In C, would !~b ever be faster than b == 0xff ?

Posted by James Morris on Stack Overflow See other posts from Stack Overflow or by James Morris
Published on 2010-05-04T15:54:54Z Indexed on 2010/05/04 15:58 UTC
Read the original article Hit count: 257

Filed under:

optimization

From a long time ago I have a memory which has stuck with me that says comparisons against zero are faster than any other value (ahem Z80).

In some C code I'm writing I want to skip values which have all their bits set. Currently the type of these values is char but may change. I have two different alternatives to perform the test:

if (!~b)
    /* skip */

and

if (b == 0xff)
    /* skip */

Apart from the latter making the assumption that b is an 8bit char whereas the former does not, would the former ever be faster due to the old compare to zero optimization trick, or are the CPUs of today way beyond this kind of thing?

Related posts about bits

given two bits in a set of four, fine position of two other bits

as seen on Stack Overflow - Search for 'Stack Overflow'
hello I am working on a simple combinatorics part, and found that I need to recover position of two bits given position of other two bits in 4-bits srring. for example, (0,1) maps to (2,3), (0,2) to (1,3), etc. for a total of six combinations. My solution is to test bits using four nested ternary… >>> More
Vmware Player 3.0 - cannot ping 32 bits guest from 64 bits (guest or host)

as seen on Server Fault - Search for 'Server Fault'
I'm stuck with what seems a bug in VmWare Player (build 203739). I'm using W7 Ultimate 64bits as host and have a CentOS 5.4 (64 bits) as a guest and a Windows XP Professional SP3 (32 bits) as another guest. From the 64 bits machines (the host and the linux guest) I cannot ping the windows XP. Off… >>> More
Vmware Player 3.0 - cannot ping 32 bits guest from 64 bits (guest or host)

as seen on Server Fault - Search for 'Server Fault'
I'm stuck with what seems a bug in VmWare Player (build 203739). I'm using W7 Ultimate 64bits as host and have a CentOS 5.4 (64 bits) as a guest and a Windows XP Professional SP3 (32 bits) as another guest. From the 64 bits machines (the host and the linux guest) I cannot ping the windows XP. Off… >>> More
NEC uPD720200 USB 3.0 not working on Ubuntu 12.04

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
I've recently installed Ubuntu 12.04 64-bit on a HP Envy 15 1104tx. Most stuff appears to be working fine with the exception of the two USB3 ports (USB2 port works fine). I've read a lot of articles but so far have not been able to find a solution. I've tried adding 'pci=nomsi' to '/etc/default/grub'… >>> More
Strip parity bits in C from 8 bits of data followed by 1 parity bit

as seen on Stack Overflow - Search for 'Stack Overflow'
I have a buffer of bits with 8 bits of data followed by 1 parity bit. This pattern repeats itself. The buffer is currently stored as an array of octets. Example (p are parity bits): 0001 0001 p000 0100 0p00 0001 00p01 1100 ... should become 0001 0001 0000 1000 0000 0100 0111 00 ... Basically… >>> More

Developer IT