> I'm planning to add a set of new performance diagnostics to the
> libstdc++ profile mode
> (http://gcc.gnu.org/onlinedocs/libstdc++/manual/profile_mode.html) and
> am trying to come up with a list of what diagnostics are most
> meaningful and most wanted.
>
> At this (brainstorming) point I'm looking for any suggestions,
> including and not limited to:
> - container selection (e.g., replace list with deque).
> - container implementation selection (e.g., string implementation).
> - algorithm selection (e.g., sort implementation)
> - data structure or algorithm parameter selection (e.g., fixed string
> reserved size value for a particular context).
>
> Please reply to this message or email me privately ([email protected]) if
> you have any suggestions for new diagnostics, or about the profile
> mode in general, or to express your support for a particular proposed
> diagnostic. For new diagnostics, it works best if you can provide:
First idea, based on a performance issue I fixed in a codebase in 2001:
> - A simple description, e.g., "replace vector with list".
"cache value of std::string::c_str() instead of multiple invocations with a
non-shareable declared std::string"
> - (optional) The instrumentation points, even if vague, e.g.,
> "instrument insertion, element access and iterators".
Instrument calls to std::string::c_str() that are allocation and invocation
context-aware.
> - (optional) How the decision is made, e.g. "there are many inserts in
> the middle, there is no random access and it's not iterated through
> many times".
1) allocation of std::string in local variable
2) calls to said local string's c_str() method within loops
3) and said loops do not modify the contents of the value returned from c_str()
Example:
#include <string>
#include <cstdio>
void notify(const char* printable) { printf("%s\n", printable); }
int main(void)
{
std::string name("bob");
for (int i = 0; i < name.length(); i++)
{
notify(name.c_str());
}
return 0;
}
Second idea, based on the same codebase as before. Removing 5 conversions
to/from std::string and char* resulted in a 10X throughput improvement in
network throughput in that codebase:
> - A simple description, e.g., "replace vector with list".
"avoid converting a std::string to a char*, just to convert it back to
std::string later in the call stack"
> - (optional) The instrumentation points, even if vague, e.g.,
> "instrument insertion, element access and iterators".
Instrument calls to std::string::c_str(), tracking the resulting value returned
by c_str().
> - (optional) How the decision is made, e.g. "there are many inserts in
> the middle, there is no random access and it's not iterated through
> many times".
1) where a value is returned by std::string::c_str()
2) and said char* value is fed back into std::string constructor, locally or
further down the call chain
3) and said char* value was not modified between the calls to
std::string::c_str() and std::string()
Example:
#include <cstdio>
#include <string>
void tally(const std::string& index) { printf("%s\n", index.c_str()); }
void notify(const char* printable) { tally(std::string(printable)); }
int main(void)
{
std::string name("bob");
notify(name.c_str());
return 0;
}
Over the years, I have seen both of these occur multiple times across multiple
teams. The constant seems to be C programmers who are passive-aggressive about
their distaste for STL, and/or teams with poor communication between module
owners.
