Re: [dpdk-dev] [PATCH] acl: fix build issue with some arm64 compiler

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

> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Sent: Monday, June 10, 2019 11:00 AM
> To: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; dev@dpdk.org
> Cc: thomas@monjalon.net; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>; msantana@redhat.com; aconole@redhat.com;
> stable@dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> nd <nd@arm.com>; nd <nd@arm.com>
> Subject: [EXT] RE: [dpdk-dev] [PATCH] acl: fix build issue with some arm64
> compiler
> 
> > > --
> > > > Subject: [dpdk-dev] [PATCH] acl: fix build issue with some arm64
> > > > compiler
> > > >
> > > > From: Jerin Jacob <jerinj@marvell.com>
> > > >
> > > > Some compilers reporting the following error, though the existing
> > > > code doesn't have any uninitialized variable case.
> > > > Just to make compiler happy, initialize the int32x4_t variable one
> > > > shot in C language.
> > > >
> > > > ../lib/librte_acl/acl_run_neon.h: In function 'search_neon_4'
> > > > ../lib/librte_acl/acl_run_neon.h:230:12: error: 'input' may be
> > > > used uninitialized in this function [-Werror=maybe-uninitialized]
> > > >   int32x4_t input;
> > > >
> > > > Fixes: 34fa6c27c156 ("acl: add NEON optimization for ARMv8")
> > > > Cc: stable@dpdk.org
> > > >
> > > > Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> > > > ---
> > > >  lib/librte_acl/acl_run_neon.h | 29 ++++++++++++-----------------
> > > >  1 file changed, 12 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/lib/librte_acl/acl_run_neon.h
> > > > b/lib/librte_acl/acl_run_neon.h index 01b9766d8..dc9e9efe9 100644
> > > > --- a/lib/librte_acl/acl_run_neon.h
> > > > +++ b/lib/librte_acl/acl_run_neon.h
> > > > @@ -165,7 +165,6 @@ search_neon_8(const struct rte_acl_ctx *ctx,
> > > > const uint8_t **data,
> > > >  	uint64_t index_array[8];
> > > >  	struct completion cmplt[8];
> > > >  	struct parms parms[8];
> > > > -	int32x4_t input0, input1;
> > > >
> > > >  	acl_set_flow(&flows, cmplt, RTE_DIM(cmplt), data, results,
> > > >  		     total_packets, categories, ctx->trans_table); @@ -181,17
> > > > +180,14 @@ search_neon_8(const struct rte_acl_ctx *ctx, const
> > > > +uint8_t
> > > > **data,
> > > >
> > > >  	while (flows.started > 0) {
> > > >  		/* Gather 4 bytes of input data for each stream. */
> > > > -		input0 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 0),
> > > > input0, 0);
> > > > -		input1 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 4),
> > > > input1, 0);
> > > > -
> > > > -		input0 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 1),
> > > > input0, 1);
> > > > -		input1 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 5),
> > > > input1, 1);
> > > > -
> > > > -		input0 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 2),
> > > > input0, 2);
> > > > -		input1 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 6),
> > > > input1, 2);
> > > > -
> > > > -		input0 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 3),
> > > > input0, 3);
> > > > -		input1 = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 7),
> > > > input1, 3);
> > > > +		int32x4_t input0 = {GET_NEXT_4BYTES(parms, 0),
> > > > +				    GET_NEXT_4BYTES(parms, 1),
> > > > +				    GET_NEXT_4BYTES(parms, 2),
> > > > +				    GET_NEXT_4BYTES(parms, 3)};
> > > > +		int32x4_t input1 = {GET_NEXT_4BYTES(parms, 4),
> > > > +				    GET_NEXT_4BYTES(parms, 5),
> > > > +				    GET_NEXT_4BYTES(parms, 6),
> > > > +				    GET_NEXT_4BYTES(parms, 7)};
> > > >
> > > This mixes the use of NEON intrinsics with GCC vector extensions.
> > > ACLE (Arm C Language Extensions) specifically recommends not to mix
> > > the two methods in section 12.2.6. IMO, Aaron's suggestion of using
> > > a temp vector
> > should be good.
> >
> > We are using this pattern across DPDK and SSE for x86 as well.
> > https://git.dpdk.org/dpdk/tree/drivers/net/i40e/i40e_rxtx_vec_neon.c#n
> > 91
> I am not sure about x86, I have not looked at a document similar to ACLE for
> x86. IMO, it is not relevant here as this is Arm specific code.

What I meant was its been already used in DPDK for arm64.
https://git.dpdk.org/dpdk/tree/drivers/net/i40e/i40e_rxtx_vec_neon.c#n91

Please see offial page vector gcc gcc documentation. The examples are using this scheme.
https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html

This is to just create 'input' variable. I am fine to use any other scheme with out additional cost
of instructions.

> 
> >
> > Since it used in fastpath, a temp variable would be additional cost
> > for no reason.
> Then, I would suggest we can go with using 'vdupq_n_s32'.

We have to form uint64x2_t with 4 x uint32_t variable, How does 'vdupq_n_s32' help here?
Can you share code snippet without any temp variable?

> 
> > If GCC supports it then I think it is fine, I think, above usage
> > matters with C++ portability.
> I did not understand the C++ portability part. Can you elaborate more?
> 
> >
> >
> > >
> > > >  		/* Process the 4 bytes of input on each stream. */
> > > >
> > > > @@ -227,7 +223,6 @@ search_neon_4(const struct rte_acl_ctx *ctx,
> > > > const uint8_t **data,
> > > >  	uint64_t index_array[4];
> > > >  	struct completion cmplt[4];
> > > >  	struct parms parms[4];
> > > > -	int32x4_t input;
> > > >
> > > >  	acl_set_flow(&flows, cmplt, RTE_DIM(cmplt), data, results,
> > > >  		     total_packets, categories, ctx->trans_table); @@ -242,10
> > > > +237,10 @@ search_neon_4(const struct rte_acl_ctx *ctx, const
> > > > +uint8_t
> > > > **data,
> > > >
> > > >  	while (flows.started > 0) {
> > > >  		/* Gather 4 bytes of input data for each stream. */
> > > > -		input = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 0), input,
> > > > 0);
> > > > -		input = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 1), input,
> > > > 1);
> > > > -		input = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 2), input,
> > > > 2);
> > > > -		input = vsetq_lane_s32(GET_NEXT_4BYTES(parms, 3), input,
> > > > 3);
> > > > +		int32x4_t input = {GET_NEXT_4BYTES(parms, 0),
> > > > +				   GET_NEXT_4BYTES(parms, 1),
> > > > +				   GET_NEXT_4BYTES(parms, 2),
> > > > +				   GET_NEXT_4BYTES(parms, 3)};
> > > >
> > > >  		/* Process the 4 bytes of input on each stream. */
> > > >  		input = transition4(input, flows.trans, index_array);
> > > > --
> > > > 2.21.0