[{"id":3685356,"web_url":"http://patchwork.ozlabs.org/comment/3685356/","msgid":"","list_archive_url":null,"date":"2026-05-03T06:49:38","subject":"Re: [PATCH v2 3/3] i386: Add peephole2 to convert highpart mul to\n mulx","submitter":{"id":4387,"url":"http://patchwork.ozlabs.org/api/people/4387/","name":"H.J. Lu","email":"hjl.tools@gmail.com"},"content":"On Sun, May 3, 2026 at 1:36 PM wrote:\n>\n> From: MITSUNARI Shigeo \n>\n> When the register allocator selects the MUL-based highpart pattern\n> (umuldi3_highpart) with the source value already in %rdx, it inserts\n> a redundant mov to %rax before the mul instruction. Add a peephole2\n> that detects this mov + mul sequence and converts it to a single mulx,\n> eliminating the extra mov.\n>\n> This improves inlined loops that perform multiple unsigned divisions\n> by constants. For example, a loop with three div-by-constant\n> operations now generates 15 instructions (matching LLVM) instead\n> of 18.\n>\n> Before (loop body excerpt):\n> mov rax, rdx\n> mul r9\n>\n> After:\n> mulx rdx, rax, r9\n\nPlease add a test with check-function-bodies, like\n\n[hjl@gnu-tgl-3 testsuite]$ cat gcc.target/i386/pr14907-3.c\n/* { dg-do compile } */\n/* { dg-options \"-O2 -g0\" } */\n/* Keep labels and directives ('.cfi_startproc', '.cfi_endproc'). */\n/* { dg-final { check-function-bodies \"x86*\" \"\" \"\" { target *-*-linux*\n*-*-gnu* } {^\\t?\\.} } } */\n\n/*\nx86*c1:\nx86*.LFB0:\nx86* .cfi_startproc\nx86* jmp c2\nx86* .cfi_endproc\nx86*...\n*/\n\nextern char c2 (char);\n\nchar\nc1 (char c)\n{\n return c2 (c);\n}\n[hjl@gnu-tgl-3 testsuite]$\n\n> gcc/ChangeLog:\n>\n> * config/i386/i386.md: Add peephole2 to convert\n> mov + umul_highpart to mulx on BMI2 targets.\n> ---\n> gcc/config/i386/i386.md | 17 +++++++++++++++++\n> 1 file changed, 17 insertions(+)\n>\n> diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md\n> index 472f9d41332..1c394690b04 100644\n> --- a/gcc/config/i386/i386.md\n> +++ b/gcc/config/i386/i386.md\n> @@ -11522,6 +11522,23 @@\n> (set_attr \"prefix\" \"vex\")\n> (set_attr \"mode\" \"\")])\n>\n> +;; Convert mov + highpart mul to mulx when the mov source is %rdx.\n> +;; mov %rdx, %rax; mulq %src -> mulx %src, %rax, %out\n> +(define_peephole2\n> + [(set (match_operand:DWIH 0 \"register_operand\")\n> + (match_operand:DWIH 1 \"register_operand\"))\n> + (parallel [(set (match_operand:DWIH 2 \"register_operand\")\n> + (umul_highpart:DWIH (match_dup 0)\n> + (match_operand:DWIH 3 \"nonimmediate_operand\")))\n> + (clobber (match_dup 0))\n> + (clobber (reg:CC FLAGS_REG))])]\n> + \"TARGET_BMI2\n> + && REGNO (operands[1]) == DX_REG\n> + && REGNO (operands[0]) != REGNO (operands[2])\"\n> + [(parallel [(set (match_dup 2)\n> + (umul_highpart:DWIH (match_dup 1) (match_dup 3)))\n> + (clobber (match_dup 0))])])\n> +\n> ;; Highpart multiplication patterns\n> (define_insn \"mul3_highpart\"\n> [(set (match_operand:DWIH 0 \"register_operand\" \"=d\")\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=mEgWAgvR;\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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