Re: git: cc4f247fb946 - main - sys: Add [u]divmoddi4 intrinsics on i386.

On 1 Jul 2023, at 20:23, John Baldwin <jhb@FreeBSD.org> wrote:
> 
> The branch main has been updated by jhb:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=cc4f247fb9468bbece5286557ab2d6810704aaf3
> 
> commit cc4f247fb9468bbece5286557ab2d6810704aaf3
> Author:     John Baldwin <jhb@FreeBSD.org>
> AuthorDate: 2023-07-01 19:22:44 +0000
> Commit:     John Baldwin <jhb@FreeBSD.org>
> CommitDate: 2023-07-01 19:22:44 +0000
> 
>    sys: Add [u]divmoddi4 intrinsics on i386.
> 
>    GCC 12 uses these in several places when building the i386 kernel.
>    They are very similar to [u]divdiv3 except that they return both
>    the quotient and the remainder.
> 
>    Reviewed by:    imp
>    Differential Revision:  https://reviews.freebsd.org/D40817
> ---
> sys/conf/files.i386      |  2 ++
> sys/libkern/divmoddi4.c  | 60 ++++++++++++++++++++++++++++++++++++++++++++++++
> sys/libkern/quad.h       |  2 ++
> sys/libkern/udivmoddi4.c | 46 +++++++++++++++++++++++++++++++++++++
> 4 files changed, 110 insertions(+)
> 
> diff --git a/sys/conf/files.i386 b/sys/conf/files.i386
> index f0aa4007d8d3..cd1aeb4850b7 100644
> --- a/sys/conf/files.i386
> +++ b/sys/conf/files.i386
> @@ -140,6 +140,7 @@ isa/vga_isa.c optional vga
> kern/imgact_aout.c optional compat_aout
> kern/subr_sfbuf.c standard
> libkern/divdi3.c standard
> +libkern/divmoddi4.c standard
> libkern/ffsll.c standard
> libkern/flsll.c standard
> libkern/memcmp.c standard
> @@ -149,6 +150,7 @@ libkern/qdivrem.c standard
> libkern/strlen.c standard
> libkern/ucmpdi2.c standard
> libkern/udivdi3.c standard
> +libkern/udivmoddi4.c standard
> libkern/umoddi3.c standard
> #
> # x86 real mode BIOS support, required by dpms/pci/vesa
> diff --git a/sys/libkern/divmoddi4.c b/sys/libkern/divmoddi4.c
> new file mode 100644
> index 000000000000..e5bd81a90ad7
> --- /dev/null
> +++ b/sys/libkern/divmoddi4.c
> @@ -0,0 +1,60 @@
> +/*-
> + * SPDX-License-Identifier: BSD-3-Clause
> + *
> + * Copyright (c) 1992, 1993
> + * The Regents of the University of California.  All rights reserved.
> + *
> + * This software was developed by the Computer Systems Engineering group
> + * at Lawrence Berkeley Laboratory under DARPA contract BG 91-66 and
> + * contributed to Berkeley.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + * 3. Neither the name of the University nor the names of its contributors
> + *    may be used to endorse or promote products derived from this software
> + *    without specific prior written permission.
> + *
> + * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
> + * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> + * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
> + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> + * SUCH DAMAGE.
> + */
> +
> +#include <libkern/quad.h>
> +
> +/*
> + * Divide two signed quads.
> + * ??? if -1/2 should produce -1 on this machine, this code is wrong
> + */
> +quad_t
> +__divmoddi4(quad_t a, quad_t b, quad_t *rem)
> +{
> + u_quad_t ua, ub, uq, urem;
> + int neg;
> +
> + if (a < 0)
> + ua = -(u_quad_t)a, neg = 1;
> + else
> + ua = a, neg = 0;
> + if (b < 0)
> + ub = -(u_quad_t)b, neg ^= 1;

I believe you need separate negq and negr so that only negq is flipped
here. Consider the four cases:

+3 / +2: q = +1, r = 1
+3 / -2: q = -1, r = 1
-3 / +2: q = -1, r = -1
-3 / -2: q = +1, r = -1

At least, that’s what I wrote when I added __[u]divmoddi4 back in 2019
for Debian GNU/kFreeBSD, my reading of the C spec agrees with that and
so do my tests (putting the above four into godbolt.org).

Jess

> + else
> + ub = b;
> + uq = __qdivrem(ua, ub, &urem);
> + if (rem != 0)
> + *rem = urem;
> + return (neg ? -uq : uq);
> +}
> diff --git a/sys/libkern/quad.h b/sys/libkern/quad.h
> index 452f331661b3..e6be1c7bde15 100644
> --- a/sys/libkern/quad.h
> +++ b/sys/libkern/quad.h
> @@ -105,10 +105,12 @@ quad_t __ashldi3(quad_t, qshift_t);
> quad_t __ashrdi3(quad_t, qshift_t);
> int __cmpdi2(quad_t a, quad_t b);
> quad_t __divdi3(quad_t a, quad_t b);
> +quad_t __divmoddi4(quad_t a, quad_t b, quad_t *rem);
> quad_t __lshrdi3(quad_t, qshift_t);
> quad_t __moddi3(quad_t a, quad_t b);
> u_quad_t __qdivrem(u_quad_t u, u_quad_t v, u_quad_t *rem);
> u_quad_t __udivdi3(u_quad_t a, u_quad_t b);
> +u_quad_t __udivmoddi4(u_quad_t a, u_quad_t b, u_quad_t *rem);
> u_quad_t __umoddi3(u_quad_t a, u_quad_t b);
> int __ucmpdi2(u_quad_t a, u_quad_t b);
> 
> diff --git a/sys/libkern/udivmoddi4.c b/sys/libkern/udivmoddi4.c
> new file mode 100644
> index 000000000000..bb9b06917e2e
> --- /dev/null
> +++ b/sys/libkern/udivmoddi4.c
> @@ -0,0 +1,46 @@
> +/*-
> + * SPDX-License-Identifier: BSD-3-Clause
> + *
> + * Copyright (c) 1992, 1993
> + * The Regents of the University of California.  All rights reserved.
> + *
> + * This software was developed by the Computer Systems Engineering group
> + * at Lawrence Berkeley Laboratory under DARPA contract BG 91-66 and
> + * contributed to Berkeley.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + * 3. Neither the name of the University nor the names of its contributors
> + *    may be used to endorse or promote products derived from this software
> + *    without specific prior written permission.
> + *
> + * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
> + * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> + * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
> + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> + * SUCH DAMAGE.
> + */
> +
> +#include <libkern/quad.h>
> +
> +/*
> + * Divide two unsigned quads.
> + */
> +u_quad_t
> +__udivmoddi4(u_quad_t a, u_quad_t b, u_quad_t *rem)
> +{
> +
> + return (__qdivrem(a, b, rem));
> +}