,
>> > but
>> > "-mtune" accepts CPU name only. So "-mcpu" is an shortcut of "-march"
>> > and
>> > "-tune". Keeping this option alive in clang is because it's still alive
>> > in
>> > gcc, and may still be used in many projects.  An warning is added to
>> > discourage use of this option.
>>
>> This is fine, and I encourage the warning. Also, -march should
>> probably default to -mtune of the same architecture. I didn't read to
>> verify, but just making sure this is the case.
>
> Currently, there's only one architecture available,  so -march will always
> default to "armv8-a+neon". We can do further when there's more and more
> architectures on AArch64 target.
>>
>>
>> >>
>> >>
>> >>
>> >>
>> >> > 1. Neon is enabled by default, and "generic" will be used if no CPU
>> >> > type
>> >> > is specified.
>> >>
>> >> Makes sense to me.
>> >>
>> >>
>> >> > 2. For most scenario, Using "-mtune=CPU" only is recommended as neon
>> >> > is
>> >> > enabled by default and all micro-architecture optimizations are
>> >> > selected,
>> >> > and it would provide great compatibility to run on most of AArch64
>> >> > devices.
>> >>
>> >> That'd be -mcpu, and we still need -march or -target.
>> >
>> > "-target" is still necessary at moment while "-march" can be omitted
>> > sometimes, because the settings of default feature can work well for
>> > most
>> > scenarios and provide good code migration. All I want to do is to get
>> > "-mcpu" supporter happy to use "-mtune" instead. They don't need to
>> > complain
>> > typing too much as splitting "-mcpu" into "-march" and "-mtune" because
>> > they
>> > can use "-mtune" only. For a standard sets of compiling flags, pair use
>> > of
>> > "-march" and "-mtune" is strongly recommended.
>>
>> This seems to be a good idea. Can you give examples of behavior you're
>> expecting to see just to verify?
>
>
> Single use of "-target aarch64-linux-gnu" equals "-target aarch64-linux-gnu
> -march=armv8-a+neon mtune=generic", which can provide correct codes but not
> fully optimized.
>
> "-target aarch64-linux-gnu -mtune=cortex-a57" euqals "-target
> aarch64-linux-gnu -march=armv8-a+neon mtune=cortex-a57" ,which can work
> quite well in most scenarios. NEON is enabled for vectorization and MI
> scheduler is selected to optimize codes for cortex-a57. And it provides good
> compatibility which allows binary running on most AArch64 devices as it
> doesn't rely on any crc or crypto support. New starters of AArch64 can
> easily start their project from these flags, and it is good enough for
> experiment purpose for experienced developer.
>
> If user wants to control more features, such as enable crc and crypto, or
> disable neon or fp, then they need to use "-target=aarch64-linux-gnu
> -march=armv8-a+[no]feature -mtune=cortex-a57". It's standard sets of flags I
> recommend to use, which explicitly select the architecture feature though
> command line.  Even if a project only require NEON, it's recommend to add
> "-march=armv8-a+neon" in command line. Because the default behavior of
> -march is unreliable, which may get change in future.
>
> To summarize, missing of "-march" can work well at moment, but should only
> be used for short term experiment. Pair using "-march" and "-mtune" is
> recommended.

I can agree with this. I also would like -march=cortex-a57 to work as
part of this patch. There's no reason to have it be different between
-march and -mtune.

Thanks.

-eric

>>
>>
>> >>
>> >>
>> >>
>> >> > 3. "-march" is designed to be used only if user wants to use crc and
>> >> > crypto instructions, or disable fp/neon. So "-march" will not be
>> >> > frequently
>> >> > used and won't bring too much finger burden.
>> >>
>> >> I thought the idea was to encourage -march... at least on new
>> >> targets...
>> >
>> > Yes, we always encourage people to specifying architecture features via
>> > "-march". Letting "-march" and "-mtune" replace "-mcpu" and "-mfpu" is
>> > what
>> > we want to do.
>>
>> Very much so.
>>
>> Thanks!
>>
>> -eric
>>
>> >>
>> >>
>> >> --renato
>> >
>> >
>> >
>> >
>> > --
>> > Best Regards,
>> >
>> > Kevin Qin
>> >
>> > _______________________________________________
>> > cfe-commits mailing list
>> > cfe-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>> >
>
>
>
>
> --
> Best Regards,
>
> Kevin Qin