> > > > > For SA outbound packets, rte_atomic64_add_return is used to
> > > > > generate SQN atomically. This introduced an unnecessary full
> > > > > barrier by calling the '__sync' builtin implemented rte_atomic_XX
> > > > > API on aarch64. This patch optimized it with c11 atomic and
> > > > > eliminated the expensive barrier for aarch64.
> > > > >
> > > > > Signed-off-by: Phil Yang <phil.y...@arm.com>
> > > > > Reviewed-by: Ruifeng Wang <ruifeng.w...@arm.com>
> > > > > Reviewed-by: Gavin Hu <gavin...@arm.com>
> > > > > ---
> > > > > lib/librte_ipsec/ipsec_sqn.h | 3 ++-
> > > > > lib/librte_ipsec/sa.h | 2 +-
> > > > > 2 files changed, 3 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/lib/librte_ipsec/ipsec_sqn.h
> > > > > b/lib/librte_ipsec/ipsec_sqn.h index 0c2f76a..e884af7 100644
> > > > > --- a/lib/librte_ipsec/ipsec_sqn.h
> > > > > +++ b/lib/librte_ipsec/ipsec_sqn.h
> > > > > @@ -128,7 +128,8 @@ esn_outb_update_sqn(struct rte_ipsec_sa *sa,
> > > > > uint32_t *num)
> > > > >
> > > > > n = *num;
> > > > > if (SQN_ATOMIC(sa))
> > > > > - sqn = (uint64_t)rte_atomic64_add_return(&sa-
> > > > >sqn.outb.atom, n);
> > > > > + sqn = __atomic_add_fetch(&sa->sqn.outb.atom, n,
> > > > > + __ATOMIC_RELAXED);
> > > >
> > > > One generic thing to note:
> > > > clang for i686 in some cases will generate a proper function call
> > > > for 64-bit __atomic builtins (gcc seems to always generate cmpxchng8b
> > > > for
> > such cases).
> > > > Does anyone consider it as a potential problem?
> > > > It probably not a big deal, but would like to know broader opinion.
> > > I had looked at this some time back for GCC. The function call is
> > > generated only if the underlying platform does not support the atomic
> > instructions for the operand size. Otherwise, gcc generates the instructions
> > directly.
> > > I would think the behavior would be the same for clang.
> >
> > From what I see not really.
> > As an example:
> >
> > $ cat tatm11.c
> > #include <stdint.h>
> >
> > struct x {
> > uint64_t v __attribute__((aligned(8))); };
> >
> > uint64_t
> > ffxadd1(struct x *x, uint32_t n, uint32_t m) {
> > return __atomic_add_fetch(&x->v, n, __ATOMIC_RELAXED); }
> >
> > uint64_t
> > ffxadd11(uint64_t *v, uint32_t n, uint32_t m) {
> > return __atomic_add_fetch(v, n, __ATOMIC_RELAXED); }
> >
> > gcc for i686 will generate code with cmpxchng8b for both cases.
> > clang will generate cmpxchng8b for ffxadd1() - when data is explicitly 8B
> > aligned, but will emit a function call for ffxadd11().
> Does it require libatomic to be linked in this case?
Yes, it does.
In fact same story even with current dpdk.org master.
To make i686-native-linuxapp-clang successfully, I have to
explicitly add EXTRA_LDFLAGS="-latomic".
To be more specific:
$ for i in i686-native-linuxapp-clang/lib/*.a; do x=`nm $i | grep __atomic_`;
if [[ -n "${x}" ]]; then echo $i; echo $x; fi; done
i686-native-linuxapp-clang/lib/librte_distributor.a
U __atomic_load_8 U __atomic_store_8
i686-native-linuxapp-clang/lib/librte_pmd_opdl_event.a
U __atomic_load_8 U __atomic_store_8
i686-native-linuxapp-clang/lib/librte_rcu.a
U __atomic_compare_exchange_8 U __atomic_load_8
As there were no complains so far, it makes me think that
probably no-one using clang for IA-32 builds.
> Clang documentation calls out unaligned case where it would generate the
> function call
> [1].
Seems so, and it treats uin64_t as 4B aligned for IA.
> On aarch64, the atomic instructions need the address to be aligned.
For that particular case (cmpxchng8b) there is no such restrictions for IA-32.
Again, as I said before, gcc manages to emit code without function calls
for exactly the same source.
>
> [1] https://clang.llvm.org/docs/Toolchain.html#atomics-library
>
> >
> > >
> > > >
> > > > > else {
> > > > > sqn = sa->sqn.outb.raw + n;
> > > > > sa->sqn.outb.raw = sqn;
> > > > > diff --git a/lib/librte_ipsec/sa.h b/lib/librte_ipsec/sa.h index
> > > > > d22451b..cab9a2e 100644
> > > > > --- a/lib/librte_ipsec/sa.h
> > > > > +++ b/lib/librte_ipsec/sa.h
> > > > > @@ -120,7 +120,7 @@ struct rte_ipsec_sa {
> > > > > */
> > > > > union {
> > > > > union {
> > > > > - rte_atomic64_t atom;
> > > > > + uint64_t atom;
> > > > > uint64_t raw;
> > > > > } outb;
> > > >
> > > > If we don't need rte_atomic64 here anymore, then I think we can
> > > > collapse the union to just:
> > > > uint64_t outb;
> > > >
> > > > > struct {
> > > > > --
> > > > > 2.7.4
