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The C23 edition of Modern C

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515 points bwidlar | 6 comments | 15 Oct 24 16:06 UTC | HN request time: 0.976s | source | bottom
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belter ◴[15 Oct 24 17:30 UTC] No.41850897[source]▶
Important reminder just in the Preface :-)

Takeaway #1: "C and C++ are different: don’t mix them, and don’t mix them up"

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jpcfl ◴[15 Oct 24 17:44 UTC] No.41851047[source]▶
Bjarne should have called it ++C.
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card_zero ◴[15 Oct 24 18:09 UTC] No.41851328[source]▶
Because people choose to use pre-increment by default instead of post-increment?

Why is that?

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wpollock ◴[15 Oct 24 21:52 UTC] No.41853498[source]▶
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.)
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1. zabzonk ◴[16 Oct 24 02:47 UTC] No.41855124[source]▶
Please explain what you mean by "a separate instruction".
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2. spc476 ◴[16 Oct 24 04:53 UTC] No.41855741[source]▶
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.
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3. wang_li ◴[16 Oct 24 15:53 UTC] No.41860562[source]▶
> 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.

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4. wpollock ◴[16 Oct 24 22:47 UTC] No.41864639[source]▶
A MOV and an INC, as opposed to just the MOV.
5. spc476 ◴[18 Oct 24 06:11 UTC] No.41876747{3}[source]▶
It hurts less if you think of it like assignment:

    al = [rsi]
    [rdi] = al
6. int_19h ◴[18 Oct 24 15:35 UTC] No.41880372{3}[source]▶
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?