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: Friday, June 7, 2019 11:05 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; Jerin
> Jacob Kollanukkaran <jerinj@marvell.com>; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; stable@dpdk.org; 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#n91

Since it used in fastpath, a temp variable would be additional cost for no reason.
If GCC supports it then I think it is fine, I think, above usage matters with C++ portability.


> 
> >  		/* 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