[PATCH] arm64: kill off the libgcc dependency

[Ard Biesheuvel]

On 15 January 2015 at 09:59, Will Deacon <will.deacon at arm.com> wrote:
> On Thu, Jan 15, 2015 at 07:07:30AM +0000, Kevin Hao wrote:
>> On Wed, Jan 14, 2015 at 01:37:42PM +0000, Will Deacon wrote:
>> > On Wed, Jan 14, 2015 at 12:52:49PM +0000, Ard Biesheuvel wrote:
>> > > IMO libgcc.a cannot be used at all in the kernel, as it is not built
>> > > with -mgeneral-regs-only so we have no guarantee that it will leave
>> > > the NEON registers alone.
>> >
>> > That's a very good point. In which case, we'd need to extend this patch
>> > to implement the bitops that we currently rely on the __builtins for.
>>
>> Sorry, I am not sure I get what you mean. These builtin functions for bitops
>> are not implemented in the libgcc, they are inlined in compiler. So why do
>> we need to reimplement these bitops when removing the link with libgcc?
>
> I'm just relaying what the tools guys told us. Namely, that they don't
> guarantee that they are inlined and could generate a branch to a libgcc
> function. As Ard says, they could also use fpsimd registers, so whether
> or not they are inlined is moot anyway.
>

Which builtins are you referring to, exactly?

~/linux-2.6$ git grep __builtin arch/arm64
arch/arm64/kernel/module.c:
__builtin_return_address(0));
arch/arm64/kernel/return_address.c:     frame.fp = (unsigned
long)__builtin_frame_address(0);
arch/arm64/kernel/stacktrace.c:         frame.fp = (unsigned
long)__builtin_frame_address(0);
arch/arm64/kernel/traps.c:              frame.fp = (unsigned
long)__builtin_frame_address(0);
arch/arm64/mm/ioremap.c:
__builtin_return_address(0));
arch/arm64/mm/ioremap.c:
__builtin_return_address(0));

none of which would ever require library support, as far as I can
judge, and the arm64 kernel builds fine with -fno-builtin added (i.e.,
builtins can only be invoked explicitly using their __builtin- alias)
and libgcc.a removed.

-- 
Ard.