[RFC PATCH] arm64: lse: use register variables instead of -ffixed, -fcall flags

[will.deacon@arm.com (Will Deacon)]

On Thu, Aug 09, 2018 at 07:07:51PM -0700, Tri Vo wrote:
> -ffixed, -fcall-used, -fcall-saved flags are used to build LL/SC
> fallback atomic implementations in such a way that:
> 1. LSE implementations only assume x16, x17, x30 to be clobbered by a
> function call to LL/SC fallbacks.
> 2. LL/SC fallbacks are responsible for saving/restoring the rest of the
> GPRs.
> 3. LL/SC fallbacks always prefer to allocate in x16, x17 since there is
> no need to save/restore them.
> 
> Same result can be achieved without -ffixed, -fcall-used, -fcall-saved
> flags by explicitly telling the compiler where to allocate each variable
> in LL/SC atomic implementations. This patch makes all functions use x16,
> x17 registers. 'fetch' variants of the functions need one more scratch
> register, so we allocate one from the PCS's set of callee-saved
> registers (in this case x19).
> 
> With this patch:
> 1. the desired register allocation is reflected in the code rather than
> in compiler flags.
> 2. LSE atomic support can be built with both clang and gcc.
> 3. Number of preserved registers in LL/SC fallbacks is unchanged.
> 
> The tradeoff is that the compiler loses flexibility in allocating
> registers for inline LL/SC atomic implementations.

This impacts the atomics code when !CONFIG_LSE, and causes the fetch*
variants to spill x19 in most cases.

Please just fix clang, or augment the lseinstr check in our Makefile to
check for ffixed support as well.

Thanks,

Will