DMD: Why bytes, etc. are promoted to ints (if not D linkage)

[Adam D. Ruppe]

On Saturday, 14 March 2020 at 15:27:50 UTC, Luhrel wrote:
>     // Promote bytes, words, etc., to ints
>     arg = integralPromotions(arg, sc);
> ---
>
> What was to purpose of this line ?

The C spec requires integers to be promoted to int in such cases. 
C style variadics will be expecting something of `int` size or 
larger.

You can printf("%hd", some_short) but C still expects some_short 
to be promoted to int before passed so it will look at 32 bits on 
the abi level.

```
#include<stdio.h>

int main() {
         short a = 10;
         printf("%hd\n", a);
}
```

gcc compiling that will disassemble to:

```
0000000000401112 <main>:
   401112:       55                      push   rbp
   401113:       48 89 e5                mov    rbp,rsp
   401116:       48 83 ec 10             sub    rsp,0x10
   40111a:       66 c7 45 fe 0a 00       mov    WORD PTR 
[rbp-0x2],0xa
   401120:       0f bf 45 fe             movsx  eax,WORD PTR 
[rbp-0x2]
   401124:       89 c6                   mov    esi,eax
   401126:       bf 04 20 40 00          mov    edi,0x402004
   40112b:       b8 00 00 00 00          mov    eax,0x0
   401130:       e8 fb fe ff ff          call   401030 <printf at plt>
   401135:       b8 00 00 00 00          mov    eax,0x0
   40113a:       c9                      leave
   40113b:       c3                      ret
   40113c:       0f 1f 40 00             nop    DWORD PTR [rax+0x0]
```

Notice the `movsx eax`, move with sign extension, into a 32 bit 
register, which is then passed as the argument, despite the 
variable itself being a 16 bit short. C promotes it too for the 
variadic, so D must do the same.