Re: GNAT not generating any code for sub‑program: known bug?

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Sun, 28 Apr 2013 09:14:55 +0200, Simon Wright <simon@pushface.org> a  
écrit:

> and I think that the problem is that Template actually has to use the
> inputs and outputs! So, while optimising, the compiler says
>
>    "int $0x80" doesn't use any of the inputs, so I can ignore them
>    "int $0x80" doesn't write to any of the outputs, so I can ignore them
>    Result isn't changed, so it's undefined
>    Therefore I don't need to do anything.

I thought about the same previously (not my first attempt), and retried  
what I tried before, with the same result:

     function Syscall_3
       (N  : Number_Type;
        P1 : P1_Type;
        P2 : P2_Type;
        P3 : P3_Type)
        return Result_Type
     is
        use System.Machine_Code;
        Result : Result_Type;
     begin
        Asm
          (Template =>
             "movl %1, %%eax"  & ASCII.LF &
             "movl %2, %%ebx"  & ASCII.LF &
             "movl %3, %%ecx"  & ASCII.LF &
             "movl %4, %%edx"  & ASCII.LF &
             "int $0x80"       & ASCII.LF &
             "movl %%eax, %0"  & ASCII.LF,
           Outputs => Result_Type'Asm_Output ("=g", Result),
           Inputs =>
             (Number_Type'Asm_Input ("g", N),
              P1_Type'Asm_Input     ("g", P1),
              P2_Type'Asm_Input     ("g", P2),
              P3_Type'Asm_Input     ("g", P3)),
           Clobber => "eax, ebx, ecx, edx",
           Volatile => True);

        return Result;

     end Syscall_3;

Here (except it's a generic to avoid error while copying similar things),  
the template contains all of the instructions loading registers, to be  
clear about the inputs the template uses. Then, I used a `Clobber` list  
[1][3] to explicitly notify the compiler of the registers used in the  
template and which must remains untouched, then I also added `Volatile =>  
True` [2][3] to notify the compiler it should avoid unwanted optimization.  
The result is still the same, and so, even if the generic instantiation is  
not inlined (the generic declaration is not inlined neither). That's  
enough for a a caller to be inlined for everything to be dropped, even if  
the above subprogram itself not inlined. I agree I must tell the compiler  
what it is dealing with and what not to do with it, but so far, I still  
feel that looks like a compiler bug (GCC's fault or GNAT's fault?). Will  
try something else later.


Side note: with the `type Low_High is array (Half) of  
Interfaces.Unsigned_32;` in your snippet, shouldn't you specify the array  
should be packed? Or may be that's already implied by the component type…  
will have to check the RM.


[1]: http://docs.adacore.com/gnat-unw-docs/html/gnat_ugn_38.html#SEC382
[2]: http://docs.adacore.com/gnat-unw-docs/html/gnat_ugn_38.html#SEC383
[3]: http://docs.adacore.com/gnat-unw-docs/html/gnat_rm_15.html#SEC626


-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University