[PATCH][AArch64] Fix default CPU configurations

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

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.
    (aarch64_override_options): Update comment about default cpu.

commit 918bb596fde24640a68e5d1febd6d94d6acbd9ab
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Mon Feb 24 09:10:03 2014 +0000

    [AArch64] Fix default CPU options

diff --git a/gcc/config.gcc b/gcc/config.gcc
index 2156640..89da61b 100644
--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -3391,6 +3391,11 @@ case "${target}" in
 				  ${srcdir}/config/aarch64/$def | \
 				  sed -e 's/^[^,]*,[ 	]*//' | \
 				  sed -e 's/,.*$//'`
+				# Extract the architecture flags from aarch64-arches.def
+				ext_mask=`grep "^$pattern(\"$base_val\"," \
+				   ${srcdir}/config/aarch64/$def | \
+				   sed -e 's/)$//' | \
+				   sed -e 's/^.*,//'`
 			  else
 				base_id=`grep "^$pattern(\"$base_val\"," \
 				  ${srcdir}/config/aarch64/$def | \
@@ -4052,10 +4057,8 @@ esac
 target_cpu_default2=
 case ${target} in
 	aarch64*-*-*)
-		if test x$target_cpu_cname = x
+		if test x"$target_cpu_cname" != x
 		then
-			target_cpu_default2=TARGET_CPU_generic
-		else
 			target_cpu_default2=$target_cpu_cname
 		fi
 		;;
diff --git a/gcc/config/aarch64/aarch64-arches.def b/gcc/config/aarch64/aarch64-arches.def
index 5028f61..4b796d8 100644
--- a/gcc/config/aarch64/aarch64-arches.def
+++ b/gcc/config/aarch64/aarch64-arches.def
@@ -26,4 +26,4 @@
    this architecture.  ARCH is the architecture revision.  FLAGS are
    the flags implied by the architecture.  */
 
-AARCH64_ARCH("armv8-a",	      cortexa53,	     8,  AARCH64_FL_FOR_ARCH8)
+AARCH64_ARCH("armv8-a",	      generic,	     8,  AARCH64_FL_FOR_ARCH8)
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 21054e5..624a7bb 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -5307,7 +5307,7 @@ aarch64_override_options (void)
 
   /* If the user did not specify a processor, choose the default
      one for them.  This will be the CPU set during configuration using
-     --with-cpu, otherwise it is "cortex-a53".  */
+     --with-cpu, otherwise it is "generic".  */
   if (!selected_cpu)
     {
       selected_cpu = &all_cores[TARGET_CPU_DEFAULT & 0x3f];
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 13c424c..d0463be 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -472,10 +472,10 @@ enum target_cpus
   TARGET_CPU_generic
 };
 
-/* If there is no CPU defined at configure, use "cortex-a53" as default.  */
+/* If there is no CPU defined at configure, use generic as default.  */
 #ifndef TARGET_CPU_DEFAULT
 #define TARGET_CPU_DEFAULT \
-  (TARGET_CPU_cortexa53 | (AARCH64_CPU_DEFAULT_FLAGS << 6))
+  (TARGET_CPU_generic | (AARCH64_CPU_DEFAULT_FLAGS << 6))
 #endif
 
 /* The processor for which instructions should be scheduled.  */