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posted by
LaminatorX
on Sunday March 16 2014, @03:28AM
from the premature-optimization-is-the-root-of-all-evil dept.
from the premature-optimization-is-the-root-of-all-evil dept.
Subsentient writes:
"I've been writing C for quite some time, but I never followed good conventions I'm afraid, and I never payed much attention to the optimization tricks of the higher C programmers. Sure, I use const when I can, I use the pointer methods for manual string copying, I even use register for all the good that does with modern compilers, but now, I'm trying to write a C-string handling library for personal use, but I need speed, and I really don't want to use inline ASM. So, I am wondering, what would other Soylenters do to write efficient, pure, standards-compliant C?"
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(Score: 1) by sjames on Sunday March 16 2014, @07:20AM
There are tradeoffs. Yes, you do extra work in some cases, but in others you do a lot less. For example, getting the length of the string becomes trivial rather than scanning the buffer (potentially wiping out your cache) Comparisons are, of coure, trivial. You do more accounting when you cat the strings, but you do less scanning and there's no chance of overrunning the buffer. It can also be a win if you have good optimization.
Which is overall better and faster will strongly depend on what you're doing. I do suspect that on average final strings will be a bit slower but safer.
(Score: 2) by maxwell demon on Sunday March 16 2014, @12:15PM
Any sane string implementation (that is, not C's) will have a length field (or, alternatively, an additional pointer to end), so you don't have to scan the string to find out its length (an O(n) operation which, as you noted, will do bad things to your cache), but can just read the length (or calculate it in O(1) by simple pointer subtraction). This also means you'll not arbitrarily restrict the characters which can be stored in your string (C applications are generally easily identified by them being tripped off by a zero byte in positions supposed to contain text).
The Tao of math: The numbers you can count are not the real numbers.