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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: how cast works?
Date: Thu, 8 Aug 2024 19:01:56 +0100
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On 08/08/2024 18:50, Thiago Adams wrote:
> On 08/08/2024 14:29, Bart wrote:
>> On 08/08/2024 17:32, Michael S wrote:
>>  > On Thu, 8 Aug 2024 14:23:44 +0100
>>  > Bart <bc@freeuk.com> wrote:
>>  >> Try godbolt.org. Type in a fragment of code that does different kinds
>>  >> of casts (it needs to be well-formed, so inside a function), and see
>>  >> what code is produced with different C compilers.
>>  >>
>>  >> Use -O0 so that the code isn't optimised out of existence, and so
>>  >> that you can more easily match it to the C ource.
>>  >>
>>  >>
>>  >
>>  >
>>  > I'd recommend an opposite - use -O2 so the cast that does nothing
>>  > optimized away.
>>  >
>>  > int foo_i2i(int x) { return (int)x; }
>>  > int foo_u2i(unsigned x) { return (int)x; }
>>  > int foo_b2i(_Bool x) { return (int)x; }
>>  > int foo_d2i(double x) { return (int)x; }
>> The OP is curious as to what's involved when a conversion is done. 
>> Hiding or eliminating code isn't helpful in that case; the results can 
>> also be misleading:
>>
>> Take this example:
>>
>>    void fred(void) {
>>     _Bool b;
>>       int i;
>>       i=b;
>>    }
>>
>> Unoptimised, it generates this code:
>>
>>          push    rbp
>>          mov     rbp, rsp
>>
>>          mov     al, byte ptr [rbp - 1]
>>          and     al, 1
>>          movzx   eax, al
>>          mov     dword ptr [rbp - 8], eax
>>
>>          pop     rbp
>>          ret
>>
>> You can see from this that a Bool occupies one byte; it is masked to 
>> 0/1 (so it doesn't trust it to contain only 0/1), then it is widened 
>> to an int size.
>>
>> With optimisation turned on, even at -O1, it produces this:
>>
>>          ret
>>
>> That strikes me as rather less enlightening!
>>
>> Meanwhile your foo_b2i function contains this optimised code:
>>
>>          mov     eax, edi
>>          ret
>>
>> The masking and widening is not present. Presumably, it is taking 
>> advantage of the fact that a _Bool argument will be converted and 
>> widened to `int` at the callsite even though the parameter type is 
>> also _Bool. So the conversion has already been done.
>>
>> You will see this if writing also a call to foo_b2i() and looking at 
>> the /non-elided/ code.
>>
>> The unoptimised code for foo_b2i is pretty awful (like masking twice, 
>> with a pointless write to memory between them). But sometimes with gcc 
>> there is no sensible middle ground between terrible code, and having 
>> most of it eliminated.
>>
>> The unoptimised code from my C compiler for foo_b2i, excluding 
>> entry/exit code, is:
>>
>>      movsx   eax, byte [rbp + foo_b2i.x]
>>
>> My compiler assumes that a _Bool type already contains 0 or 1.
>>
>>
>>
> 
> If you are doing constant expression in your compiler, then you have the 
> same problem (casts) I am solving in cake.
> 
> For instance
> static_assert((unsigned char)1234 == 210);
> 
> is already working in my cake. I had to simulate this cast.
> 
> Previously, I was doing all computations with bigger types for constant 
> expressions. Then I realize compile time  must work as the runtime.
> 
> For constexpr the compiler does not accept initialization invalid types.
> for instance.
> 
>   constexpr char s = 12345;
> 
> <source>:6:21: error: constexpr initializer evaluates to 12345 which is 
> not exactly representable in type 'const char'
>      6 |  constexpr char s = 12345;
> 
> 
> I am also checking all wraparound and overflow in constant expressions.
> I have a warning when the computed value is different from the math value.

In my C compiler I have no constexpr (don't know why you got that 
impression). And I don't check that initialisers for integer types 
overflow their destination.

This is because within the language in general:

     char c; int i;

     c = i;

Such an assignment is not checked at runtime (and I don't know if this 
can be warned against, or if a runtime check can be added).

This is just how C works: too-large values are silently truncated (there 
are worse aspects of the language, like being able to do `int i; 
(&i)[12345];`).

But you are presumably superimposing a new stricter language on top. In 
the case, if my `c = i` assignment was not allowed, how do I get around 
that; by a cast?