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Re: Massive performance regression from switching to gcc 4.5


On Fri, Jun 25, 2010 at 1:02 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> 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"?

Thus, do you have a testcase where your patch helps?

Richard.

> Richard.
>
>> --
>> Eric Botcazou
>>
>


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