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References: <bug-29789-10053@http.gcc.gnu.org/bugzilla/>
Subject: [Bug tree-optimization/29789] Missed invariant out of the loop with conditionals and shifts
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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #5 from rguenth at gcc dot gnu dot org  2007-04-20 15:33 -------
Note that this does fix the loop invariant motion only in the case of two
ifs can be merged (because that re-instantiates the A & (1 << B) form).  The
following parts are still not resolved:

void quantum_cnot(int control, int target, unsigned long *state, int size)
{
  int i;

  for(i=0; i<size; i++)
    {
      if (state[i] & ((unsigned long) 1 << control))
        state[i] ^= ((unsigned long) 1 << target);
    }
}

(and more similar loops in libquantum).  It would be nice if rtl loop-invariant
motion could detect this form:

(insn 23 22 24 4 (parallel [
            (set (reg:DI 67)
                (lshiftrt:DI (reg:DI 62 [ D.1992 ])
                    (subreg:QI (reg/v:SI 63 [ control ]) 0)))
            (clobber (reg:CC 17 flags))
        ]) 470 {*lshrdi3_1_rex64} (nil)
    (nil))

(insn 24 23 25 4 (parallel [
            (set (reg:SI 68)
                (and:SI (subreg:SI (reg:DI 67) 0)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) 301 {*andsi_1} (nil)
    (nil))

and move the invariant (1 << control).  It does move the (1 << target) which
looks like

(insn 30 28 31 5 (set (reg:DI 70)
        (const_int 1 [0x1])) 82 {*movdi_1_rex64} (nil)
    (nil))

(insn 31 30 32 5 (parallel [
            (set (reg:DI 69)
                (ashift:DI (reg:DI 70)
                    (subreg:QI (reg/v:SI 64 [ target ]) 0)))
            (clobber (reg:CC 17 flags))
        ]) 411 {*ashldi3_1_rex64} (nil)
    (expr_list:REG_EQUAL (ashift:DI (const_int 1 [0x1])
            (subreg:QI (reg/v:SI 64 [ target ]) 0))
        (nil)))


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29789