[Bug target/36713] [4.4 regression] r137252 breaks -O2 optimization on x86_64-unknown-linux-gnu

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #12 from rguenther at suse dot de  2008-07-03 12:54 -------
Subject: Re:  [4.4 regression] r137252 breaks -O2 optimization
 on x86_64-unknown-linux-gnu

On Thu, 3 Jul 2008, dfranke at gcc dot gnu dot org wrote:

> ------- Comment #11 from dfranke at gcc dot gnu dot org  2008-07-03 12:45 -------
>> I guess the assembly is different, right?
>
> Yes, it is. Comparing *.s files from FCFLAGS="-O2 -save-temps", there are
> differences in one file - though not the one suspected in comment #7.
>
> There is one difference in the .optimized file that I could easily identify:
> -  D.1935 = dummy_atom_model_inertia_tensor (this, phase);
> -  __result_dummy_atom_model_rad.21 = inertia_tensor_to_rg (&D.1935);
> +  D.1935 = dummy_atom_model_inertia_tensor (this, phase) [return slot
> optimization];
> +  __result_dummy_atom_model_rad.21 = inertia_tensor_to_rg (&D.1935) [tail
> call];

Ok, that is an interesting difference.  Can you try if 
-fno-optimize-sibling-calls fixes it (that should remove the [tail call]).
Unfortunately there is no option to disable the return slot optimization,
so you have to disable that in the source (tree-nrv.c, just return early
from execute_return_slot_opt).

Obviously the above may be a valid extra optimization ...

Richard.

>
> And there are plenty in *.alias, e.g.:
>  Aliased symbols
>
>  -D.1935, UID D.1935, struct inertia_tensor, is addressable, score: 8, direct
> reads: 0, direct writes: 0, indirect reads: 0, indirect writes: 2, call
> clobbered (passed to call)
>  +D.1935, UID D.1935, struct inertia_tensor, is addressable, score: 0, direct
> reads: 0, direct writes: 0, indirect reads: 0, indirect writes: 0
>
> The difference file has 3000+ lines here, one could argue that the dumps are
> significantly different ;)
>
>
>


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36713