[PATCH] Add emulated gather capability to the vectorizer

Kewen.Lin linkw@linux.ibm.com
Mon Aug 2 06:07:49 GMT 2021


on 2021/7/30 下午10:04, Kewen.Lin via Gcc-patches wrote:
> Hi Richi,
> 
> on 2021/7/30 下午7:34, Richard Biener wrote:
>> 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.
>>
> 
> Wow, it sounds promising!
> 
>> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>>
>> Any comments?  I still plan to run this over full SPEC and
>> I have to apply TLC to the followup patch before I can post it.
>>
>> I think neither power nor z has gather so I'm curious if the
>> patch helps 510.parest_r there, I'm unsure about neon/advsimd.
> 
> Yes, Power (latest Power10) doesn't support gather load.
> I just measured 510.parest_r with this patch on Power9 at option
> -Ofast -mcpu=power9 {,-funroll-loops}, both are neutral.
> 
> It fails to vectorize the loop in vect-gather-1.c:
> 
> vect-gather.c:12:28: missed:  failed: evolution of base is not affine.
> vect-gather.c:11:46: missed:   not vectorized: data ref analysis failed _6 = *_5;
> vect-gather.c:12:28: missed:   not vectorized: data ref analysis failed: _6 = *_5;
> vect-gather.c:11:46: missed:  bad data references.
> vect-gather.c:11:46: missed: couldn't vectorize loop
> 

By further investigation, it's due to that rs6000 fails to make
maybe_gather true in:

	  bool maybe_gather
	    = DR_IS_READ (dr)
	      && !TREE_THIS_VOLATILE (DR_REF (dr))
	      && (targetm.vectorize.builtin_gather != NULL
		  || supports_vec_gather_load_p ());

With the hacking defining TARGET_VECTORIZE_BUILTIN_GATHER (as
well as TARGET_VECTORIZE_BUILTIN_SCATTER) for rs6000, the
case gets vectorized as expected.

But re-evaluated 510.parest_r with this extra hacking, the
runtime performance doesn't have any changes.

BR,
Kewen

>> Both might need the followup patch - I was surprised about
>> the speedup without it on Zen (the followup improves runtime
>> to 198s there).
>>
>> Thanks,
>> Richard.
>>
>> 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.
>> 	* 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                     |  29 +++--
>>  gcc/tree-vect-patterns.c                      |   2 +-
>>  gcc/tree-vect-stmts.c                         | 100 ++++++++++++++++--
>>  5 files changed, 136 insertions(+), 15 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..0279e75fa8e 100644
>> --- a/gcc/tree-vect-data-refs.c
>> +++ b/gcc/tree-vect-data-refs.c
>> @@ -4007,8 +4007,26 @@ 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))))
>>  	    {
>>  	      off = op0;
>>  	      offtype = TREE_TYPE (off);
>> @@ -4036,7 +4054,8 @@ 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 +4069,6 @@ 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;
>> 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));
>>
> 


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