Performance std::strstr vs. std::string::find

Question

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C++ string::find complexity

Recently I noticed that the function std::string::find is an order of magnitude slower than the function std::strstr - in my environment with GCC 4.7 on Linux. The performance difference depends on the lengths of the strings and on the hardware architecture.

There seems to be a simple reason for the difference: std::string::find basically calls std::memcmp in a loop - with time complexity O(m * n). In contrast, std::strstr is highly optimized for the hardware architecture (e.g. with SSE instructions) and uses a more sophisticated string matching algorithm (apparently Knuth-Morris-Pratt).

I was also surprised not to find the time complexities of these two functions in the language documents (i.e. drafts N3290 and N1570). I only found time complexities for char_traits. But that doesn't help, because there is no function for substring search in char_traits.

I would expect, that std::strstr and memmem contain similar optimizations with almost identical performance. And until recently, I assumed that std::string::find uses memmem internally.

The questions are: Is there any good reason, why std::string::find does not use std::memmem? And does it differ in other implementations?

The question is not: What is the best implementation of this function? It is really difficult to argue for C++, if it is slower than C. I wouldn't matter if both implementations would be slow. It is the performance difference that really hurts.

Answer 1

First, what's memmem? I can't find this in the C++ standard, nor the Posix standard (which contains all of the standard C functions).

Second, any measurement values will depend on the actual data. Using KMP, for example, will be a pessimisation in a lot of cases; probably most of the cases where the member functions of std::string are used; the time to set up the necessary tables will often be more than the total time of the straightforeward algorithm. Things like O(m*n) don't mean much when the typical length of the string is short.