linux-kernel - Re: [PATCH v2] arm64: support _

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Message-ID: <20171103134223.GD13499@arm.com>
Date:   Fri, 3 Nov 2017 13:42:24 +0000
From:   Will Deacon <will.deacon@....com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, wangkefeng.wang@...wei.com,
        ard.biesheuvel@...aro.org
Subject: Re: [PATCH v2] arm64: support __int128 on gcc 5+

Hi Jason,

[+Ard]

On Thu, Nov 02, 2017 at 06:43:22PM +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, such as __ashlti3 and __ashrti3,
> that require libgcc, which is fine. So, we also link to libgcc for these
> functions when needed, which is what several other architectures already
> have been doing for a long time.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  arch/arm64/Makefile | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

We used to link against libgcc way back when, but that dependency was
removed in commit d67703a8a69e ("arm64: kill off the libgcc dependency")
and I'm really not keen to add it back. I also think that there might
be licensing concerns if you link against it and make use of GCC plugins,
but IANAL.

Shouldn't we just provide our own implementations of __ashlti3 and
__ashrti3 instead?

Cheers,

Will

> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 939b310913cf..70c7c0c1bccb 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -53,6 +53,8 @@ KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst)
>  KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
>  KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
>  
> +KBUILD_CFLAGS	+= $(call cc-ifversion, -ge, 0500, -DCONFIG_ARCH_SUPPORTS_INT128)
> +
>  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
>  KBUILD_CPPFLAGS	+= -mbig-endian
>  CHECKFLAGS	+= -D__AARCH64EB__
> @@ -108,7 +110,7 @@ core-$(CONFIG_NET) += arch/arm64/net/
>  core-$(CONFIG_KVM) += arch/arm64/kvm/
>  core-$(CONFIG_XEN) += arch/arm64/xen/
>  core-$(CONFIG_CRYPTO) += arch/arm64/crypto/
> -libs-y		:= arch/arm64/lib/ $(libs-y)
> +libs-y		:= arch/arm64/lib/ $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name) $(libs-y)
>  core-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>  
>  # Default target when executing plain make
> -- 
> 2.14.2
>