C++ compiler questions regarding increment operators -

why considered bad practice use prefix increment operator versus postfix operator old compilers? understanding modern compilers optimize old ones not. there particular reason 1 slower other, , reason left optimized in compiled code? if provide other details regarding operators , compilers i'd grateful, thanks,

by "old" must mean old.

the reason was, in days before real optimization, main system c/unix development pdp-11 , later vax-11. pdp , vax have pre-decrement , post increment addressing modes.

   movl r0, -(r1) ; decrements r1 4 , move value of r0 location.    movl (r1)+, r2 ; move value @ r1 r2 add 4 r1. 

these c equivalent of

  *(--a) = b ;   b = *(a++) ; 

in days of little optimization, these mapped underlying assembly instructions.

at same time these addressing modes did not exist opposite:

  movl (r0)-, r1   movl +(r0), r1 

therefore, there no hardware mapping

*(++a) = b ; b = *(a--) ; 

the -(rx) , (rx)+ addressing modes made easy implement downwards growing stacks.