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NNTP-Posting-Date: Tue, 26 Nov 2024 03:59:30 +0000
Subject: Re: Joy of this, Joy of that
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From: "186282@ud0s4.net" <186283@ud0s4.net>
Organization: wokiesux
Date: Mon, 25 Nov 2024 22:59:29 -0500
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On 11/25/24 4:07 AM, Richard Kettlewell wrote:
> "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.
> 

   Heh ... much of my 'C' looks like the top example, all
   straight-forward and readable. As I said somewhere, 'C'
   was the neat-o new lang back when I got started in things
   so I strongly trend towards the K&R look and feel even now.
   SO easy to write incomprehensible 'C' !

   I agree that the optimization tech has become VERY good
   these days. That final example shows how tight that
   particular bit CAN be made. How the compiler figures
   that out - NO idea ! Messing with microcontrollers
   can be helpful in teaching how to shrink ASM - ya always
   wanna save a few bytes, a few cycles.

   "Tight memory" means "Only four gigabytes" to the
   Gen-Z/A2 crowd  :-)