Re: [PATCH v2] arm64: support __int128 on gcc 5+

[Ard Biesheuvel]

> On 3 Nov 2017, at 13:42, Will Deacon <will.deacon@xxxxxxx> wrote:
> 
> 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@xxxxxxxxx>
>> ---
>> 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.
> 

Is it guaranteed that libgcc doesnât use neon/vfp? We canât use it in the kernel otherwise.


> 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
>>