[PATCH] arm64: Add support ARCH_SUPPORTS_INT128

[Catalin Marinas]

On Wed, Jun 22, 2016 at 01:43:23PM +0100, Will Deacon wrote:
> On Wed, Jun 22, 2016 at 11:57:50AM +0100, Catalin Marinas 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.
> 
> Linking agianst libgcc is pretty scary if we're starting to use plugins
> for the compiler[1], but IANAL.

At a quick grep, there are 9 other architectures linking the kernel
image against libgcc. I wonder what they do about it.

For the time being, I'm reverting this patch. It's probably not that
hard to add the int128 variants of these functions to the kernel (in a
GPLv2 variant).

-- 
Catalin