Finding substring in assembly

I'm wondering if there is a more efficient method to finding a substring in assembly then what I am currently planning to do.

I know the string instruction "scansb/scasw/scads" 开发者_JS百科can compare a value in EAX to a value addressed by EDI. However, as far as I understand, I can only search for one character at a time using this methodology.

So, if I want to find the location of "help" in string "pleasehelpme", I could use scansb to find the offset of the h, then jump to another function where I compare the remainder. If the remainder isn't correct, I jump back to scansb and try searching again, this time after the previous offset mark.

However, I would hate to do this and then discover there is a more efficient method. Any advice? Thanks in advance

There are indeed more efficient ways, both instruction-wise and algorithmically.

If you have the hardware you can use the sse 4.2 compare string functions, which are very fast. See an overview http://software.intel.com/sites/products/documentation/studio/composer/en-us/2009/compiler_c/intref_cls/common/intref_sse42_comp.htm and an example using the C instrinsics http://software.intel.com/en-us/articles/xml-parsing-accelerator-with-intel-streaming-simd-extensions-4-intel-sse4/

If you have long substrings or multiple search patterns, the Boyer-Moore, Knuth-Morris-Pratt and Rabin-Karp algorithms may be more efficient.

I don't think there is a more efficient method (only some optimizations that can be done to this method). Also this might be of interest.

scansb is the assembly variant for strcmp, not for strstr. if you want a really efficient method, then you have to use better algorithm.

For example, if you search in a long string, then you could try some special algorithms: http://en.wikipedia.org/wiki/String_searching_algorithm