Re: [RFC][Scalar masks 1/x] Introduce GEN_MASK_EXPR.

[Jeff Law <law at redhat dot com>]

On 08/17/2015 10:22 AM, Ilya Enkovich wrote:
> Hi,
>
> This patch starts a series introducing scalar masks support in the vectorizer.  It was discussed on the recent Cauldron and changes overiew is available here: https://gcc.gnu.org/wiki/cauldron2015?action=AttachFile&do=view&target=Vectorization+for+Intel+AVX-512.pdf.  Here is shortly a list of changes introduced by this series:
>
>   - Add new tree expr to produce scalar masks in a vectorized code
>   - Fix-up if-conversion to use bool predicates instead of integer masks
>   - Disable some bool patterns to avoid bool to int conversion where masks can be used
>   - Support bool operands in vectorization factor computation
>   - Support scalar masks in MASK_LOAD, MASK_STORE and VEC_COND_EXPR by adding new optabs
>   - Support vectorization for statements which are now not transformed by bool patterns
>   - Add target support (hooks, optabs, expands)
>
> This patch introduces GEN_MASK_EXPR code.  Intitially I wanted to use a comparison as an operand for it directly mapping it into AVX-512 comparison instruction.  But a feedback was to simplify new code's semantics and use it for converting vectors into scalar masks.  Therefore if we want to compare two vectors into a scalar masks we use two statements:
>
>    vect.18_87 = vect__5.13_81 > vect__6.16_86;
>    mask__ifc__23.17_88 = GEN_MASK <vect.18_87>;
>
> Trying it in practice I found it producing worse code. The problem is that on target first comparison is expanded into two instructions: cmp with mask result + masked move to get a vector. GEN_MASK is then expanded into another comparison with zero vector.  Thus I get two comparisons + move instead of a single comparison and have to optimize this out on a target side (current optimizers can't handle it).  That's actually what I wanted to avoid.  For now I changed GEN_MASK_EXPR to get a vector value as an operand but didn't change expand pattern which has four opernads: two vectors to compare + cmp operator + result.  On expand I try to detect GEN_MASK uses a result of comparison and thus avoid double comparison generation.
>
> Patch series is not actually fully finished yet.  I still have several type conversion tests not being vectorized and it wasn't widely tested.  That's what I'm working on now.
>
> Will be glad to any comments.
>
> Thanks,
> Ilya
> --
> 2015-08-17  Ilya Enkovich  <enkovich.gnu@gmail.com>
>
> 	* expr.c (expand_expr_real_2): Support GEN_MASK_EXPR.
> 	* gimple-pretty-print.c (dump_unary_rhs): Likewise.
> 	* gimple.c (get_gimple_rhs_num_ops): Likewise.
> 	* optabs.c: Include gimple.h.
> 	(vector_compare_rtx): Add OPNO arg.
> 	(get_gen_mask_icode): New.
> 	(expand_gen_mask_expr_p): New.
> 	(expand_gen_mask_expr): New.
> 	(expand_vec_cond_expr): Adjust vector_compare_rtx call.
> 	* optabs.def (gen_mask_optab): New.
> 	(gen_masku_optab): New.
> 	* optabs.h (expand_gen_mask_expr_p): New.
> 	(expand_gen_mask_expr): New.
> 	* tree-cfg.c (verify_gimple_assign_unary): Support GEN_MASK_EXPR.
> 	* tree-inline.c (estimate_operator_cost): Likewise.
> 	* tree-pretty-print.c (dump_generic_node): Likewise.
> 	* tree-ssa-operands.c (get_expr_operands): Likewise.
> 	* tree.def (GEN_MASK_EXPR): New.
A general question, would any of this likely help Yuri's work to 
optimize MASK_STORES?

> diff --git a/gcc/optabs.c b/gcc/optabs.c
> index a6ca706..bf466ca 100644
> --- a/gcc/optabs.c
> +++ b/gcc/optabs.c
> @@ -51,6 +51,7 @@ along with GCC; see the file COPYING3.  If not see
>   #include "recog.h"
>   #include "reload.h"
>   #include "target.h"
> +#include "gimple.h"
Hmm, part of me doesn't want to see optabs.c depending on gimple.h.

How painful would it be to have this stuff live in expr.c?

> +
> +/* Generate insns for a GEN_MASK_EXPR, given its TYPE and operand.  */
> +
> +rtx
> +expand_gen_mask_expr (tree type, tree op0, rtx target)
> +{
> +  struct expand_operand ops[4];
> +  enum insn_code icode;
> +  rtx comparison;
> +  machine_mode mode = TYPE_MODE (type);
> +  machine_mode cmp_op_mode;
> +  bool unsignedp;
> +  tree op0a, op0b;
> +  enum tree_code tcode;
> +  gimple def_stmt;
> +
> +  /* Avoid double comparison.  */
> +  if (TREE_CODE (op0) == SSA_NAME
> +      && (def_stmt = SSA_NAME_DEF_STMT (op0))
> +      && is_gimple_assign (def_stmt)
> +      && TREE_CODE_CLASS (gimple_assign_rhs_code (def_stmt)) == tcc_comparison)
> +    {
> +      op0a = gimple_assign_rhs1 (def_stmt);
> +      op0b = gimple_assign_rhs2 (def_stmt);
> +      tcode = gimple_assign_rhs_code (def_stmt);
> +    }
> +  else
> +    {
> +      op0a = op0;
> +      op0b = build_zero_cst (TREE_TYPE (op0));
> +      tcode = NE_EXPR;
> +    }
> +
> +  unsignedp = TYPE_UNSIGNED (TREE_TYPE (op0a));
> +  cmp_op_mode = TYPE_MODE (TREE_TYPE (op0a));
> +
> +  gcc_assert (GET_MODE_BITSIZE (mode) >= GET_MODE_NUNITS (cmp_op_mode));
> +
> +  icode = get_gen_mask_icode (cmp_op_mode, unsignedp);
> +  if (icode == CODE_FOR_nothing)
> +    return 0;
So if the target doesn't have suitable insns, what happens?  I suspect 
the answer is nothing useful.  In which case the question becomes what 
prevents the optimizers from generating a GEN_MASK_EXPR?  Maybe that'll 
become clear as I read the rest of the patches.



> diff --git a/gcc/tree-inline.c b/gcc/tree-inline.c
> index e1ceea4..052c055 100644
> --- a/gcc/tree-inline.c
> +++ b/gcc/tree-inline.c
> @@ -3850,6 +3850,7 @@ estimate_operator_cost (enum tree_code code, eni_weights *weights,
>       case COMPLEX_EXPR:
>       case PAREN_EXPR:
>       case VIEW_CONVERT_EXPR:
> +    case GEN_MASK_EXPR:
>         return 0;
That seems wrong to me :-)

Jeff