Re: [PATCH] AArch64: Do not increase the vect reduction latency by multiplying count [PR110625]

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jul 26, 2023 at 4:02 AM Hao Liu OS via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> > When was STMT_VINFO_REDUC_DEF empty?  I just want to make sure that we're not papering over an issue elsewhere.
>
> Yes, I also wonder if this is an issue in vectorizable_reduction.  Below is the the gimple of "gcc.target/aarch64/sve/cost_model_13.c":
>
>   <bb 3>:
>   # res_18 = PHI <res_15(7), 0(6)>
>   # i_20 = PHI <i_16(7), 0(6)>
>   _1 = (long unsigned int) i_20;
>   _2 = _1 * 2;
>   _3 = x_14(D) + _2;
>   _4 = *_3;
>   _5 = (unsigned short) _4;
>   res.0_6 = (unsigned short) res_18;
>   _7 = _5 + res.0_6;                             <-- The current stmt_info
>   res_15 = (short int) _7;
>   i_16 = i_20 + 1;
>   if (n_11(D) > i_16)
>     goto <bb 7>;
>   else
>     goto <bb 4>;
>
>   <bb 7>:
>   goto <bb 3>;
>
> It looks like that STMT_VINFO_REDUC_DEF should be "res_18 = PHI <res_15(7), 0(6)>"?
> The status here is:
>   STMT_VINFO_REDUC_IDX (stmt_info): 1
>   STMT_VINFO_REDUC_TYPE (stmt_info): TREE_CODE_REDUCTION
>   STMT_VINFO_REDUC_VECTYPE (stmt_info): 0x0

Not all stmts in the SSA cycle forming the reduction have
STMT_VINFO_REDUC_DEF set,
only the last (latch def) and live stmts have at the moment.

Richard.

> Thanks,
> Hao
>
> ________________________________________
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Tuesday, July 25, 2023 17:44
> To: Hao Liu OS
> Cc: GCC-patches@gcc.gnu.org
> Subject: Re: [PATCH] AArch64: Do not increase the vect reduction latency by multiplying count [PR110625]
>
> Hao Liu OS <hliu@os.amperecomputing.com> writes:
> > Hi,
> >
> > Thanks for the suggestion.  I tested it and found a gcc_assert failure:
> >     gcc.target/aarch64/sve/cost_model_13.c (internal compiler error: in info_for_reduction, at tree-vect-loop.cc:5473)
> >
> > It is caused by empty STMT_VINFO_REDUC_DEF.
>
> When was STMT_VINFO_REDUC_DEF empty?  I just want to make sure that
> we're not papering over an issue elsewhere.
>
> Thanks,
> Richard
>
>   So, I added an extra check before checking single_defuse_cycle. The updated patch is below.  Is it OK for trunk?
> >
> > ---
> >
> > The new costs should only count reduction latency by multiplying count for
> > single_defuse_cycle.  For other situations, this will increase the reduction
> > latency a lot and miss vectorization opportunities.
> >
> > Tested on aarch64-linux-gnu.
> >
> > gcc/ChangeLog:
> >
> >       PR target/110625
> >       * config/aarch64/aarch64.cc (count_ops): Only '* count' for
> >       single_defuse_cycle while counting reduction_latency.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * gcc.target/aarch64/pr110625_1.c: New testcase.
> >       * gcc.target/aarch64/pr110625_2.c: New testcase.
> > ---
> >  gcc/config/aarch64/aarch64.cc                 | 13 ++++--
> >  gcc/testsuite/gcc.target/aarch64/pr110625_1.c | 46 +++++++++++++++++++
> >  gcc/testsuite/gcc.target/aarch64/pr110625_2.c | 14 ++++++
> >  3 files changed, 69 insertions(+), 4 deletions(-)
> >  create mode 100644 gcc/testsuite/gcc.target/aarch64/pr110625_1.c
> >  create mode 100644 gcc/testsuite/gcc.target/aarch64/pr110625_2.c
> >
> > diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> > index 560e5431636..478a4e00110 100644
> > --- a/gcc/config/aarch64/aarch64.cc
> > +++ b/gcc/config/aarch64/aarch64.cc
> > @@ -16788,10 +16788,15 @@ aarch64_vector_costs::count_ops (unsigned int count, vect_cost_for_stmt kind,
> >      {
> >        unsigned int base
> >       = aarch64_in_loop_reduction_latency (m_vinfo, stmt_info, m_vec_flags);
> > -
> > -      /* ??? Ideally we'd do COUNT reductions in parallel, but unfortunately
> > -      that's not yet the case.  */
> > -      ops->reduction_latency = MAX (ops->reduction_latency, base * count);
> > +      if (STMT_VINFO_REDUC_DEF (stmt_info)
> > +       && STMT_VINFO_FORCE_SINGLE_CYCLE (
> > +         info_for_reduction (m_vinfo, stmt_info)))
> > +     /* ??? Ideally we'd use a tree to reduce the copies down to 1 vector,
> > +        and then accumulate that, but at the moment the loop-carried
> > +        dependency includes all copies.  */
> > +     ops->reduction_latency = MAX (ops->reduction_latency, base * count);
> > +      else
> > +     ops->reduction_latency = MAX (ops->reduction_latency, base);
> >      }
> >
> >    /* Assume that multiply-adds will become a single operation.  */
> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr110625_1.c b/gcc/testsuite/gcc.target/aarch64/pr110625_1.c
> > new file mode 100644
> > index 00000000000..0965cac33a0
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/aarch64/pr110625_1.c
> > @@ -0,0 +1,46 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-Ofast -mcpu=neoverse-n2 -fdump-tree-vect-details -fno-tree-slp-vectorize" } */
> > +/* { dg-final { scan-tree-dump-not "reduction latency = 8" "vect" } } */
> > +
> > +/* Do not increase the vector body cost due to the incorrect reduction latency
> > +    Original vector body cost = 51
> > +    Scalar issue estimate:
> > +      ...
> > +      reduction latency = 2
> > +      estimated min cycles per iteration = 2.000000
> > +      estimated cycles per vector iteration (for VF 2) = 4.000000
> > +    Vector issue estimate:
> > +      ...
> > +      reduction latency = 8      <-- Too large
> > +      estimated min cycles per iteration = 8.000000
> > +    Increasing body cost to 102 because scalar code would issue more quickly
> > +      ...
> > +    missed:  cost model: the vector iteration cost = 102 divided by the scalar iteration cost = 44 is greater or equal to the vectorization factor = 2.
> > +    missed:  not vectorized: vectorization not profitable.  */
> > +
> > +typedef struct
> > +{
> > +  unsigned short m1, m2, m3, m4;
> > +} the_struct_t;
> > +typedef struct
> > +{
> > +  double m1, m2, m3, m4, m5;
> > +} the_struct2_t;
> > +
> > +double
> > +bar (the_struct2_t *);
> > +
> > +double
> > +foo (double *k, unsigned int n, the_struct_t *the_struct)
> > +{
> > +  unsigned int u;
> > +  the_struct2_t result;
> > +  for (u = 0; u < n; u++, k--)
> > +    {
> > +      result.m1 += (*k) * the_struct[u].m1;
> > +      result.m2 += (*k) * the_struct[u].m2;
> > +      result.m3 += (*k) * the_struct[u].m3;
> > +      result.m4 += (*k) * the_struct[u].m4;
> > +    }
> > +  return bar (&result);
> > +}
> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr110625_2.c b/gcc/testsuite/gcc.target/aarch64/pr110625_2.c
> > new file mode 100644
> > index 00000000000..7a84aa8355e
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/aarch64/pr110625_2.c
> > @@ -0,0 +1,14 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-Ofast -mcpu=neoverse-n2 -fdump-tree-vect-details -fno-tree-slp-vectorize" } */
> > +/* { dg-final { scan-tree-dump "reduction latency = 8" "vect" } } */
> > +
> > +/* The reduction latency should be multiplied by the count for
> > +   single_defuse_cycle.  */
> > +
> > +long
> > +f (long res, short *ptr1, short *ptr2, int n)
> > +{
> > +  for (int i = 0; i < n; ++i)
> > +    res += (long) ptr1[i] << ptr2[i];
> > +  return res;
> > +}