Using SSE instructions

c++ optimization assembly processor sse
Naveen picture Naveen · Feb 25, 2009 · Viewed 35.3k times · Source

I have a loop written in C++ which is executed for each element of a big integer array. Inside the loop, I mask some bits of the integer and then find the min and max values. I heard that if I use SSE instructions for these operations it will run much faster compared to a normal loop written using bitwise AND , and if-else conditions. My question is should I go for these SSE instructions? Also, what happens if my code runs on a different processor? Will it still work or these instructions are processor specific?

Answer

Niki picture Niki · Feb 25, 2009
  1. SSE instructions are processor specific. You can look up which processor supports which SSE version on wikipedia.
  2. If SSE code will be faster or not depends on many factors: The first is of course whether the problem is memory-bound or CPU-bound. If the memory bus is the bottleneck SSE will not help much. Try simplifying your integer calculations, if that makes the code faster, it's probably CPU-bound, and you have a good chance of speeding it up.
  3. Be aware that writing SIMD-code is a lot harder than writing C++-code, and that the resulting code is much harder to change. Always keep the C++ code up to date, you'll want it as a comment and to check the correctness of your assembler code.
  4. Think about using a library like the IPP, that implements common low-level SIMD operations optimized for various processors.

Related questions

Why does C++ code for testing the Collatz conjecture run faster than hand-written assembly?

I wrote these two solutions for Project Euler Q14, in assembly and in C++. They implement identical brute force approach for testing the Collatz conjecture. The assembly solution was assembled with: nasm -felf64 p14.asm && gcc p14.o …

Read More
Using Assembly Language in C/C++

I remember reading somewhere that to really optimize & speed up certain section of the code, programmers write that section in Assembly language. My questions are - Is this practice still done? and How does one do this? Isn't writing …

Read More
Is there a way to flush the entire CPU cache related to a program?

On x86-64 platforms, the CLFLUSH assembly instruction allows to flush the cache line corresponding to a given address. Instead of flushing the cache related to a specific address, would there be a way to flush the entire cache (either …

Read More