I happened to notice today that the initializer macro for dlist_head
variables is
#define DLIST_STATIC_INIT(name) {{&(name).head, &(name).head}}
However, all the functions that work with dlists are prepared to handle
a dlist_head that starts out as zeroes, so that this could also be
#define DLIST_STATIC_INIT(name) {{NULL, NULL}}
I submit that we'd be better off with the latter. The number of cycles
that the linker and loader expend on getting those non-constant values
correctly set up (especially in PIE builds) probably dwarf what it
costs for the first dlist access to initialize them. It's especially
obviously a loss in processes that never touch the particular dlist
at all.
Another thought is that maybe we should deprecate the use of the
[DS]LIST_STATIC_INIT macros altogether, and just write
static dlist_header myheader;
leaving the compiler to drop such variables into a BSS area instead
of an initialized-data area. I'm not very sure how much that saves,
but I bet it saves something.
regards, tom lane