Laurent GUERBY - Re: Why doesn't combine like volatiles? (volatile_ok again, sorry!) (original) (raw)

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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

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