Comment by wpollock

15 hours ago

The PDP-11 that C originally targeted had address modes to support the stack. Pre-increment and post-decrement therefore did not require a separate instruction; they were free. After the PDP-11 went the way of the dodo, both forms took a machine cycle so it (mostly) became a stylistic issue. (The two operators have different semantics, but the trend to avoid side-effects in expressions means that both are most often used in a single expression statement like "++x;" or "x++;", so it comes down to your preferred style.)

2 comments

wpollock

zabzonk 10 hours ago

Please explain what you mean by "a separate instruction".

spc476 7 hours ago

Some idiomatic C code to copy a string (I'm not saying this is good C code, but it's just an example):

    while(*d++ = *s++)
      ;

On the Motorola 68000 (based somewhat on the PDP-11) the code would look like:

    loop:       move.b  (a0)+,d0
                move.b  d0,(a1)+
                bne     loop

while on the x86 line, it would be:

    loop:       mov     al,[rsi]
                mov     [rdi],al
                inc     rsi     ; extra instruction!
                inc     rdi     ; extra instruction!
                cmp     al,0
                jne     loop

Yes, there are better ways to write that code for both the 68K and x86, but I hope this gets the point across.