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

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Date:   Mon, 6 Nov 2017 15:59:18 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        wangkefeng.wang@...wei.com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4] arm64: support __int128 on gcc 5+

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).

Will is managing the upcoming merging window for arm64 but is travelling
this week. I'll wait for couple of days, he'll probably catch up with
emails but my suggestion is that we revert the patch and push it back
again into -next after the 4.15 merging window.

-- 
Catalin