[Bug rtl-optimization/70825] x86_64: __atomic_compare_exchange_n() accesses stack unnecessarily

[ghalliday at hpccsystems dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70825

ghalliday at hpccsystems dot com changed:

           What    |Removed                     |Added
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                 CC|                            |ghalliday at hpccsystems dot com

--- Comment #4 from ghalliday at hpccsystems dot com ---
I have also hit this problem on x86 and Power8, and am adding a comment about
its significance.  Although it seems a minor bug it can have a very significant
effect on performance.

I have code which uses __sync_bool_compare_and_swap() to implement a lock free
linked list (inside a memory manager).  Replacing it with
__atomic_compare_exchange_n should allow better performance - by avoiding
reloading the expected value (and also selecting a less restrictive memory
order).

However on one example test query, the new code using
__atomic_compare_exchange_n is over 40% *slower* on x86.  It is also slower on
Power8 despite using a lwsync instead of sync in the generated code.