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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Thu, 8 Aug 2024 22:41:47 +0100
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On 08/08/2024 21:34, Thiago Adams wrote:
> On 08/08/2024 16:42, Keith Thompson wrote:
>> Thiago Adams <thiago.adams@gmail.com> writes:
>>> On 07/08/2024 17:00, Dan Purgert wrote:
>>>> On 2024-08-07, Thiago Adams wrote:
>> [...]
>>>>> How about floating point?
>>>> Floating point is a huge mess, and has a few variations for
>>>> encoding;
>>>> though I think most C implementations use the one from the IEEE on 1985
>>>> (uh, IEEE754, I think?)
>>>
>>> I didn't specify properly , but my question was more about floating
>>> point registers. I think in this case they have specialized registers.
>>
>> Who is "they"?
>>
>> Some CPUs have floating-point registers, some don't.  C says nothing
>> about registers.
>>
>> What exactly is your question?  Is it not already answered by reading
>> the "Conversions" section of the C standard?
>>
> 
> 
> This part is related with the previous question about the origins of 
> integer promotions.
> 
> We don't have "char" register or signed/unsigned register. But I believe 
> we may have double and float registers. So float does not need to be 
> converted to double.
> 
> There is no specif question here, just trying to understand the 
> rationally behind the conversions rules.

The rules have little to do with concrete machines with registers.

Your initial post showed come confusion about how conversions work. They 
are not performed 'in-place', any more than writing `a + 1` changes the 
value of `a`.

Take:

     int a; double x;

     x = (double)a;

The cast is implicit here but I've written it out to make it clear. My C 
compiler produces intermediate code like this before converting it to 
native code:

     push x   r64                   # r64 means float64
     fix      r64 -> i32
     pop  a   i32

I could choose to interprete this code just as it is. Then, in this 
execution model, there are no registers at all, only a stack that can 
hold data of any type.

The 'fix' instruction pops the double value from the stack, converts it 
to int (which involves changing both the bit-pattern, and the 
bit-width), and pushes it back onto the stack.

Registers come into it when running it directly on a real machine. But 
you seem more concerned with safety and correctness than performance, so 
there's probably no real need to look at actual generated native code.

That'll just be confusing (especially if you follow the advice to 
generate only optimised code).