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Re: Massive performance regression from switching to gcc 4.5
On Fri, Jun 25, 2010 at 12:45 PM, Eric Botcazou <firstname.lastname@example.org> 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
void bar(struct X);
void foo(struct X&x)
will appear as a single call stmt while
void bar (int);
void foo(int &x)
will have a load that is not supposed to be "free"?
> Eric Botcazou