[PATCH] arm64: Add support ARCH_SUPPORTS_INT128

[Ard Biesheuvel]

On 22 June 2016 at 12:57, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Tue, Jun 21, 2016 at 02:55:23PM +0800, Kefeng Wang wrote:
>> The gcc support __SIZEOF_INT128__ and __int128 in arm64, thus,
>> enable ARCH_SUPPORTS_INT128 to make mul_u64_u32_shr() a bit
>> more efficient in scheduler.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang at huawei.com>
>> ---
>>  arch/arm64/Kconfig | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 5a0a691..ab319eb 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -11,6 +11,7 @@ config ARM64
>>       select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
>>       select ARCH_USE_CMPXCHG_LOCKREF
>>       select ARCH_SUPPORTS_ATOMIC_RMW
>> +     select ARCH_SUPPORTS_INT128
>>       select ARCH_SUPPORTS_NUMA_BALANCING
>>       select ARCH_WANT_OPTIONAL_GPIOLIB
>>       select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
>
> With this patch and UBSAN+KASAN enabled, Linux fails to link with:
>
> lib/built-in.o: In function `get_signed_val':
> lib/ubsan.c:93: undefined reference to `__ashlti3'
> lib/ubsan.c:93: undefined reference to `__ashrti3'
>
> It succeeds if I revert commit d67703a8a69e ("arm64: kill off the libgcc
> dependency"). We may have to revisit that decision or implement the
> required library functions in the kernel. In general, I'd like to reduce
> the amount of code duplication if the functionality can be found
> elsewhere like in libgcc. There is a risk of not knowing precisely what
> ligbcc relies on (like libc functions), though there are other
> architectures linking against it.
>

I wonder if this change is such a huge win if we simply end up
invoking support routines in libgcc to implement the 128-bit
arithmetic. Do we have any benchmark results?