[2.26 COMMITTED] aarch64: add HWCAP_ATOMICS to HWCAP_IMPORTANT

[2.26 COMMITTED] aarch64: add HWCAP_ATOMICS to HWCAP_IMPORTANT

[Wilco Dijkstra]

This enables searching shared libraries in atomics/ when the hardware
supports LSE atomics of armv8.1 so one can provide optimized variants
of libraries in a portable way.

LSE atomics does not affect library abi, the new instructions can
interoperate with old ones.

I considered the earlier comments on the patch

https://sourceware.org/ml/libc-alpha/2018-04/msg00400.html
https://sourceware.org/ml/libc-alpha/2018-04/msg00625.html

It turns out that the way glibc dynamic linker decides on the search
path is not very flexible: it wants to use hwcap bits and associated
strings.  So some targets reuse hwcap bits for glibc internal purposes
to affect the search logic.  But hwcap is an interface with the kernel,
glibc should not allocate bits in it for its internal logic as that
limits future hwcap extensions and confusing to users who expect to see
hwcap bits in ifunc resolvers.  Instead of rewriting the dynamic linker
path logic (which affects all targets) this patch just uses the existing
mechanism, however this means that the path name has to be the hwcap
name "atomics" and cannot be changed to something more meaningful to
users.

It is hard to tell how much performance benefit this can give, in
principle armv8.1 atomics can be better optimized in the hardware, so it
can make a difference for synchronization heavy code.  On some systems
such multilib setup may be the only viable way to get optimized
libraries used.

        * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h (HWCAP_IMPORTANT): Add
        HWCAP_ATOMICS.

(cherry picked from commit 397c54c1afa531242602fe3ac7bb47eff0e909f9)
---
 ChangeLog                                     | 5 +++++
 sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 732e89d..b10a76f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2018-07-06  Szabolcs Nagy  <szabolcs.nagy@arm.com>

+       * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h (HWCAP_IMPORTANT): Add
+       HWCAP_ATOMICS.
+
+2018-07-06  Szabolcs Nagy  <szabolcs.nagy@arm.com>
+
         * sysdeps/unix/sysv/linux/aarch64/cpu-features.c (init_cpu_features):
         Use dl_hwcap without masking.
         * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h (HWCAP_IMPORTANT):
diff --git a/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h b/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
index 63bdf75..a72eaae 100644
--- a/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
+++ b/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
@@ -34,7 +34,7 @@
 #define _DL_HWCAP_LAST 15

 /* No additional library search paths.  */
-#define HWCAP_IMPORTANT 0
+#define HWCAP_IMPORTANT HWCAP_ATOMICS

 static inline const char *
 __attribute__ ((unused))
--
2.7.4
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