Re: Massive performance regression from switching to gcc 4.5

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, Jun 25, 2010 at 12:45 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> I do think so.
>
> Huh? ?What do your version and mine return for the following assignment?
>
> void foo (int i)
> {
> ?struct S s;
> ?s.a = i;
> }
>
>> Which in the following example makes i = *p not likely eliminated
>> but makes j = *q likely eliminated.
>>
>> void foo (int *p, struct X *q)
>> {
>> ? int i;
>> ? struct X j;
>> ? i = *p;
>> ? j = *q;
>> ? bar (&i, &q);
>> }
>>
>> That doesn't make sense.
>
> Yet that's what's supposed to be implemented, see the comment: "loads from
> parameters passed by reference".
>
>> What makes sense is that all scalar (thus gimple_reg_typed)
>> loads/stores to/from parameters or the result are free.
>
> Precisely not, they aren't free, otherwise they wouldn't exist in the first
> place. ?Scalar loads/stores are never free, aggregate loads/stores may be
> free if they are created only to pass the object around.

Err.  aggregate loads/stores do not appear because aggregate
uses can appear in calls.

Scalar uses cannot appear in calls and thus you see them as
separate statements.

Thus,

struct X;
void bar(struct X);
void foo(struct X&x)
{
  bar (x);
}

will appear as a single call stmt while

void bar (int);
void foo(int &x)
{
  bar (x);
}

will have a load that is not supposed to be "free"?

Richard.

> --
> Eric Botcazou
>