Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc

From: Matthias Kaehlcke <mka@chromium.org>
To: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Nick Desaulniers <ndesaulniers@google.com>,
	Manoj Gupta <manojgupta@chromium.org>,
	Tiancong Wang <tcwang@chromium.org>,
	Stephen Hines <srhines@google.com>,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc
Date: Mon, 18 Mar 2019 14:31:13 -0700	[thread overview]
Message-ID: <20190318213113.GI112750@google.com> (raw)
In-Reply-To: <CAG5bF+S6OvBnsaR6UpMCqjDR9_hMo6qRMHGiW+iCaRYQW4C3YA@mail.gmail.com>

On Fri, Mar 15, 2019 at 01:54:50PM -0700, Matthias Kaehlcke wrote:
> The compiler may emit calls to __lshrti3 from the compiler runtime
> library, which results in undefined references:
> 
> arch/x86/kvm/x86.o: In function `mul_u64_u64_shr':
>   include/linux/math64.h:186: undefined reference to `__lshrti3'
> 
> Add a copy of the __lshrti3 libgcc routine (from gcc v4.9.2).
> 
> Include the function for x86 builds with clang, which is the
> environment where the above error was observed.
> 
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>

With "Revert "kbuild: use -Oz instead of -Os when using clang"
(https://lore.kernel.org/patchwork/patch/1051932/) the above
error is fixed, a few comments inline for if the patch is
resurrected in the future because __lshrti3 is emitted in a
different context.

> diff --git a/include/linux/libgcc.h b/include/linux/libgcc.h
> index 32e1e0f4b2d0..a71036471838 100644
> --- a/include/linux/libgcc.h
> +++ b/include/linux/libgcc.h
> @@ -22,15 +22,26 @@
>  #include <asm/byteorder.h>
> 
>  typedef int word_type __attribute__ ((mode (__word__)));
> +typedef int TItype __attribute__ ((mode (TI)));

Consider using __int128 instead. Definition and use need a
'defined(__SIZEOF_INT128__)' guard  (similar for mode (TI)), since
these 128 bit types aren't supported on all platforms.

>  #ifdef __BIG_ENDIAN
>  struct DWstruct {
>  	int high, low;
>  };
> +
> +struct DWstruct128 {
> +	long long high, low;
> +};

This struct isn't needed, struct DWstruct can be used.

> diff --git a/lib/lshrti3.c b/lib/lshrti3.c
> new file mode 100644
> index 000000000000..2d2123bb3030
> --- /dev/null
> +++ b/lib/lshrti3.c
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/export.h>
> +#include <linux/libgcc.h>
> +
> +long long __lshrti3(long long u, word_type b)

use TItype for input/output, which is what gcc does, though the above
matches the interface in the documentation.

> +{
> +	DWunion128 uu, w;
> +	word_type bm;
> +
> +	if (b == 0)
> +		return u;
> +
> +	uu.ll = u;
> +	bm = 64 - b;
> +
> +	if (bm <= 0) {
> +		w.s.high = 0;
> +		w.s.low = (unsigned long long) uu.s.high >> -bm;

include <linux/types.h> and use u64 instead of unsigned long long.