On Mon, 2005-11-28 at 14:10 +0100, Paolo Bonzini wrote:
> Then, I don't know if it would be legal to optimize
>
> struct r {
> unsigned int x : 7;
> volatile unsigned int y : 1;
> };
>
> struct r my_reg;
>
> So that my_reg.x is accessed with a non-volatile mem, and my_reg.y is
> accessed with a volatile one. Would such an optimization be possible
> within the Ada compile-time rules?
procedure P is
type T7 is mod 2**7;
type T1 is mod 2;
pragma Volatile (T1);
type R is record
X : T7;
Y : T1;
end record;
for R'Size use 8;
for R'Alignment use 1;
for R use record
X at 0 range 0 .. 6;
Y at 0 range 7 .. 7;
end record;
Z : R;
A : T7;
B : T1;
begin
Z.X := 127;
Z.Y := 1;
A := Z.X;
B := Z.Y;
end P;
trunk gcc -O2 -S p.adb on x86-linux gives:
_ada_p:
pushl %ebp
movl %esp, %ebp
subl $16, %esp
movb $1, -1(%ebp)
leave
ret
My understanding is that B := Z.Y implied memory read should not be
optimized away since it is a volatile read and thus an external effect,
so this looks like a bug to me. (If this is not supported, GNAT should
fail at compile time.)
Z.X write and read are correctly optimized away: non volatile
read and write have no external effects here.
Laurent
