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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: question about linker
Date: Tue, 26 Nov 2024 20:00:48 +0000
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On 26/11/2024 19:38, Thiago Adams wrote:
> On 26/11/2024 16:18, Bart wrote:
>> On 26/11/2024 18:35, Thiago Adams wrote:
>>>
>>> (I think I know the answer but I would like to learn more.)
>>>
>>> I am using C89 as "compiler backend intermediate language".
>>>
>>> I want a very simple output that could facilitate the construction of 
>>> a simple C89 compiler focused on code generation.
>>>
>>> I am removing these features from the generated code.
>>>
>>>   - typedef
>>>   - enum
>>>   - preprocessor
>>>   - const
>>
>> I don't use them in generated code either. (Only in a brief section at 
>> the top to define my prefered type designations.)
> 
> 
> I am generating the prototypes for all functions called.
> No includes, no macros.
> 
> The generated code is depending on compiler flags , platform and 
> headers. It is intended to direct usage like in the pipeline
> 
> use my_compiler -> C89 -> CC
> 
> 
>>
>>>
>>> At this output, I am generating the prototypes for the functions I call.
>>>
>>> For instance,
>>> int strcmp( const char* lhs, const char* rhs );
>>>
>>> is generated as
>>>
>>> int strcmp( char* lhs, char* rhs );
>>
>> I don't use #include either, not even for standard headers, although 
>> gcc doesn't like it when I define my own std library functions. There 
>> are ways to shut it up though.
>>
>>
>>> I am also generating the structs as required (on demand). For the 
>>> structs I am renaming it because I am generating all structs at 
>>> global scope.
>>>
>>> Question:
>>> Does the compiler/linkers? Cares If I am lying about const? Or If 
>>> rename the structs?
>>>
>>> I think it does not care, and it seems to work (it compiles and run).
>>
>> I don't know why the linker would care about anything. All it sees are 
>> symbol imports and exports.
>>
>> A compiler might care about lack of 'const' in that it could stop it 
>> doing doing some optimisations.
>>
>> But it can't report a type mismatch between 'const' and non-'const' 
>> types if 'const' has been banished completely.
>>
>>
> 
> I think GCC has some builtin functions and he can complain if the 
> function prototype differs.
> 
> Do you have any idea what else can be simplified when creating a C 
> compiler?

What are you asking; are you thinking of writing one? Because C 
compilers already exist!

If so, think of what you would find troublesome. I could create a long 
list of things that makes C harder to compile than my own language.

> 
> I was thinking about literal strings.
> 
> Instead of
> 
> f("abc");
> 
> Generating something like
> 
> char literal_string_1[] = {'a', 'b', 'c', '\0' }; //or numbers global
> 
> f(literal_string_1);

I wouldn't bother with this. How hard is it to deal with string 
constants? Even a 4KB BASIC from a microcomputer had them!

If you want to simplify, perhaps get rid of 'A' constants. (Unless you 
think you're likely to be running on an EBCDIC system. However in my 
compilers, 'A' gets turned into 65 early on anyway.)

> The reason I believe compilers already have to do this right? (put 
> strings in a data section)

Data like {'a', 'b', 'c', 0} will be put into in a .data segment. But 
"abc" will be put by gcc into a .rodata segment, so it's safer. Maybe 
'const' will fix the former, but you no longer have that.


> So this was one extra simplification I was thinking about.
> Also remove  loops (for , while) and switches.

This sort of simplification is of more benefit when /generating/ C code. 
It's not usually something to worry about in the tool that will turn 
that intermediate C into native code.

This comes back to the question above.