Re: SH runtime switchable atomics - proposed design

[Oleg Endo <oleg.endo@t-online.de>]

On Tue, 2016-01-19 at 15:28 -0500, Rich Felker wrote:
> I've been working on the new version of runtime-selected SH atomics
> for musl, and I think what I've got might be appropriate for GCC's
> generated atomics too. I know Oleg was not very excited about doing
> this on the gcc side from a cost/benefit perspective

I am just not keen on making this the default atomic model for SH.
If you have a system built around this atomic model and want to add it
to GCC, please send in patches.  Just a few comments below...

> Inputs:
> - R0: Memory address to operate on
> - R1: Address of implementation function, loaded from a global
> - R2: Comparison value
> - R3: Value to set on success
> 
> Outputs:
> - R3: Old value read, ==R2 iff cas succeeded.

> Preserved: R0, R2.
> 
> Clobbered: R1, PR, T.

The T bit is obviously the result of the cas operation.  So you could
use it as an output directly instead of the implicit R3 == R2
condition.

> 
> This call (performed from __asm__ for musl, but gcc would do it as SH
> "SFUNC") is highly compact/convenient for inlining because it avoids
> clobbering any of the argument registers that are likely to already
> be
> in use by the caller, and it preserves the important values that are
> likely to be reused after the cas operation.
> 
> For J2 and future J4, the function pointer just points to:
> 
> 	rts
> 	 cas.l r2,r3,@r0
> 

> and the only costs vs an inline cas.l are loading the address of the
> function (done in the caller; involves GOT access) and clobbering R1
> and PR.
> 
> This is still a draft design and the version in musl is subject to
> change at any time since it's not a public API/ABI, but I think it
> could turn into something useful to have on the gcc side with a
> -matomic-model=libfunc option or similar. Other ABI considerations
> for
> gcc use would be where to store the function pointer and how to
> initialize it. To be reasonably efficient with FDPIC the caller needs
> to be responsible for loading the function pointer (and it needs to
> always point to code, not a function descriptor) so that the callee
> does not need a GOT pointer passed in.

Obviously the ABI has been constructed around the J-core's cas.l
instruction.  Do you have plans to add other atomic operations (like
arithmetic)?  If not, then I'd suggest to name the atomic model
"libfunc-musl-cas".

Cheers,
Oleg