[wpollock]

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.)

[zabzonk]

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

[spc476]

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.

[wang_li]

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

...

> loop: mov al,[rsi] mov [rdi],al

This hurts my brain. When we invent time machines I'm going to use it to go back and slap whoever at intel came up with that operand order.

[int_19h]

memcpy etc also take the destination as their first argument, and it mirrors the usual way we write assignments. Personally I always found Intel syntax to be more straightforward, but I think it's ultimately down to whatever one was exposed to first.

I wonder sometimes why we keep insisting on the "OP ARG, ARG" format in general for assembly. Why not something like `MOV X -> Y` that would make it absolutely clear and unambiguous? For that matter, why not `COPY X -> Y`, since that's what it actually does?

[spc476]

It hurts less if you think of it like assignment:

    al = [rsi]
    [rdi] = al

[wpollock]

A MOV and an INC, as opposed to just the MOV.