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

[Kevin Qin]

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.

>
> >>
> >>
> >>
> >> > 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
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