In typical executions, nearly 50% of the time was spent in libunwind
get_proc_name.
The algorithm to find the function for a given IP (instruction
pointer) was:
(proc_name, offset) = get_proc_name(ip) // Slow!
dwarf_ip = ip - offset
function = functions_by_name[proc_name]
We added a structure mapping IP ranges to functions and the algorithm
is now:
function = functions_by_ip[ip]
Instead of relying on libunwind, we use the DWARF information to find
the corresponding DWARF TAG_subprogram DIEs directly.
The secution time on some MPICH tests is nearly halved.
Notes:
* It was necessary to disable the support for inlined_subprograms
which was broken anyway: the inlined_subprogram entries should be
stored as children of their parent subprogram (as a block). We need
to add support for scope blocks inside a suprogram to handle this
correctly.
* Currently the translation between process virtual addresses and
DWARF virtual addresses is handled in many different places. We
should change this to process it only when parsing the DWARF DIEs
and be done with it.
