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From: BGB <cr88192@gmail.com>
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Subject: =?UTF-8?Q?Re=3A_technology_discussion_=E2=86=92_does_the_world_need?=
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Date: Wed, 10 Jul 2024 12:59:59 -0500
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On 7/10/2024 2:41 AM, David Brown wrote:
> On 09/07/2024 23:43, BGB wrote:
>> On 7/9/2024 3:22 PM, James Kuyper wrote:
>>> On 7/9/24 14:55, BGB wrote:
>>> ...
>>>> The pass by reference, in this context, was referring to the ABI, not
>>>> to C itself.
>>>>
>>>> It looks from C's POV as-if it were by-value.
>>>>
>>>>
>>>> Which it is, depends on if one is looking at things at the language
>>>> level, ABI level, or IR level, ...
>>>
>>> The C standard doesn't explicitly specify pass by value, or pass by
>>> reference, or anything other passing mechanism. What it does say is what
>>> a programmer needs to know to use the passing mechanism. It says that
>>> the value of a function parameter that is seen by the code inside that
>>> function is a copy of the value passed as an argument to the function.
>>> The copy can be modified without changing the original. When a C
>>> function's declaration looks as though it takes an array as an argument,
>>> what that declaration actually means is that it takes a pointer value as
>>> an argument, and it is a copy of that pointer's value which is seen
>>> inside the function, and can be modified. The memory it points at is the
>>> same as the memory pointed at by the corresponding argument.
>>>
>>>
>>
>>
>> We can probably agree that, in C:
>>    typedef struct Foo_s Foo;
>>    struct Foo_s {
>>      int x, y, z, a, b, c;
>>    };
>>
>>    int FooFunc(Foo obj)
>>    {
>>      obj.z = obj.x + obj.y;
>>      return(obj.z);
>>    }
>>
>>    int main()
>>    {
>>      Foo obj;
>>      int z1;
>>      obj.x=3;
>>      obj.y=4;
>>      obj.z=0;
>>      z1=FooFunc(obj);
>>      printf("%d %d\n", obj.z, z1);
>>    }
>>
>> Should print "0 7" regardless of how the structure is passed in the ABI.
>>
> 
> ABI's are irrelevant to how the language is defined and how these 
> expressions are evaluated.  ABI's go along with details of the target - 
> they can affect implementation-dependent behaviour but no more than 
> that.  (A clear example would be that alignment of fundamental types 
> would normally be specified by an ABI.)
> 
> So code that does not depend on implementation-dependent behaviour, such 
> as your code here, will necessarily give the same results on all 
> conforming C implementations.
> 

These sorts of things may bleed through depending on implementation 
choices (and "optimizations"). Part of the compiler writing fun is 
trying to get good performance while also not breaking C semantics.

But, a language more aggressively tuned for performance would have even 
more wonky semantic edge cases than it does already.


Though, one thing is that some of my code (both my Boot ROM) and 
kernel/shell program, has a bunch of sanity check stuff to try to detect 
if ISA features are broken or if the compiler breaks C semantics in 
various ways. But, there are limits to how much coverage there is with 
such checks.


>>
>> Though, one possibility being to relax the language such that both "0 
>> 7" and "7 7" are valid possibilities (the latter potentially allowing 
>> more performance by not needing to make a temporary local copy). 
>> Though, AFAIK, C doesn't really allow for this.
> 
> It continues to astound me that people who claim to have written C 
> compilers themselves - such as you and Bart - regularly show 
> misunderstandings or ignorance about things that are clear in the C 
> standards and which I would expect any experienced C programmer to know.
> 
> All arguments to function calls in C are passed by value.  This is in 
> 6.5.2.2p4 - it is a two sentence paragraph that you really ought to have 
> read before even considering writing a C compiler.
> 
> Some languages do have pass by reference (or other types of parameter 
> passing systems), and would give "7 7".  C is not such a language.  (And 
> I have never heard of a language that would either result.)
> 

Note that I did say: "C doesn't really allow for this".

As-in, such a tweak would allow things to be slightly faster, but would 
violate the language rules as it stands.


>>
>> An implementation could be clever though and only make local copies in 
>> cases where the structure is modified by the callee, as in the example 
>> above.
>>
> 
> A clever implementation would turn the whole of main() into a single 
> puts("0 7") call.
> 
> Compliers can generate whatever code they like, as long as the results 
> are correct hin the end.
> 

Not all forms of "clever" are equal though...

In this case, cleverness is more at the level of things like:
Using LEA rather than memory loads if the address is statically known;
Turning small memcpy into register loads and stores;
Caching values in registers rather than always loading/storing from memory;
....

Though, ironically, despite having a lot of significant limitations in 
my compiler (like, it not being smart enough to figure out loop 
unrolling or inline functions), performance isn't going too poorly (at 
least vs GCC+RV64G, particularly in PIC/PIE mode).

Though, much of this is because the lack of register-indexed addressing 
is a bit of a boat anchor for many programs.

Also the way PIC and PIE are designed penalizes the use of global 
variables (by first requiring the address to be loaded from the GOT 
before it can then be accessed), which some amount of the code I have 
ported uses extensively.


>