Re: [Algorithms] Shifting or Multiplication....

[Pierre T. <p.t...@wa...>]

On plain Pentiums, shifting was 0.5 cycle in the U pipe only (so there =
was a 0.5 cycle latency if in the V pipe). Integer multiplications were =
about ~10 cycles. Nowadays, even if rules have changed, you shouldn't =
discard bitshifts :

1) you never know where your program will run in the end
2) using a bitshift for masks and similar things is conceptually clearer =
anyway. Keep your code clean!
3) you probably code in C/C++, and most of the time the compiler handles =
this for you anyway (*)
4) there's more to optimization than this :)
5) you can't do the same as SHLD / SHRD with muls, ah!

(*) As a reminder, even "slow" things like:

    int Value;
    ...
    Value*=3D5;

...are cleverly compiled using the good old LEA instruction (that is: =
without shifts or muls). Nowadays I would leave all those things to the =
compiler.


Pierre Terdiman      *   Home: p.t...@wa...
Coder in the dark    *   Zappy's Lair:  www.codercorner.com


  ----- Original Message -----=20
  From: Alex Morano=20
  To: gda...@li...=20
  Sent: Saturday, December 23, 2000 9:38 PM
  Subject: [Algorithms] Shifting or Multiplication....


      I was wondering if anyone could clear up something. =
Algorithmically, is bitshifting or using multiplication faster?? Now, I =
know this sounds like a subjective question, as some will undoubtadly =
say, it depends on the compiler and what it translates the code into. =
Lets take it down to the ASM level.
  =20
      Shifting, to me, looks like a 1 cycle operation.
  =20
      In this age, if the CPU can handle most MUL's almost as rapidly, =
is there a place for bitshifts? If so, then for what kinds of algo =
applications? Do we still need them to mask off bits in pixels colors?