Re: Incorrect bitfield aliasing with Tree SSA

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

> Because it wouldn't be pruning it if the alias sets conflicted!

Well, just look at the first RTL dump for:

struct S { int i; };

int f (struct S *s, int *p)
{
  s->i = 0;
  *p = 1;
  return s->i;
}

and

package P is

  type S is record
    i : Integer;
  end record;

  type SP is access all S;

  type IP is access all Integer;

  function f (s : SP; p : IP) return Integer;

end P;
package body P is

  function f (s : SP; p : IP) return Integer is
  begin
    s.i := 0;
    p.all := 1;
    return s.i;
  end;

end P;

compiled at -O2 -gnatp -fno-tree-dominator-opts -fno-tree-store-ccp.


For the C testcase:

;; s->i = 0
(insn 7 6 0 t.c:5 (set (mem/s:SI (reg/v/f:DI 59 [ s ]) [3 <variable>.i+0 S4 
A32])
        (const_int 0 [0x0])) -1 (nil))

;; *p = 1
(insn 8 7 0 t.c:6 (set (mem:SI (reg/v/f:DI 60 [ p ]) [3 S4 A32])
        (const_int 1 [0x1])) -1 (nil))

;; return s->i
(insn 9 8 10 t.c:6 (set (reg:SI 62)
        (mem/s:SI (reg/v/f:DI 59 [ s ]) [3 <variable>.i+0 S4 A32])) -1 (nil))

i.e. everything is accessed with the alias set of 'int'.


For the Ada testcase:

;; s->i = 0
(insn 7 6 0 p.adb:5 (set (mem/s/j:SI (reg/v/f:DI 59 [ s ]) [4 <variable>.i+0 
S4 A32])
        (const_int 0 [0x0])) -1 (nil))

;; *p = 1
(insn 8 7 0 p.adb:6 (set (mem:SI (reg/v/f:DI 60 [ p ]) [2 S4 A32])
        (const_int 1 [0x1])) -1 (nil))

;; return s->i
(insn 9 8 10 p.adb:6 (set (reg:SI 62)
        (mem/s/j:SI (reg/v/f:DI 59 [ s ]) [4 <variable>.i+0 S4 A32])) -1 
(nil))

i.e. s->i is accessed with the alias set of 'S'.


Now put 'aliased' on the field

  type S is record
    i : aliased Integer;
  end record;

and you get:

;; s->i = 0
(insn 7 6 0 p.adb:5 (set (mem/s:SI (reg/v/f:DI 59 [ s ]) [2 <variable>.i+0 S4 
A32])
        (const_int 0 [0x0])) -1 (nil))

;; *p = 1
(insn 8 7 0 p.adb:6 (set (mem:SI (reg/v/f:DI 60 [ p ]) [2 S4 A32])
        (const_int 1 [0x1])) -1 (nil))

;; return s->i
(insn 9 8 10 p.adb:6 (set (reg:SI 62)
        (mem/s:SI (reg/v/f:DI 59 [ s ]) [2 <variable>.i+0 S4 A32])) -1 (nil))

like in C.  The discrepancy purely stems from DECL_NONADDRESSABLE_P.


> > I didn't invent it either, but everything is more or less documented.
>
> No, not really.

Yes, it is, that's how I've understood how this stuff works.

-- 
Eric Botcazou