[PATCH][AArch64] Fix default CPU configurations

[Kyrill Tkachov]

Ping?

On 25/02/14 10:08, Kyrill Tkachov wrote:
> Hi all,
>
> The problem solved in this patch is that when gcc is configured with
> --with-arch=armv8-a gcc will go into  aarch64-arches.def, pick the
> representative CPU (Cortex-A53 for ARMv8-A) and use that CPUs ISA flags. Now
> that we specified that Cortex-A53 has CRC and crypto though, this means that gcc
> will choose by default to enable CRC and Crypto.
>
> What it should be doing though is to use the 4th field in the AARCH64_ARCH macro
> that specifies the ISA flags implied by the architecture. This patch does that
> by looking in aarch64-arches.def and extracting the 4th field appropriately and
> using that as the ext_mask when processing a --with-arch option.
>
> Furthermore, if no --with-arch or --with-cpu directives are specified config.gcc
> will set TARGET_DEFAULT_CPU to TARGET_CPU_generic. What it should be doing, is
> leaving it undefined so that the backend in aarch64.h can define its own default
> with the correct ISA options (currently we have this scheme where the
> TARGET_CPU_<core> is encoded in the first 6 bits of TARGET_DEFAULT_CPU and the
> ISA flags are encoded in the upper part of it. We should clean that up in the
> next release). Before this patch, the code in aarch64.h that does that
> initialisation was never even exercised because TARGET_CPU_DEFAULT was always
> defined by config.gcc no matter what! config.gcc defined it as
> TARGET_CPU_generic but without encoding the appropriate ISA flags in the upper
> bits, leading to a cpu configured without fp or simd.
>
> After a discussion with Richard, this patch sets the default CPU (if no
> -mcpu,-march,--with-cpu,--with-arch is given) to be generic+fp+simd. The generic
> CPU already schedules like the Cortex-A53, so it should give a decent generic
> tuning.
>
> This patch should improve the current situation a bit.
> With this patch:
> - If --with-arch=armv8-a is specified we will use generic+fp+simd as the CPU
> (without the patch it's cortex-a53+fp+simd+crc+crypto)
> - If no arch or cpu options specified anywhere, we will use the generic+fp+simd
> CPU (without the patch it would be just generic)
>
> Tested aarch64-none-elf on a model and checked the .cpu directive in the
> generated assembly for a variety of --with-cpu, --with-arch combinations.
>
> I'm proposing this patch as an alternative to
> http://gcc.gnu.org/ml/gcc-patches/2014-02/msg01072.html.
>
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> 2014-02-25  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>       * config.gcc (aarch64*-*-*): Use ISA flags from aarch64-arches.def.
>       Do not define target_cpu_default2 to generic.
>       * config/aarch64/aarch64.h (TARGET_CPU_DEFAULT): Use generic cpu.
>       * config/aarch64/aarch64.c (aarch64_override_options): Update comment.
>       * config/aarch64/aarch64-arches.def (armv8-a): Use generic cpu.