Re: [PATCH] Tree SRA and atomicity/volatility

["Richard Guenther" <richard.guenther@gmail.com>]

On 1/12/07, Richard Guenther <richard.guenther@gmail.com> wrote:
> On 1/12/07, Eric Botcazou <ebotcazou@adacore.com> wrote:
> > [Sorry for the delay, pretty hot week]
> >
> > > On a second thought this cannot be true.  If you have "all the magic
> > > to support atomicity" and generate "the full-word access on" your own --
> > > why do you expose the individual fields to the middle-end at all??
> >
> > Well, the type has fields and you can access these fields individually in the
> > source so they must be present in the IL to generate debug info for them.
> >
> > > I claim you _cannot_ generate full-word access this way (unless
> > > using memcpy, but even that is nowadays lowered).
> >
> > Do not underestimate the cleverness of Gigi. :-)
>
> I don't see anything "clever" in
>
> Q ()
> {
>   typedef q__byte q__byte;
>   typedef struct q__word q__word;
>   struct q__word T1b = {.first=0, .second=0, .third=0, .fourth=0};
>   struct q__word tmp;
>   struct q__word external = VIEW_CONVERT_EXPR<struct q__word>(SAVE_EXPR <T1b>);
>
>   tmp = VIEW_CONVERT_EXPR<struct q__word>(external);
>   tmp.first = 0;
>   external = VIEW_CONVERT_EXPR<struct q__word>(tmp);
>   return;
> }
>
> for this testcase (t02.original dump, gcc 4.1.2) .  Now if I change the
> testcase to something I requested (direct write to a component), I get
>
> Q ()
> {
>   typedef q__byte q__byte;
>   typedef struct q__word q__word;
>   struct q__word T1b = {.first=0, .second=0, .third=0, .fourth=0};
>   struct q__word external = VIEW_CONVERT_EXPR<struct q__word>(SAVE_EXPR <T1b>);
>
>   VIEW_CONVERT_EXPR<struct q__word>(external).first = 0;
>   return;
> }
>
> and after gimplification we can see all it broken already (that is, you
> exposed the component write to the middle-end - no cleverness
> prevented this):
>
> Q ()
> {
>   struct q__word T1b.0;
>   typedef q__byte q__byte;
>   typedef struct q__word q__word;
>   struct q__word T1b;
>   struct q__word external;
>
>   T1b.first = 0;
>   T1b.second = 0;
>   T1b.third = 0;
>   T1b.fourth = 0;
>   T1b.0 = T1b;
>   external = T1b.0;
>   external.first = 0;
>   return;
> }
>
> The testcase now looks like:
>
> procedure Q is
>
>    type Byte is mod 2**8;
>    for Byte'Size use 8;
>
>    type Word is
>       record
>          First, Second, Third, Fourth : Byte;
>       end record;
>
>    External : Word := (others => 0);
>    pragma Atomic (External);
>
> begin
>    External.First := 0;
> end;
>

Oh, and of course wrong (?) assembly is created for this:

_ada_q:
.LFB3:
        pushl   %ebp
.LCFI0:
        movl    %esp, %ebp
.LCFI1:
        subl    $16, %esp
.LCFI2:
        movl    $0, -4(%ebp)
        movb    $0, -4(%ebp)
        leave
        ret

Richard.