The atomic builtins page doesn't indicate that gcc supports 16-byte compare-and-swap instructions. But gcc does in fact support this instruction, so the page should indicate that the functionality can be enabled with -mcex16 (or, presumably, with the appropriate -march flag).
I don't think the atomics page should reference a target specific option.
I think the following sentence makes that clear that the builtins are dependent on target options as they can change what the target processor is:
Not all operations are supported by all target processors.
"Not all operations are supported by all target processors" isn't the same as "not all operations supported by target processors are listed here."
If you want to be vague and say "some target processors support other operations or other operand sizes, not listed here", I guess that would be an improvement. But from a user's perspective, I'd like the manual to tell me which builtins GCC supports, and under which circumstances those operations are available. In this case, the manual's omission of 16-bit cex suggested to me that gcc didn't support it at all.
Actually it says the target processors might not include all of the builtins. What is a target processor is up to each target and likewise on how to change the processor.
This should allow you to read the target options for how to change the processor GCC is targeting. This is the same issue as -march=i386 does not support any of them vs -march=i686 supports the majority of them.
The x86 specific options explicitly references the atomic builtins. Again I don't think we should reference target specific options in the generic part of the manual. Doing that makes the manual even more confusing. Maybe adding the following sentence will help:
Each target defines how a specific processor is selected, see the target options pages on how to select the specific processor.
> Actually it says the target processors might not include all of the builtins.
Maybe I'm not making sense. My point is that there's a builtin (*) that is supported by some target processors but is not listed in the manual. So the statement that some target processors might not include all the builtins isn't helpful.
* The builtin is compare_and_swap with 16-byte operands. The manual explicitly says "GCC will allow any integral scalar or pointer type that is 1, 2, 4 or 8 bytes in length" but should indicate that 16-byte operands are supported for compare_and_swap, at least under some circumstances.