[Vm-dev] [OpenSmalltalk/opensmalltalk-vm] passing struct by value does not respect x86_64 SysV ABI (#443)

[Nicolas Cellier]

For passing parameters by value, my understanding is that we can pass up to 8 _eigthbytes_ individually for each struct (either via register if struct fits on two eightbytes, and if we still have available registers, or via stack...). Only for more than 8 eightbytes, we will pass by MEMORY (a pointer to struct).

Here is a little example:

```
double adi(sdi x) { return (double) x.a + (double) x.b ; }
double adi_2(sdi x,sdi y) { return adi(x) + adi(y); }
double adi_4(sdi x,sdi y,sdi z,sdi t) { return adi_2(x,y) + adi_2(z,t); }

double aslf(sslf x) { return (double) x.a + (double) x.b + (double) x.c ; }
double aslf_2(sslf x,sslf y) { return aslf(x) + aslf(y); }
double aslf_4(sslf x,sslf y,sslf z,sslf t) { return aslf_2(x,y) + aslf_2(z,t); }
```

double-int are passed by registers (by value) because fitting on 2 eightbytes
short-long-float are passed via stack (by value) because requiring 3 eightbytes
We should try longer (> 8 eightbytes) and verify that they are passed by pointer...

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