Re: [dpdk-dev] [EXT] [PATCH v3 1/3] eal/arm64: add 128-bit atomic compare exchange

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

> -----Original Message-----
> From: Jerin Jacob Kollanukkaran
> Sent: Wednesday, July 3, 2019 5:56 PM
> To: Phil Yang <phil.yang@arm.com>; dev@dpdk.org
> Cc: thomas@monjalon.net; hemant.agrawal@nxp.com;
> Honnappa.Nagarahalli@arm.com; gavin.hu@arm.com; nd@arm.com;
> gage.eads@intel.com
> Subject: RE: [EXT] [PATCH v3 1/3] eal/arm64: add 128-bit atomic compare
> exchange
> 
> > -----Original Message-----
> > From: Phil Yang <phil.yang@arm.com>
> > Sent: Friday, June 28, 2019 1:42 PM
> > To: dev@dpdk.org
> > Cc: thomas@monjalon.net; Jerin Jacob Kollanukkaran
> > <jerinj@marvell.com>; hemant.agrawal@nxp.com;
> > Honnappa.Nagarahalli@arm.com; gavin.hu@arm.com; nd@arm.com;
> > gage.eads@intel.com
> > Subject: [EXT] [PATCH v3 1/3] eal/arm64: add 128-bit atomic compare
> > exchange
> >
> > Add 128-bit atomic compare exchange on aarch64.
> >
> > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > Tested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > ---
> > v3:
> > 1. Avoid duplication code with macro. (Jerin Jocob) 2. Make invalid
> > memory order to strongest barrier. (Jerin Jocob) 3. Update
> > doc/guides/prog_guide/env_abstraction_layer.rst. (Eads Gage) 4. Fix
> > 32-bit x86 builds issue. (Eads Gage) 5. Correct documentation issues
> > in UT. (Eads Gage)
> >
> >  .../common/include/arch/arm/rte_atomic_64.h        | 165
> > +++++++++++++++++++++
> >  .../common/include/arch/x86/rte_atomic_64.h        |  12 --
> >  lib/librte_eal/common/include/generic/rte_atomic.h |  17 ++-
> >  3 files changed, 181 insertions(+), 13 deletions(-)
> >
> > diff --git a/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> > b/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> > index 97060e4..2080c4d 100644
> > --- a/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> > +++ b/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> > @@ -1,5 +1,6 @@
> >  /* SPDX-License-Identifier: BSD-3-Clause
> >   * Copyright(c) 2015 Cavium, Inc
> > + * Copyright(c) 2019 Arm Limited
> >   */
> >
> >  #ifndef _RTE_ATOMIC_ARM64_H_
> > @@ -14,6 +15,9 @@ extern "C" {
> >  #endif
> >
> >  #include "generic/rte_atomic.h"
> > +#include <rte_branch_prediction.h>
> > +#include <rte_compat.h>
> > +#include <rte_debug.h>
> >
> >  #define dsb(opt) asm volatile("dsb " #opt : : : "memory")  #define
> > dmb(opt) asm volatile("dmb " #opt : : : "memory") @@ -40,6 +44,167 @@
> > extern "C" {
> >
> >  #define rte_cio_rmb() dmb(oshld)
> >
> > +/*------------------------ 128 bit atomic operations
> > +-------------------------*/
> > +
> > +#define RTE_HAS_ACQ(mo) ((mo) != __ATOMIC_RELAXED && (mo) !=
> > +__ATOMIC_RELEASE) #define RTE_HAS_RLS(mo) ((mo) ==
> __ATOMIC_RELEASE
> > || \
> > +			 (mo) == __ATOMIC_ACQ_REL || \
> > +			 (mo) == __ATOMIC_SEQ_CST)
> > +
> > +#define RTE_MO_LOAD(mo)  (RTE_HAS_ACQ((mo)) \
> > +		? __ATOMIC_ACQUIRE : __ATOMIC_RELAXED) #define
> > RTE_MO_STORE(mo)
> > +(RTE_HAS_RLS((mo)) \
> > +		? __ATOMIC_RELEASE : __ATOMIC_RELAXED)
> > +
> > +#ifdef __ARM_FEATURE_ATOMICS
> > +#define __ATOMIC128_CAS_OP(cas_op_name, op_string)                          \
> > +static inline rte_int128_t                                                  \
> > +cas_op_name(rte_int128_t *dst, rte_int128_t old,                            \
> > +		rte_int128_t updated)                                               \
> > +{                                                                           \
> > +	/* caspX instructions register pair must start from even-numbered
> > +	 * register at operand 1.
> > +	 * So, specify registers for local variables here.
> > +	 */                                                                     \
> > +	register uint64_t x0 __asm("x0") = (uint64_t)old.val[0];                \
> 
> I understand CASP limitation on register has to be even and odd.
> Is there anyway to remove explicit x0 register allocation and choose compiler to
> decide the register. Some reason with optimize(03) gcc makes correctly but not
> clang.
> 
> Hardcoding to specific register makes compiler to not optimize the stuff,
> especially if it is inline function.

It look like the limitation fixed recently in gcc.
https://patches.linaro.org/patch/147991/

Not sure about old gcc and clang. ARM compiler experts may know the exact status