Re: [PATCH 1/2] Add emulated gather capability to the vectorizer

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <rguenther@suse.de> writes:
> This adds a gather vectorization capability to the vectorizer
> without target support by decomposing the offset vector, doing
> sclar loads and then building a vector from the result.  This
> is aimed mainly at cases where vectorizing the rest of the loop
> offsets the cost of vectorizing the gather.
>
> Note it's difficult to avoid vectorizing the offset load, but in
> some cases later passes can turn the vector load + extract into
> scalar loads, see the followup patch.
>
> On SPEC CPU 2017 510.parest_r this improves runtime from 250s
> to 219s on a Zen2 CPU which has its native gather instructions
> disabled (using those the runtime instead increases to 254s)
> using -Ofast -march=znver2 [-flto].  It turns out the critical
> loops in this benchmark all perform gather operations.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> 2021-07-30  Richard Biener  <rguenther@suse.de>
>
> 	* tree-vect-data-refs.c (vect_check_gather_scatter):
> 	Include widening conversions only when the result is
> 	still handed by native gather or the current offset
> 	size not already matches the data size.
> 	Also succeed analysis in case there's no native support,
> 	noted by a IFN_LAST ifn and a NULL decl.
> 	(vect_analyze_data_refs): Always consider gathers.
> 	* tree-vect-patterns.c (vect_recog_gather_scatter_pattern):
> 	Test for no IFN gather rather than decl gather.
> 	* tree-vect-stmts.c (vect_model_load_cost): Pass in the
> 	gather-scatter info and cost emulated gathers accordingly.
> 	(vect_truncate_gather_scatter_offset): Properly test for
> 	no IFN gather.
> 	(vect_use_strided_gather_scatters_p): Likewise.
> 	(get_load_store_type): Handle emulated gathers and its
> 	restrictions.
> 	(vectorizable_load): Likewise.  Emulate them by extracting
>         scalar offsets, doing scalar loads and a vector construct.
>
> 	* gcc.target/i386/vect-gather-1.c: New testcase.
> 	* gfortran.dg/vect/vect-8.f90: Adjust.
> ---
>  gcc/testsuite/gcc.target/i386/vect-gather-1.c |  18 ++++
>  gcc/testsuite/gfortran.dg/vect/vect-8.f90     |   2 +-
>  gcc/tree-vect-data-refs.c                     |  34 ++++--
>  gcc/tree-vect-patterns.c                      |   2 +-
>  gcc/tree-vect-stmts.c                         | 100 ++++++++++++++++--
>  5 files changed, 138 insertions(+), 18 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/i386/vect-gather-1.c
>
> diff --git a/gcc/testsuite/gcc.target/i386/vect-gather-1.c b/gcc/testsuite/gcc.target/i386/vect-gather-1.c
> new file mode 100644
> index 00000000000..134aef39666
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/vect-gather-1.c
> @@ -0,0 +1,18 @@
> +/* { dg-do compile } */
> +/* { dg-options "-Ofast -msse2 -fdump-tree-vect-details" } */
> +
> +#ifndef INDEXTYPE
> +#define INDEXTYPE int
> +#endif
> +double vmul(INDEXTYPE *rowstart, INDEXTYPE *rowend,
> +	    double *luval, double *dst)
> +{
> +  double res = 0;
> +  for (const INDEXTYPE * col = rowstart; col != rowend; ++col, ++luval)
> +        res += *luval * dst[*col];
> +  return res;
> +}
> +
> +/* With gather emulation this should be profitable to vectorize
> +   even with plain SSE2.  */
> +/* { dg-final { scan-tree-dump "loop vectorized" "vect" } } */
> diff --git a/gcc/testsuite/gfortran.dg/vect/vect-8.f90 b/gcc/testsuite/gfortran.dg/vect/vect-8.f90
> index 9994805d77f..cc1aebfbd84 100644
> --- a/gcc/testsuite/gfortran.dg/vect/vect-8.f90
> +++ b/gcc/testsuite/gfortran.dg/vect/vect-8.f90
> @@ -706,5 +706,5 @@ END SUBROUTINE kernel
>  
>  ! { dg-final { scan-tree-dump-times "vectorized 24 loops" 1 "vect" { target aarch64_sve } } }
>  ! { dg-final { scan-tree-dump-times "vectorized 23 loops" 1 "vect" { target { aarch64*-*-* && { ! aarch64_sve } } } } }
> -! { dg-final { scan-tree-dump-times "vectorized 2\[23\] loops" 1 "vect" { target { vect_intdouble_cvt && { ! aarch64*-*-* } } } } }
> +! { dg-final { scan-tree-dump-times "vectorized 2\[234\] loops" 1 "vect" { target { vect_intdouble_cvt && { ! aarch64*-*-* } } } } }
>  ! { dg-final { scan-tree-dump-times "vectorized 17 loops" 1 "vect" { target { { ! vect_intdouble_cvt } && { ! aarch64*-*-* } } } } }
> diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
> index 6995efba899..3c29ff04fd8 100644
> --- a/gcc/tree-vect-data-refs.c
> +++ b/gcc/tree-vect-data-refs.c
> @@ -4007,8 +4007,27 @@ vect_check_gather_scatter (stmt_vec_info stmt_info, loop_vec_info loop_vinfo,
>  	      continue;
>  	    }
>  
> -	  if (TYPE_PRECISION (TREE_TYPE (op0))
> -	      < TYPE_PRECISION (TREE_TYPE (off)))
> +	  /* Include the conversion if it is widening and we're using
> +	     the IFN path or the target can handle the converted from
> +	     offset or the current size is not already the same as the
> +	     data vector element size.  */
> +	  if ((TYPE_PRECISION (TREE_TYPE (op0))
> +	       < TYPE_PRECISION (TREE_TYPE (off)))
> +	      && ((!use_ifn_p
> +		   && (DR_IS_READ (dr)
> +		       ? (targetm.vectorize.builtin_gather
> +			  && targetm.vectorize.builtin_gather (vectype,
> +							       TREE_TYPE (op0),
> +							       scale))
> +		       : (targetm.vectorize.builtin_scatter
> +			  && targetm.vectorize.builtin_scatter (vectype,
> +								TREE_TYPE (op0),
> +								scale))))
> +		  || (!use_ifn_p
> +		      && !operand_equal_p (TYPE_SIZE (TREE_TYPE (off)),
> +					   TYPE_SIZE (TREE_TYPE (vectype)),
> +					   0))
> +		  || use_ifn_p))

Looks like this would be slightly simpler to read/understand as:

    && (use_ifn_p
        || (DR_IS_READ …)
        || !operand_equal_p …)

(Sorry for the unsubstantive comment.)

Richard

>  	    {
>  	      off = op0;
>  	      offtype = TREE_TYPE (off);
> @@ -4036,7 +4055,8 @@ vect_check_gather_scatter (stmt_vec_info stmt_info, loop_vec_info loop_vinfo,
>        if (!vect_gather_scatter_fn_p (loop_vinfo, DR_IS_READ (dr), masked_p,
>  				     vectype, memory_type, offtype, scale,
>  				     &ifn, &offset_vectype))
> -	return false;
> +	ifn = IFN_LAST;
> +      decl = NULL_TREE;
>      }
>    else
>      {
> @@ -4050,10 +4070,6 @@ vect_check_gather_scatter (stmt_vec_info stmt_info, loop_vec_info loop_vinfo,
>  	  if (targetm.vectorize.builtin_scatter)
>  	    decl = targetm.vectorize.builtin_scatter (vectype, offtype, scale);
>  	}
> -
> -      if (!decl)
> -	return false;
> -
>        ifn = IFN_LAST;
>        /* The offset vector type will be read from DECL when needed.  */
>        offset_vectype = NULL_TREE;
> @@ -4283,9 +4299,7 @@ vect_analyze_data_refs (vec_info *vinfo, poly_uint64 *min_vf, bool *fatal)
>          {
>  	  bool maybe_gather
>  	    = DR_IS_READ (dr)
> -	      && !TREE_THIS_VOLATILE (DR_REF (dr))
> -	      && (targetm.vectorize.builtin_gather != NULL
> -		  || supports_vec_gather_load_p ());
> +	      && !TREE_THIS_VOLATILE (DR_REF (dr));
>  	  bool maybe_scatter
>  	    = DR_IS_WRITE (dr)
>  	      && !TREE_THIS_VOLATILE (DR_REF (dr))
> diff --git a/gcc/tree-vect-patterns.c b/gcc/tree-vect-patterns.c
> index 743fd3f5414..25de97bd9b0 100644
> --- a/gcc/tree-vect-patterns.c
> +++ b/gcc/tree-vect-patterns.c
> @@ -4811,7 +4811,7 @@ vect_recog_gather_scatter_pattern (vec_info *vinfo,
>       function for the gather/scatter operation.  */
>    gather_scatter_info gs_info;
>    if (!vect_check_gather_scatter (stmt_info, loop_vinfo, &gs_info)
> -      || gs_info.decl)
> +      || gs_info.ifn == IFN_LAST)
>      return NULL;
>  
>    /* Convert the mask to the right form.  */
> diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
> index 05085e1b110..9d51b476db6 100644
> --- a/gcc/tree-vect-stmts.c
> +++ b/gcc/tree-vect-stmts.c
> @@ -1084,6 +1084,7 @@ static void
>  vect_model_load_cost (vec_info *vinfo,
>  		      stmt_vec_info stmt_info, unsigned ncopies, poly_uint64 vf,
>  		      vect_memory_access_type memory_access_type,
> +		      gather_scatter_info *gs_info,
>  		      slp_tree slp_node,
>  		      stmt_vector_for_cost *cost_vec)
>  {
> @@ -1172,9 +1173,17 @@ vect_model_load_cost (vec_info *vinfo,
>    if (memory_access_type == VMAT_ELEMENTWISE
>        || memory_access_type == VMAT_GATHER_SCATTER)
>      {
> -      /* N scalar loads plus gathering them into a vector.  */
>        tree vectype = STMT_VINFO_VECTYPE (stmt_info);
>        unsigned int assumed_nunits = vect_nunits_for_cost (vectype);
> +      if (memory_access_type == VMAT_GATHER_SCATTER
> +	  && gs_info->ifn == IFN_LAST && !gs_info->decl)
> +	/* For emulated gathers N offset vector element extracts
> +	   (we assume the scalar scaling and ptr + offset add is consumed by
> +	   the load).  */
> +	inside_cost += record_stmt_cost (cost_vec, ncopies * assumed_nunits,
> +					 vec_to_scalar, stmt_info, 0,
> +					 vect_body);
> +      /* N scalar loads plus gathering them into a vector.  */
>        inside_cost += record_stmt_cost (cost_vec,
>  				       ncopies * assumed_nunits,
>  				       scalar_load, stmt_info, 0, vect_body);
> @@ -1184,7 +1193,9 @@ vect_model_load_cost (vec_info *vinfo,
>  			&inside_cost, &prologue_cost, 
>  			cost_vec, cost_vec, true);
>    if (memory_access_type == VMAT_ELEMENTWISE
> -      || memory_access_type == VMAT_STRIDED_SLP)
> +      || memory_access_type == VMAT_STRIDED_SLP
> +      || (memory_access_type == VMAT_GATHER_SCATTER
> +	  && gs_info->ifn == IFN_LAST && !gs_info->decl))
>      inside_cost += record_stmt_cost (cost_vec, ncopies, vec_construct,
>  				     stmt_info, 0, vect_body);
>  
> @@ -1866,7 +1877,8 @@ vect_truncate_gather_scatter_offset (stmt_vec_info stmt_info,
>        tree memory_type = TREE_TYPE (DR_REF (dr));
>        if (!vect_gather_scatter_fn_p (loop_vinfo, DR_IS_READ (dr), masked_p,
>  				     vectype, memory_type, offset_type, scale,
> -				     &gs_info->ifn, &gs_info->offset_vectype))
> +				     &gs_info->ifn, &gs_info->offset_vectype)
> +	  || gs_info->ifn == IFN_LAST)
>  	continue;
>  
>        gs_info->decl = NULL_TREE;
> @@ -1901,7 +1913,7 @@ vect_use_strided_gather_scatters_p (stmt_vec_info stmt_info,
>  				    gather_scatter_info *gs_info)
>  {
>    if (!vect_check_gather_scatter (stmt_info, loop_vinfo, gs_info)
> -      || gs_info->decl)
> +      || gs_info->ifn == IFN_LAST)
>      return vect_truncate_gather_scatter_offset (stmt_info, loop_vinfo,
>  						masked_p, gs_info);
>  
> @@ -2355,6 +2367,27 @@ get_load_store_type (vec_info  *vinfo, stmt_vec_info stmt_info,
>  			     vls_type == VLS_LOAD ? "gather" : "scatter");
>  	  return false;
>  	}
> +      else if (gs_info->ifn == IFN_LAST && !gs_info->decl)
> +	{
> +	  if (vls_type != VLS_LOAD)
> +	    {
> +	      if (dump_enabled_p ())
> +		dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> +				 "unsupported emulated scatter.\n");
> +	      return false;
> +	    }
> +	  else if (!TYPE_VECTOR_SUBPARTS (vectype).is_constant ()
> +		   || !known_eq (TYPE_VECTOR_SUBPARTS (vectype),
> +				 TYPE_VECTOR_SUBPARTS
> +				   (gs_info->offset_vectype)))
> +	    {
> +	      if (dump_enabled_p ())
> +		dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> +				 "unsupported vector types for emulated "
> +				 "gather.\n");
> +	      return false;
> +	    }
> +	}
>        /* Gather-scatter accesses perform only component accesses, alignment
>  	 is irrelevant for them.  */
>        *alignment_support_scheme = dr_unaligned_supported;
> @@ -8692,6 +8725,15 @@ vectorizable_load (vec_info *vinfo,
>  			     "unsupported access type for masked load.\n");
>  	  return false;
>  	}
> +      else if (memory_access_type == VMAT_GATHER_SCATTER
> +	       && gs_info.ifn == IFN_LAST
> +	       && !gs_info.decl)
> +	{
> +	  if (dump_enabled_p ())
> +	    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> +			     "unsupported masked emulated gather.\n");
> +	  return false;
> +	}
>      }
>  
>    if (!vec_stmt) /* transformation not required.  */
> @@ -8725,7 +8767,7 @@ vectorizable_load (vec_info *vinfo,
>  
>        STMT_VINFO_TYPE (orig_stmt_info) = load_vec_info_type;
>        vect_model_load_cost (vinfo, stmt_info, ncopies, vf, memory_access_type,
> -			    slp_node, cost_vec);
> +			    &gs_info, slp_node, cost_vec);
>        return true;
>      }
>  
> @@ -9438,7 +9480,8 @@ vectorizable_load (vec_info *vinfo,
>  		    unsigned int misalign;
>  		    unsigned HOST_WIDE_INT align;
>  
> -		    if (memory_access_type == VMAT_GATHER_SCATTER)
> +		    if (memory_access_type == VMAT_GATHER_SCATTER
> +			&& gs_info.ifn != IFN_LAST)
>  		      {
>  			tree zero = build_zero_cst (vectype);
>  			tree scale = size_int (gs_info.scale);
> @@ -9456,6 +9499,51 @@ vectorizable_load (vec_info *vinfo,
>  			data_ref = NULL_TREE;
>  			break;
>  		      }
> +		    else if (memory_access_type == VMAT_GATHER_SCATTER)
> +		      {
> +			/* Emulated gather-scatter.  */
> +			gcc_assert (!final_mask);
> +			unsigned HOST_WIDE_INT const_nunits
> +			  = nunits.to_constant ();
> +			vec<constructor_elt, va_gc> *ctor_elts;
> +			vec_alloc (ctor_elts, const_nunits);
> +			gimple_seq stmts = NULL;
> +			tree idx_type = TREE_TYPE (TREE_TYPE (vec_offset));
> +			tree scale = size_int (gs_info.scale);
> +			align
> +			  = get_object_alignment (DR_REF (first_dr_info->dr));
> +			tree ltype = build_aligned_type (TREE_TYPE (vectype),
> +							 align);
> +			for (unsigned k = 0; k < const_nunits; ++k)
> +			  {
> +			    tree boff = size_binop (MULT_EXPR,
> +						    TYPE_SIZE (idx_type),
> +						    bitsize_int (k));
> +			    tree idx = gimple_build (&stmts, BIT_FIELD_REF,
> +						     idx_type, vec_offset,
> +						     TYPE_SIZE (idx_type),
> +						     boff);
> +			    idx = gimple_convert (&stmts, sizetype, idx);
> +			    idx = gimple_build (&stmts, MULT_EXPR,
> +						sizetype, idx, scale);
> +			    tree ptr = gimple_build (&stmts, PLUS_EXPR,
> +						     TREE_TYPE (dataref_ptr),
> +						     dataref_ptr, idx);
> +			    ptr = gimple_convert (&stmts, ptr_type_node, ptr);
> +			    tree elt = make_ssa_name (TREE_TYPE (vectype));
> +			    tree ref = build2 (MEM_REF, ltype, ptr,
> +					       build_int_cst (ref_type, 0));
> +			    new_stmt = gimple_build_assign (elt, ref);
> +			    gimple_seq_add_stmt (&stmts, new_stmt);
> +			    CONSTRUCTOR_APPEND_ELT (ctor_elts, NULL_TREE, elt);
> +			  }
> +			gsi_insert_seq_before (gsi, stmts, GSI_SAME_STMT);
> +			new_stmt = gimple_build_assign (NULL_TREE,
> +							build_constructor
> +							  (vectype, ctor_elts));
> +			data_ref = NULL_TREE;
> +			break;
> +		      }
>  
>  		    align =
>  		      known_alignment (DR_TARGET_ALIGNMENT (first_dr_info));