[{"id":3683441,"web_url":"http://patchwork.ozlabs.org/comment/3683441/","msgid":"","list_archive_url":null,"date":"2026-04-28T12:56:54","subject":"Re: [PATCH v3] BB SLP: Enabling reduction root finding for\n sum-of-diff\n kind of patterns","submitter":{"id":4338,"url":"http://patchwork.ozlabs.org/api/people/4338/","name":"Richard Biener","email":"rguenther@suse.de"},"content":"On Tue, 28 Apr 2026, Raghesh Aloor wrote:\n\n> Add an optional parameter allow_alt_code to vect_slp_linearize_chain\n> (default true). When false, do not follow into MINUS_EXPR when\n> building a PLUS reduction chain; treat MINUS results as leaves.\n> \n> This will allow \"sum of diffs\" (d_i = a[i]-b[i], sum = d0+...+dN)\n> kind of pattern to be recognized and vectorized. Pure PLUS chains\n> will still work; other callers of vect_slp_linearize_chain keep the\n> default. Once support for MINUS_EXPR in the chain is added, this\n> call site can be switched to allow_alt_code true.\n>\n> gcc/ChangeLog:\n> \n> \t* tree-vect-slp.cc (vect_slp_linearize_chain): Optional parameter\n> allow_alt_code added (default true), check added not to follow\n> MINUS_EXPR, when false\n> \t(vect_slp_check_for_roots): Calls vect_slp_linearize_chain with\n> parameter allow_alt_code set to false\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* gcc.dg/vect/bb-slp-sum-of-diffs.c: New test.\n> \n> ---\n> [Public]\n> \n> Hi Richard,\n> \n> I have addressed your comments. Added the target string to the test case, removed unwanted spaces and calls vect_slp_linearize_chain with 'false' set to the parameter vect_slp_linearize_chain. Refactoring the code is avoided for now as you suggested.\n> \n> The patch is tested using contrib/check_GNU_style.sh and this version is send via git send-email to avoid any indentation errors.\n> \n> Bootstrapped and tested on x86.\n\nOK.\n\nThanks,\nRichard.\n\n> \n> Thank You,\n> Raghesh\n> \n> .../gcc.dg/vect/bb-slp-sum-of-diffs.c | 25 +++++++++++++++++++\n> gcc/tree-vect-slp.cc | 12 ++++++---\n> 2 files changed, 33 insertions(+), 4 deletions(-)\n> create mode 100644 gcc/testsuite/gcc.dg/vect/bb-slp-sum-of-diffs.c\n> \n> diff --git a/gcc/testsuite/gcc.dg/vect/bb-slp-sum-of-diffs.c b/gcc/testsuite/gcc.dg/vect/bb-slp-sum-of-diffs.c\n> new file mode 100644\n> index 00000000000..6236e31289f\n> --- /dev/null\n> +++ b/gcc/testsuite/gcc.dg/vect/bb-slp-sum-of-diffs.c\n> @@ -0,0 +1,25 @@\n> +/* { dg-do compile } */\n> +/* { dg-require-effective-target vect_int } */\n> +/* { dg-additional-options \"-O3 -fdump-tree-slp2\" } */\n> +\n> +/* Test that SLP recognizes \"sum of diffs\" (d_i = a[i]-b[i], sum = d0+...+d7)\n> + as a reduction chain and vectorizes the basic block. */\n> +\n> +int __attribute__((noipa))\n> +sum_of_diffs (int *a, int *b)\n> +{\n> + int d0 = a[0] - b[0];\n> + int d1 = a[1] - b[1];\n> + int d2 = a[2] - b[2];\n> + int d3 = a[3] - b[3];\n> + int d4 = a[4] - b[4];\n> + int d5 = a[5] - b[5];\n> + int d6 = a[6] - b[6];\n> + int d7 = a[7] - b[7];\n> +\n> + int sum = d0 + d1 + d2 + d3 + d4 + d5 + d6 + d7;\n> + return sum;\n> +}\n> +\n> +/* Check that the reduction was SLP-vectorized (REDUC_PLUS appears in the tree). */\n> +/* { dg-final { scan-tree-dump \"\\.REDUC_PLUS\" \"slp2\" { target x86_64-*-* aarch64*-*-* } } } */\n> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc\n> index 185ff6fb542..82ab2b7e196 100644\n> --- a/gcc/tree-vect-slp.cc\n> +++ b/gcc/tree-vect-slp.cc\n> @@ -1746,7 +1746,8 @@ dt_sort_cmp (const void *op1_, const void *op2_, void *)\n> filling CHAIN with the result and using WORKLIST as intermediate storage.\n> CODE_STMT and ALT_CODE_STMT are filled with the first stmt using CODE\n> or MINUS_EXPR. *CHAIN_STMTS if not NULL is filled with all computation\n> - stmts, starting with START. */\n> + stmts, starting with START. When ALLOW_ALT_CODE is false, do not\n> + follow into MINUS_EXPR when building a PLUS chain (treat MINUS as leaf). */\n> \n> static void\n> vect_slp_linearize_chain (vec_info *vinfo,\n> @@ -1754,7 +1755,8 @@ vect_slp_linearize_chain (vec_info *vinfo,\n> \t\t\t vec &chain,\n> \t\t\t enum tree_code code, gimple *start,\n> \t\t\t gimple *&code_stmt, gimple *&alt_code_stmt,\n> -\t\t\t vec *chain_stmts)\n> +\t\t\t vec *chain_stmts,\n> +\t\t\t bool allow_alt_code = true)\n> {\n> /* For each lane linearize the addition/subtraction (or other\n> uniform associatable operation) expression tree. */\n> @@ -1790,7 +1792,8 @@ vect_slp_linearize_chain (vec_info *vinfo,\n> \t && single_imm_use (op, &use_p, &use_stmt)\n> \t && is_gimple_assign (def_stmt_info->stmt)\n> \t && (gimple_assign_rhs_code (def_stmt_info->stmt) == code\n> -\t\t || (code == PLUS_EXPR\n> +\t\t || (allow_alt_code\n> +\t\t && code == PLUS_EXPR\n> \t\t && (gimple_assign_rhs_code (def_stmt_info->stmt)\n> \t\t\t == MINUS_EXPR))))\n> \t {\n> @@ -10049,7 +10052,8 @@ vect_slp_check_for_roots (bb_vec_info bb_vinfo)\n> \t continue;\n> \t vect_slp_linearize_chain (bb_vinfo, worklist, chain, code, assign,\n> \t\t\t\t /* ??? */\n> -\t\t\t\t code_stmt, alt_code_stmt, &chain_stmts);\n> +\t\t\t\t code_stmt, alt_code_stmt, &chain_stmts,\n> +\t\t\t\t false);\n> \t if (chain.length () > 1)\n> \t {\n> \t /* Sort the chain according to def_type and operation. */\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=suse.de header.i=@suse.de header.a=rsa-sha256\n header.s=susede2_rsa header.b=s26alLu2;\n\tdkim=pass header.d=suse.de header.i=@suse.de header.a=ed25519-sha256\n header.s=susede2_ed25519 header.b=HEfAxjxM;\n\tdkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de\n header.a=rsa-sha256 header.s=susede2_rsa header.b=URFWrwqP;\n\tdkim=neutral header.d=suse.de header.i=@suse.de header.a=ed25519-sha256\n header.s=susede2_ed25519 header.b=CF+WBWv4;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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