Re: technology discussion → does the world need a "new" C ?

On 09/07/2024 15:31, David Brown wrote:

On 08/07/2024 19:39, BGB wrote:

Though, this one seems to be a common point of divergence between "SysV" and "Microsoft" ABIs. Sometimes a target will have an ABI defined, and the MS version was almost the same, just typically differing in that it passes structs by reference and provides a spill space for register arguments.
>

 I don't think it is helpful that you keep mixing /logical/ terms with /implementation/ terms.
 In C, there is no "pass by reference" or "return by reference".  It is all done by value.

Arrays are passed by reference:
   void F(int a[20]) {}
   int main(void) {
     int x[20];
     F(x);
   }
Although the type of 'a' inside 'F' will be int* rather than int(*)[20].

So if you have these structs and declarations :
 struct small { uint16_t a; uint16_t b; };
struct big { uint32_t xs[10]; };
 struct small foos(struct small y);
struct big foob(struct big y);
 Then compilers will typically implement "x = foos(y)" as though it were:
      extern uint32_t foos(uint32_t ab);
     uint32_t _1 = foos(y.a << 16) | (y.b);
     struct small x = { _1 >> 16, _1 & 0xffff };
 And they will typically implement "x = foosb(y)" as though it were:
      extern void foob(struct big * ret, const struct big * xs);
     struct big x;
     foob(&x, &y);

 From what I've seen, structs that are not small enough to be passed in registers, are copied to a temporary, and the address of that temporary is passed.
This seems to be the case even when the struct param is marked 'const'.
(My compiler won't create a copy when the parameter is 'const'. I assumed that was how gcc did it; I was wrong.)
This is for Win64 ABI, however an ABI will only say they are passed by reference; it will not stipulate making a copy. That is up to the language implementation.