paradox re: constraints for ReadWrite memory in asm instruction

[dw <limegreensocks@xxxxxxxxx>]

I am trying to figure out the proper constraints for __asm__ 
instructions on x64 that both read and write memory (for example "bts" 
or "lock xadd").  I'm ok writing the assembler, I'm just struggling with 
writing the constraints.  My problem is that the gcc docs forbid every 
permutation of constraint I can think of that would define this 
behavior.  You have 3 mutually exclusive requirements:

1) Since the value being read+written is memory, the memory must be 
specified as both input and output.
2) The docs say: "You should only use read-write operands when the 
constraints for the operand [...] allow a register."
3) The docs say: "You may not write a clobber description in a way that 
overlaps with an input or output operand. [...] There is no way for you 
to specify that an input operand is modified without also specifying it 
as an output operand."

In summary: You have to specify something for output, but you can't have 
"=m" (#1) or "+m" (#2) for output, and you can't fake the output by 
using a clobber constraint against the address (#3).

Which effectively means there is *no* way to specify a memory constraint 
as read+write.  Since I don't believe that is true, I can only assume 
the docs are wrong.  The question is:  which part?

I have seen a bunch of code that uses constraints that violate these 
documented requirements.  I am sure that most of them work (at least 
most of the time).  But I don't want to base my program on code that I 
know violates the docs.  That's just asking for trouble.

I'm looking for the officially supported and documented way to specify 
memory constraints as read+write.  And obviously I want what I write to 
produce the most performant code consistent with supported standards.

Thanks.