Re: Joy of this, Joy of that

"186282@ud0s4.net" <186283@ud0s4.net> writes:

  Note though ... almost NO compu-geeks these days
  know ASM. As such they will not be enlightened
  about 'C' in that fashion. Today's geeks mostly
  start with Python and MIGHT go a little further,
  likely Rust.
>
  Just sayin'
>
  Things changed between 1984 and 2024.
>
  We Old Guys can kinda look at 'C' and see
  the ASM it's going to become. Later gens
  do not.

It’s a common mental model for C, but it’s not accurate.  The language
spec leaves an awful lot of wiggle room for the generated code to
diverge from the “I can see the assembler” model and compilers take full
advantage of it.

A simple example, based on a historical Linux kernel vulnerability
(CVE-2009-1897):

    int f(int *xp) {
        int x = *xp;
        if(!xp)
          return 0;
        return x;
    }

In the “assembler” model it would compile to something like this:

       mov eax,dword ptr [rdi]
       cmp rdi,0
       jne L1
       mov eax,0
    L1:
       ret

In fact at -O2 the test on xp is optimized out:

       mov eax, dword ptr [rdi]
       ret

https://godbolt.org/z/caKeTMTxf to play further.

--
https://www.greenend.org.uk/rjk/