parse() taking a list of elements is quadratic (during compile time) in
that it generates recursive calls to itself, each time with one fewer
parameter. The total size of the parameter lists in all these generated
functions is quadratic in the initial parameter list size.
It's also exponential if we ignore inlining limits, since each .then()
call expands to two branches - a ready future branch and a non-ready
future branch. If the compiler did not give up, we'd have 2^list_len
branches. For sure the compiler does not do so indefinitely, but the effort
getting there is wasted.
Simplify by using a fold expression over the comma operator. Instead
of passing the remaining parameter list in each step, we pass only
the parameter we are processing now, making processing linear, and not
generating unnecessary functions.
It would be better expressed using pack expansion statements, but these
are part of C++26.
The largest offender is probably stats_metadata, with 21 elements.
dev-mode sstables.o:
text data bss dec hex filename
1760059 1312 7673 1769044 1afe54 sstables.o.before
1745533 1312 7673 1754518 1ac596 sstables.o.after
We save about 15k of text with presumably a corresponding (small)
decrease in compile time.
Closesscylladb/scylladb#26735
9b6ce030d0