Cache the last result of dwarf_getscopes

This one function accounted for ~30% of my callgrind profile of
"stap -l 'syscall.*'", even though it was only called ~1200 times.  We
call dwarf_getscopes for each $target variable, with the same parameters
within a given probe.  Since they're no nicely grouped, it's easy to
just cache the most recent call, and the next few calls will be a hit.

Overall this cuts the number of calls down to about 300, for an easy
speed gain.

1 files changed, 5 insertions, 0 deletions

diff --git a/dwflpp.h b/dwflpp.h
index 554e02d4..314e7583 100644
--- a/dwflpp.h
+++ b/dwflpp.h
@@ -382,6 +382,11 @@ private:
   bool blacklist_enabled;
   void build_blacklist();
   std::string get_blacklist_section(Dwarf_Addr addr);
+
+  Dwarf_Addr pc_cached_scopes;
+  int num_cached_scopes;
+  Dwarf_Die *cached_scopes;
+  int dwarf_getscopes_cached (Dwarf_Addr pc, Dwarf_Die **scopes);
 };
 
 #endif // DWFLPP_H