micro-optimization

Imagine you want to align a series of x86 assembly instructions to certain boundaries. For example, you may want to align loops to a 16 or 32-byte boundary, or pack instructions so they are efficiently placed in the uop cache or whatever. The simplest way to achieve this is single-byte NOP instructions, followed closely by multi-byte NOPs . Although the latter is generally more efficient, neither method is free: NOPs use front-end execution resources, and also count against your 4-wide 1 rename limit on modern x86. Another option is to somehow lengthen some instructions to get the alignment

Just to make it clear, I'm not going for any sort of portability here, so any solutions that will tie me to a certain box is fine. Basically, I have an if statement that will 99% of the time evaluate to true, and am trying to eke out every last clock of performance, can I issue some sort of compiler command (using GCC 4.1.2 and the x86 ISA, if it matters) to tell the branch predictor that it should cache for that branch? Drakosha Yes. http://kerneltrap.org/node/4705 The __builtin_expect is a method that gcc (versions >= 2.96) offer for programmers to indicate branch prediction information to

I've been trying to optimize some extremely performance-critical code (a quick sort algorithm that's being called millions and millions of times inside a monte carlo simulation) by loop unrolling. Here's the inner loop I'm trying to speed up: // Search for elements to swap. while(myArray[++index1] < pivot) {} while(pivot < myArray[--index2]) {} I tried unrolling to something like: while(true) { if(myArray[++index1] < pivot) break; if(myArray[++index1] < pivot) break; // More unrolling } while(true) { if(pivot < myArray[--index2]) break; if(pivot < myArray[--index2]) break; // More unrolling }

This question is merely for me as I always like to write optimized code that can run also on cheap slow servers (or servers with A LOT of traffic) I looked around and I was not able to find an answer. I was wondering what is faster between those two examples keeping in mind that the array's keys in my case are not important (pseudo-code naturally): <?php $a = array(); while($new_val = 'get over 100k email addresses already lowercased'){ if(!in_array($new_val, $a){ $a[] = $new_val; //do other stuff } } ?> <?php $a = array(); while($new_val = 'get over 100k email addresses already lowercased'){