[PATCH] rtlanal: Correct cost regularization in pattern_cost

[HAO CHEN GUI]

Hi,
   The cost return from set_src_cost might be zero. Zero for
pattern_cost means unknown cost. So the regularization converts the zero
to COSTS_N_INSNS (1).

   // pattern_cost
   cost = set_src_cost (SET_SRC (set), GET_MODE (SET_DEST (set)), speed);
   return cost > 0 ? cost : COSTS_N_INSNS (1);

   But if set_src_cost returns a value less than COSTS_N_INSNS (1), it's
untouched and just returned by pattern_cost. Thus "zero" from set_src_cost
is higher than "one" from set_src_cost.

  For instance, i386 returns cost "one" for zero_extend op.
    //ix86_rtx_costs
    case ZERO_EXTEND:
      /* The zero extensions is often completely free on x86_64, so make
         it as cheap as possible.  */
      if (TARGET_64BIT && mode == DImode
          && GET_MODE (XEXP (x, 0)) == SImode)
        *total = 1;

  This patch fixes the problem by converting all costs which are less than
COSTS_N_INSNS (1) to COSTS_N_INSNS (1).

  Bootstrapped and tested on x86 and powerpc64-linux BE and LE with no
regressions. Is it OK for the trunk?

Thanks
Gui Haochen

ChangeLog
rtlanal: Correct cost regularization in pattern_cost

For the pattern_cost (insn_cost), the smallest known cost is
COSTS_N_INSNS (1) and zero means the cost is unknown.  The method calls
set_src_cost which might returns 0 or a value less than COSTS_N_INSNS (1).
For these cases, pattern_cost should always return COSTS_N_INSNS (1).
Current regularization is wrong and a value less than COSTS_N_INSNS (1)
but larger than 0 will be returned.  This patch corrects it.

gcc/
        * rtlanal.cc (pattern_cost): Return COSTS_N_INSNS (1) when the cost
        is less than COSTS_N_INSNS (1).

patch.diff
diff --git a/gcc/rtlanal.cc b/gcc/rtlanal.cc
index 4158a531bdd..f7b3d7d72ce 100644
--- a/gcc/rtlanal.cc
+++ b/gcc/rtlanal.cc
@@ -5762,7 +5762,7 @@ pattern_cost (rtx pat, bool speed)
     return 0;

   cost = set_src_cost (SET_SRC (set), GET_MODE (SET_DEST (set)), speed);
-  return cost > 0 ? cost : COSTS_N_INSNS (1);
+  return cost > COSTS_N_INSNS (1) ? cost : COSTS_N_INSNS (1);
 }

 /* Calculate the cost of a single instruction.  A return value of zero