## Macro: SB-EXT:ATOMIC-DECF
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> Atomically decrements PLACE by DIFF, and returns the value of PLACE before
> the decrement.
> 
> PLACE must access one of the following:
> - a DEFSTRUCT slot with declared type (UNSIGNED-BYTE 64)
> or AREF of a (SIMPLE-ARRAY (UNSIGNED-BYTE 64) (*))
> The type SB-EXT:WORD can be used for these purposes.
> - CAR or CDR (respectively FIRST or REST) of a CONS.
> - a variable defined using DEFGLOBAL with a proclaimed type of FIXNUM.
> Macroexpansion is performed on PLACE before expanding ATOMIC-DECF.
> 
> Decrementing is done using modular arithmetic,
> which is well-defined over two different domains:
> - For structures and arrays, the operation accepts and produces
> an (UNSIGNED-BYTE 64), and DIFF must be of type (SIGNED-BYTE 64).
> ATOMIC-DECF of #x0 by one results in #xFFFFFFFFFFFFFFFF being stored in PLACE.
> - For other places, the domain is FIXNUM, and DIFF must be a FIXNUM.
> ATOMIC-DECF of #x-4000000000000000 by one results in #x3FFFFFFFFFFFFFFF
> being stored in PLACE.
> 
> DIFF defaults to 1.
> 
> EXPERIMENTAL: Interface subject to change.

### Lambda list
```cl
(sb-impl::place &optional sb-impl::diff)
```