[{"id":3685503,"web_url":"http://patchwork.ozlabs.org/comment/3685503/","msgid":"","list_archive_url":null,"date":"2026-05-04T05:45:32","subject":"Re: [PATCH v3 2/3] i386: Add BMI2 MULX pattern for highpart-only\n multiplication","submitter":{"id":4387,"url":"http://patchwork.ozlabs.org/api/people/4387/","name":"H.J. Lu","email":"hjl.tools@gmail.com"},"content":"On Mon, May 4, 2026 at 7:58 AM wrote:\n>\n> From: MITSUNARI Shigeo \n>\n> Add a new instruction pattern that uses MULX to compute only the high\n> part of an unsigned multiplication on BMI2 targets. Previously, when\n> only the high part was needed, GCC would emit MULQ followed by a MOV\n> to retrieve the result from RDX. With this pattern, MULX writes the\n> high part directly to the destination register, saving one instruction.\n>\n> This benefits unsigned 32-bit integer division by constants that require\n> 33-bit magic multipliers (e.g., division by 7), reducing the sequence\n> from 4 instructions to 3 on BMI2 targets.\n>\n> Before:\n> movabsq $2635249153617166336, %rcx\n> movl %edi, %eax\n> mulq %rcx\n> movl %edx, %eax\n>\n> After:\n> movabsq $2635249153617166336, %rax\n> movl %edi, %edx\n> mulx %rax, %rdx, %rax\n>\n> gcc/ChangeLog:\n>\n> * config/i386/i386.md (*bmi2_umul3_highpart): New pattern.\n\nNeed the ChangeLog entry for the test.\n\n> ---\n> gcc/config/i386/i386.md | 15 +++++++++++++++\n> .../gcc.target/i386/bmi2-mulx-highpart-1.c | 18 ++++++++++++++++++\n> 2 files changed, 33 insertions(+)\n> create mode 100644 gcc/testsuite/gcc.target/i386/bmi2-mulx-highpart-1.c\n>\n> diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md\n> index e514809453d..472f9d41332 100644\n> --- a/gcc/config/i386/i386.md\n> +++ b/gcc/config/i386/i386.md\n> @@ -11507,6 +11507,21 @@\n> (set (match_dup 5)\n> (umul_highpart:DWIH (match_dup 2) (match_dup 3)))])])\n>\n> +;; BMI2 MULX highpart-only pattern. Uses MULX to get only the high part,\n> +;; discarding the low part into a scratch register. This avoids the\n> +;; mov from rdx after mulq when only the high part is needed.\n> +(define_insn \"*bmi2_umul3_highpart\"\n> + [(set (match_operand:DWIH 0 \"register_operand\" \"=r\")\n> + (umul_highpart:DWIH\n> + (match_operand:DWIH 1 \"register_operand\" \"d\")\n> + (match_operand:DWIH 2 \"nonimmediate_operand\" \"rm\")))\n> + (clobber (match_scratch:DWIH 3 \"=r\"))]\n> + \"TARGET_BMI2\"\n> + \"mulx\\t{%2, %3, %0|%0, %3, %2}\"\n> + [(set_attr \"type\" \"imulx\")\n> + (set_attr \"prefix\" \"vex\")\n> + (set_attr \"mode\" \"\")])\n> +\n> ;; Highpart multiplication patterns\n> (define_insn \"mul3_highpart\"\n> [(set (match_operand:DWIH 0 \"register_operand\" \"=d\")\n> diff --git a/gcc/testsuite/gcc.target/i386/bmi2-mulx-highpart-1.c b/gcc/testsuite/gcc.target/i386/bmi2-mulx-highpart-1.c\n> new file mode 100644\n> index 00000000000..dd7b32aa0b0\n> --- /dev/null\n> +++ b/gcc/testsuite/gcc.target/i386/bmi2-mulx-highpart-1.c\n> @@ -0,0 +1,18 @@\n> +/* { dg-do compile { target { ! ia32 } } } */\n> +/* { dg-options \"-O2 -mbmi2\" } */\n> +/* { dg-final { check-function-bodies \"**\" \"\" \"\" { target *-*-linux* *-*-gnu* } } } */\n> +\n> +/*\n> +**div7:\n> +** movabsq \\$2635249153617166336, %rax\n> +** movl %edi, %edx\n> +** mulx %rax, %rdx, %rax\n> +** ret\n> +**...\n> +*/\n> +\n> +unsigned int\n> +div7 (unsigned int x)\n> +{\n> + return x / 7;\n> +}\n> --\n> 2.43.0\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 (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=f7kFi910;\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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