Text iteration, Assembly versus C++ - c++

Question

I am making a program in which is is frequently reading chunks of text received from the web looking for specific characters and parsing the data accordingly. I am becoming fairly skilled with C++, and have made it work well, however, is Assembly going to be faster than a
for(size_t len = 0;len != tstring.length();len++) {
if(tstring[len] == ',')
stuff();
}
Would an inline-assembly routine using cmp and jz/jnz be faster? I don't want to waste my time working with asm for the fact being able to say I used it, but for true speed purposes.
Thank you,

Answer 1

No way. Your loop is so simple, the cost of the optimizer losing the ability to reason about your code is going to be way higher than any performance you could gain. This isn't SSE intrinsics or a bootloader, it's a trivial loop.

Answer 2

An inline assembly routine using "plain old" jz/jnz is unlikely to be faster than what you have; that said, you have a few inefficiencies in your code:
you're retrieving tstring.length() once per loop iteration; that's unnecessary.
you're using random indexing, tstring[len] which might be a more-expensive operation than using a forward iterator.
you're calling stuff() during the loop; depending on what exactly that does, it might be faster to just let the loop build a list of locations within the string first (so that the scanned string as well as the scanning code stays cache-hot and is not evicted by whatever stuff() does), and only afterwards iterate over those results.
There's already a likely low-level optimized standard library function available,strchr(), for exactly that kind of scanning. The C++ STL std::string::find() is also likely to have been optimized for the purpose (and/or might use strchr() in the char specialization).
In particular, strchr() has SSE2 (using pcmpeqb, maskmov... and bsf) or SSE4.2 (using the string op pcmpistri) implementations; for examples/actual SSE code doing this, check e.g. strchr() in GNU libc (as used on Linux). See also the references and comments here (suitably named website ...).
My advice: Check your library implementation / documentation, and/or the actual generated assembly code for your program. You might well be using fast code already ... or would be if you'd switch from your hand-grown character-by-character simple search to just using std::string::find() or strchr().
If this is ultra-speed-critical, then inlining assembly code for strchr() as used by known/tested implementations (watch licensing) would eliminate function calls and gain a few cycles. Depends on your requirements ... code, benchmark, vary, benchmark again, ...

Answer 3

Checking characters one by one is not the fastest thing to do. Maybe you should try something like this and find out if it's faster.
string s("xxx,xxxxx,x,xxxx");
string::size_type pos = s.find(',');
while(pos != string::npos){
do_stuff(pos);
pos = s.find(',', pos+1);
}
Each iteration of the loop will give you the next position of a ',' character so the program will need only few loops to finish the job.

Answer 4

Would an inline-assembly routine using cmp and jz/jnz be faster?
Maybe, maybe not. It depends upon what stuff() does, what the type and scope of tstring is, and what your assembly looks like.
First, measure the speed of the maintainable C++ code. Only if this loop dominates your program's speed should you consider rewriting it.
If you choose to rewrite it, keep both implementations available, and comparatively measure them. Only use the less maintainable version if it is faster, and if the speed increase matters. Also, since you have the original version in place, future readers will be able to understand your intent even if they don't know asm that well.