[PATCH, i386]: Implement atomic_fetch_sub

[Andrew MacLeod]

On 08/03/2012 02:52 PM, Andrew MacLeod wrote:
>
> I'll take a look at it next week unless someone gets to it first.
>
> Andrew

OK, so maybe I sort of forgot about implementing the generic 
transformation until now.

This patch bootstraps and passes all the testsuite cases.  Factored out 
a little code to make sure we only generate the add if there is a direct 
opcode for it.
Also implemented the reverse since it was trivial... if an atomic_add 
doesn't generate anything, it'll try an atomic_sub to see if thats possible.

If that all fails, then we continue to regress to a compare and swap loop.

bootstraps with no new regressions and verified to fix the PR with no 
side-effects

OK for mainline?

Andrew
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