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

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

On Fri, Aug 10, 2018 at 03:04:55PM -0700, Tri Vo wrote:
> On Fri, Aug 10, 2018 at 2:46 AM, Will Deacon <will.deacon@arm.com> wrote:
> > 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.
> 
> Is it OK if I wrap that x19 register variable in an #ifdef so it's
> only allocated to x19 when CONFIG_LSE and is allocated normally for
> !CONFIG_LSE? That should keep the number of spills unchanged for both
> inline and out-of-line implementations.

I would much prefer you to:

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

Will