linux-kernel - Re: [PATCH v4] arm64: support _

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Message-ID: <20171106161459.bm4b2c7ahljluuda@armageddon.cambridge.arm.com>
Date:   Mon, 6 Nov 2017 16:14:59 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Mark Rutland <mark.rutland@....com>, wangkefeng.wang@...wei.com,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4] arm64: support __int128 on gcc 5+

On Mon, Nov 06, 2017 at 03:59:18PM +0000, Catalin Marinas wrote:
> On Mon, Nov 06, 2017 at 10:31:51AM +0100, Jason A. Donenfeld wrote:
> > Versions of gcc prior to gcc 5 emitted a __multi3 function call when
> > dealing with TI types, resulting in failures when trying to link to
> > libgcc, and more generally, bad performance. However, since gcc 5,
> > the compiler supports actually emitting fast instructions, which means
> > we can at long last enable this option and receive the speedups.
> > 
> > The gcc commit that added proper Aarch64 support is:
> > https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=d1ae7bb994f49316f6f63e6173f2931e837a351d
> > This commit appears to be part of the gcc 5 release.
> > 
> > There are still a few instructions, __ashlti3 and __ashrti3, which
> > require libgcc, which is fine. Rather than linking to libgcc, we
> > simply provide them ourselves, since they're not that complicated.
> > 
> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> 
> According to Arnd, linux-next (with this patch included) fails to build
> with gcc-7 (config: https://pastebin.com/raw/sgvPe96e).

Actually, it fails with gcc-6 as well (gcc (Debian 6.3.0-18) 6.3.0
20170516) with this error:

kernel/sched/fair.o: In function `__calc_delta':
fair.c:(.text+0x84c): undefined reference to `__lshrti3'
kernel/time/timekeeping.o: In function `timekeeping_resume':
timekeeping.c:(.text+0x2cac): undefined reference to `__lshrti3'

So I'm for reverting the commit and we should allow more randconfig
tests for the 4.16 merging window.

-- 
Catalin