[Bug rtl-optimization/47270] [4.4/4.5/4.6 Regression] GCC produces unnecessary/wrong code on -O2 and -O3 levels

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47270

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P4
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-01-17 12:06:11 UTC ---
Why do you think it is wrong?  If %esi is non-zero upon entry, then it is just
moved to %eax and back, so it isn't changed, if it was zero upon entry, then 0
is loaded into it.
The reason for the extra code is that pre attempts to optimize it (register
vars are not SSA vars), so we get at *.optimized:
  int prephitmp.4;
  int r.0;

<bb 2>:
  r.0_1 = r;
  if (r.0_1 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  __asm__("sar  %0" : "=r" r : "0" r.0_1);
  prephitmp.4_2 = r;

<bb 4>:
  # prephitmp.4_8 = PHI <0(2), prephitmp.4_2(3)>
  __asm__("sar  %0" : "=r" r : "0" prephitmp.4_8);
and RTL optimizations aren't able to undo that.  I doubt anything can be done
about this easily, with -fno-tree-pre you get your expected output.