[PATCH] Fix dwarf2 unwinding through futex functions

[Roland McGrath]

DW_CFA_val_expression was introduced in DWARF 3.

The effect of these subsection uses is to move an unlikely code path out of
the straight line hot path.  Making the hot path be straight-line (branch
not taken) is part of the benefit, but moving the slow path away so it is
less likely to pollute I-cache may be a more important benefit.  If you
want to change that you need to demonstrate conclusively that you are not
harming performance.  In fact, the obvious change would be to change
".subsection 1" to ".section .text.cold", which would move the slow path
code even farther away (the subsection is just the end of the text in the
same .o file, while .text.cold would be all the way at the end of the DSO's
text) so as to make I-cache pollution even less likely.