Strange AV in asm mode (code only for amd64)

[Eugene Wissner]

On Sunday, 5 November 2017 at 13:43:15 UTC, user1234 wrote:
> Hello, try this:
>
> ---
> import std.stdio;
>
> alias Proc = size_t function();
>
> size_t allInnOne()
> {
>     asm pure nothrow
>     {
>         mov RAX, 1;
>         ret;
>         nop;nop;nop;nop;nop;nop;nop;
>         mov RAX, 2;
>         ret;
>     }
> }
>
> void main()
> {
>     Proc proc1 = &allInnOne;
>     Proc proc2 = cast(Proc) (cast(void*)proc1 + 16);
>     writeln(proc1(), " ", proc2());
> }
> ---
>
> The call to proc1() gens a SEGFAULT after the first RET.
> Remove the call to proc1() and it works.
>
> Why that ?

One of the problems is that "naked" is missing in your assembly. 
If you write

asm pure nothrow
{
      naked;
      mov RAX, 1;
      ret;
      nop;nop;nop;nop;nop;nop;nop;
      mov RAX, 2;
      ret;
}

writeln(proc1()) works. Without "naked" dmd generates the 
prologue and epilogue for your function. Inside the assembly you 
return from the function without restoring the stack. It causes 
the segfault. So you have to write the prologue before returning 
or use nacked assembly.
With "naked" and "Proc proc2 = cast(Proc) (cast(void*)proc1 + 
8);" the example works.