[Info-vax] Rust as a HS language, was: Re: Quiet?

[Arne Vajhøj]

On 4/8/2022 12:13 PM, Johnny Billquist wrote:
> On 2022-04-08 15:33, Arne Vajhøj wrote:
>> But I was talking about something different. That the
>> C code and the generated instructions sort of "match".
>>
>> If you in C write:
>>
>> c = a + b;
>>
>> then you expect that it does an addition of
>> two pieces of data (integer or FP of some size
>> determined by declaration) and not anything else.
>>
>> In languages with operator overload and extension methods
>> that code may download a bitcoin mining program and
>> run it for 10 minutes.
> 
> Actually, you might get surprised about what C compilers might do with 
> this in some circumstances. If the size of the destination (c) is 
> smaller than a and/or b, and the compiler knows that the result is going 
> to be too large to fit, then you are again in UB. Which means the 
> compiler is allowed to do anything, including do nothing at all (even 
> though it might just be simple integer operations).
> So that "c = a + b;" might actually end up being optimized away totally.
> Which probably would surprise quite a number of people who wrote such a 
> line.
> 
> Or, as I had to explain to a colleague of mine at one point.
> If you have two unsigned values multiplied, and the resulting value is 
> stored in a signed destination, any check for a value less than zero can 
> be optimzied away, because obviously the value can never be less than 
> zero. Disregarding the fact that the actual hardware, if giving such a 
> multiplication, might wrap, and end up with a value that is actually 
> less than zero.
> The colleague had the test in his code, and it never triggered, but 
> printing the value out for sure showed that it was less than zero.
> 
> The compiler was technically allowed to do that optimization, since it's 
> UB. The writer of the code wanted to know if the value had wrapped. 
> Sorry, C does not actually allow you to find that out.

Yes.

But my point was not about something being optimized away
but about too much happening under the hood.

Arne