c-backward-syntactic-ws is a Lisp macro in `cc-defs.el'.

(c-backward-syntactic-ws &optional LIMIT)

Backward skip over syntactic whitespace.
Syntactic whitespace is defined as whitespace characters, comments,
and preprocessor directives. However if point starts inside a comment
or preprocessor directive, the content of it is not treated as

LIMIT sets a lower limit of the backward movement, if specified. If
LIMIT is reached inside a line comment or preprocessor directive then
the point is moved into it past the whitespace at the end.

Note that this function might do hidden buffer changes. See the
comment at the start of cc-engine.el for more info.