On Wed, Apr 03, 2019 at 12:29:29PM -0700, Andre McCurdy wrote: > On Tue, Apr 2, 2019 at 11:23 PM Adrian Bunk <b...@stusta.de> wrote: > > On Tue, Apr 02, 2019 at 09:26:46PM +0100, Richard Purdie wrote: > > >... > > > "armv4t" is defined in the arm tune files to mean "add -march=armv4t" > > > which is the convention used throughout all the tune files. > > >... > > > > Unfortunately this is not true. > > > > OE has both armv7a and armv7at tunes. > > > > There is no armv7a without Thumb support, > > so no -march=armv7-at exists in gcc. > > > > Both armv7a and armv7at tunes pass the same march to gcc, > > but [1] is not true: > > Default to using the Thumb-2 instruction set for armv7a and above. > > > > The hardware supports Thumb-2 in any case, the actual difference between > > the armv7a and armv7at OE tunes is whether OE tells the compiler to > > generate ARM or Thumb-2 code. > > > > OE has both armv6 and armv6t tunes. > > > > There is no armv6 without Thumb support > > so no -march=armv6t exists in gcc. > > > > Some v6 support only Thumb-1 and some v6 support also Thumb-2, > > so what gcc does have is an -march=armv6t2. > > But OE lacks tunes for that. > > > > For matching the gcc options it would be correct to remove all > > armv6t and armv7at tunes that have no coresponding gcc options, > > and add armv6t2 tunes. > > Aligning the tuning options exposed via the machine config files to > those supported by gcc seems like a worthy goal... but would be a big > upheaval at this point. > > Note that the problem isn't specific to ARM. There are similar issues > for x86, but there we seem happy to provide a very minimal abstraction > with no attempt to track gcc. e.g. "corei7" hasn't been a documented > -march option since gcc 4.8 and we (somewhat arbitrarily) map it to > -march=nehalem to hide that fact from end users. > > So the high level question seems to be: should DEFAULTTUNE even > attempt to provide a full featured mapping to the options provided by > gcc? Are we happy to expose a limited subset without a 1:1 mapping for > the options we do expose (current ARM approach) or is it better for > DEFAULTTUNE to hide away all the complexity of the options provided by > gcc (current x86 approach).
The current 32bit ARM[1] approach seems to be an attempt of a 1:1 mapping. For ARMv8 it is already obvious that DEFAULTTUNE is not long-term maintainable, and duplicating all the gcc rules regarding feature flags[2] also sounds like a pointless exercise. What are actually the benefits of DEFAULTTUNE with all the tune files, compared to just let the user provide a string that is passed to -march? cu Adrian [1] ARM <= v7, not the differing 32bit ABI of ARMv8 [2] example: 'fp16fml' Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for '-march=armv8.4-a'. Use of this option with architectures prior to Armv8.2-A is not supported. -- "Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed -- _______________________________________________ Openembedded-core mailing list Openembedded-core@lists.openembedded.org http://lists.openembedded.org/mailman/listinfo/openembedded-core