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

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> >
> > ----------------------------------------------------------------------
> > > 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
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.

> 
> 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'.

> 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