[Bug rtl-optimization/50904] [4.7 regression] pessimization when -fno-protect-parens is enabled by -Ofast

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904

--- Comment #37 from rguenther at suse dot de <rguenther at suse dot de> 2011-12-05 08:18:00 UTC ---
On Fri, 2 Dec 2011, burnus at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904
> 
> --- Comment #34 from Tobias Burnus <burnus at gcc dot gnu.org> 2011-12-02 17:06:57 UTC ---

[...]

> * * *
> 
> Back to the comment 0 issue: I still do not quite understand what the double
> evaluation (on tree level) of __builtin_pow in
>   D.1959_82 = ((D.2115_81));
>   D.1960_83 = __builtin_pow (D.1959_82, 2.0e+0);
>   D.1978_168 = __builtin_pow (D.2115_81, 2.0e+0);
> has to do with the -Ofast slow down. If I have understood it correctly, on tree
> level, there is no reason for it while the slow-down happens on RTL level.

Indeed I can find no other difference on the tree level (thus, no
invariant motion missed optimization that isn't present with both
-f[no-]protect-parens).

> That -fprotect-parens makes it faster is a mere coincidence. Is that a correct rough
> summary?

Yes.

Thus, I think if at the RTL level we see a missed invariant motion then
this is a RTL level bug (esp. if it only triggers with 
-fno-protect-parens).

Richard.