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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Fri, 9 Aug 2024 07:57:17 -0300
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Em 8/8/2024 6:41 PM, Bart escreveu:
> 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).
> 
> 
> 
>
This part was always clear to me:

"They  are not performed 'in-place', any more than writing `a + 1` 
changes the value of `a`."

Lets take double to int.

In this case the bits of double needs to be reinterpreted (copied to) int.

So the answer "how it works" can be

always/generally machine has a instruction to do this

or.. this is defined by the IIE ... standard as ...