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

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #22 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-12-02 11:50:39 UTC ---
One thing I notice (and that's the only difference I can spot at the tree
level) is that we do not CSE the **2s of


      a = sqrt((rect_inductor%v2%x - rect_inductor%v4%x)**2 +
(rect_inductor%v2%y -         &
                       rect_inductor%v4%y)**2 + (rect_inductor%v2%z -
rect_inductor%v4%z)**2)

and

      xxvec = rect_inductor%v2%x - rect_inductor%v4%x
      xyvec = rect_inductor%v2%y - rect_inductor%v4%y
      xzvec = rect_inductor%v2%z - rect_inductor%v4%z
      magnitude = sqrt(xxvec**2 + xyvec**2 + xzvec**2)

because while the former has PAREN_EXPRs the latter does not and we do not
consider

<bb 8>:
  D.2113_79 = rect_inductor_78(D)->v2.x;
  D.2114_80 = rect_inductor_78(D)->v4.x;
  D.2115_81 = D.2113_79 - D.2114_80;
  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);

D.1960_83 and D.1978_168 as equivalent (they are, value-wise, but we cannot
easily replace one with the other using our current value-numbering
machinery).  We could clevery see that ((x))**2 is equal to ((x**2))
but that would not help for seeing the CSE opportunity of the following
sum and sqrt either.

I wonder if Fortran, with -fprotect-parens, really has different
semantics for

 tem = 2 * a;
 c = b / tem;

vs.

 c = b / (2 * a);

?  Thus, is not every statement supposed to be wrapped in parens with
-fprotect-parens?  So that

 tem = 2 * a;

becomes

 tem = ( 2 * a );

implicitely?  I see that placing ()s at the toplevel of the relevant
stmts in the source has the desired effect of enabling CSE and
the tree level optimization differences vanish.

Thus, this is a question of 1) correctness of the -fprotect-parens
implementation in the frontend, 2) a question on what optimizations
we want to perform on protected expressions.

Relevant transform is CSE

 sqrt (x*x + y*y + z*z)

and

 sqrt (((x))*((x)) + ((y))*((y)) + ((z))*((z)))