4 ms·

I actually prefer the variation: char c; for (int i = 0; (c = str[i]) != '\0'; i++) ...loopcontents using c... this should be faster than all

I actually prefer the variation: char c; for (int i = 0; (c = str[i]) != '\0'; i++) ...loopcontents using c... this should be faster than all the rest of the implementations as you might break early and you save a memory dereference of every character. in all honesty though, these optimizations are highly unlikely to be the constraining factor on any loop so the question really is moot.

vidarh 13y ago
If you're going to do that, why are you wasting instructions maintaining an offset and adding it to the string pointer on every iteration, instead of just iterating the pointer (or a copy of it) every iteration instead?
- chacham15 13y ago
  I was trying to provide a semantically equivalent construct (you might need the current index for some computation). Furthermore, the difference between the two really is negligible. The current solution involves an increment to i, an increment to the pointer, and a dereference whereas the solution you describe only involves an increment to the pointer and a dereference. The difference is an increment to a number, i.e. a single instruction which is probably the fastest instruction in the ISA.
  - vidarh 13y ago
    Providing a semantically equivalent construct is a fair enough reason. > Furthermore, the difference between the two really is negligible. > ... > The difference is an increment to a number, i.e. a single instruction which is probably the fastest instruction in the ISA. Whether or not it matters will depend greatly on how often this loop is executed, and what else it does, and the ISA and available address modes, as well as whether or not the pointer and/or counter can fit in available registers. If the pointer and counter is not in registers, and you don't have a suitable indirect indexed type load/test instruction, you can easily end up with 3+ extra memory reads. Memory access gets expensive if/when those kind of loops are run often enough. I've seen speed improvements of 30%-50% from a couple of hours of work from production systems just from trivial rewrites to reduce memory accesses like these. Some fixes like that have paid my salary for months in reduction in server investments.

vidarh 13y ago

If you're going to do that, why are you wasting instructions maintaining an offset and adding it to the string pointer on every iteration, instead of just iterating the pointer (or a copy of it) every iteration instead?

chacham15 13y ago
I was trying to provide a semantically equivalent construct (you might need the current index for some computation). Furthermore, the difference between the two really is negligible. The current solution involves an increment to i, an increment to the pointer, and a dereference whereas the solution you describe only involves an increment to the pointer and a dereference. The difference is an increment to a number, i.e. a single instruction which is probably the fastest instruction in the ISA.
- vidarh 13y ago
  Providing a semantically equivalent construct is a fair enough reason. > Furthermore, the difference between the two really is negligible. > ... > The difference is an increment to a number, i.e. a single instruction which is probably the fastest instruction in the ISA. Whether or not it matters will depend greatly on how often this loop is executed, and what else it does, and the ISA and available address modes, as well as whether or not the pointer and/or counter can fit in available registers. If the pointer and counter is not in registers, and you don't have a suitable indirect indexed type load/test instruction, you can easily end up with 3+ extra memory reads. Memory access gets expensive if/when those kind of loops are run often enough. I've seen speed improvements of 30%-50% from a couple of hours of work from production systems just from trivial rewrites to reduce memory accesses like these. Some fixes like that have paid my salary for months in reduction in server investments.