[PATCH] D17950: Implement is_always_lock_free

[Craig, Ben via cfe-commits]

I know that MIPS does that, and an out-of-tree implementation of hexagon 
implements 1-byte cmpxchg in terms of the 4-byte version. The emulation 
code isn't particularly small, and it seems reasonable to make it a 
libcall.  The emulation code seems sketchy from a correctness 
perspective, as you end up generating unsolicited loads and stores on 
adjacent bytes.  Take a look at the thread on cfe-dev and llvm-dev named 
"the as-if rule / perf vs. security" for some of the ramifications and 
concerns surrounding unsolicited loads and stores.

On 3/17/2016 10:05 AM, James Y Knight wrote:
>
> > A 4 byte cmpxchg could be lock free, while a 1 byte cmpxchg may not 
> be lock free.
>
> That's not possible: if you have a 4-byte cmpxchg instruction, you can 
> use it to implement a 1-byte cmpxchg, too. Many targets do this 
> already. (It would be better if that was available generically so that 
> code didn't need to be written separately fit each target, but that's 
> not the case at the moment.)
>

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