Re: [musl] SH runtime switchable atomics - proposed design

[Rich Felker <dalias at libc dot org>]

On Tue, Jan 19, 2016 at 03:28:52PM -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, but I think my
> approach is actually preferable over inline atomics from a code size
> perspective. It uses a single "cas" function with an "SFUNC" type ABI
> (not standard calling convention) with the following constraints:
> 
> 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.
> 
> 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.

Attached is my current draft of the implementations of the cas 'sfunc'
for musl. Forgot to include it before.

Rich

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