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.