On Tue, Jul 8, 2014 at 11:51 PM, Kevin Qin <[email protected]> wrote: > HI Eric, > > Thanks for your feedback. Below is my comments. > > > 2014-07-09 2:25 GMT+08:00 Eric Christopher <[email protected]>: > >> >> > 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 >> > [email protected] >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits >> > > > > > > -- > Best Regards, > > Kevin Qin _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
