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Re: Some wide-int review comments

On 11/11/2013 10:29 AM, Richard Biener wrote:
On Mon, Nov 11, 2013 at 4:04 PM, Kenneth Zadeck
<> wrote:
On 11/11/2013 09:42 AM, Richard Biener wrote:
On Mon, Nov 11, 2013 at 3:26 PM, Kenneth Zadeck
<> wrote:
On 11/11/2013 06:49 AM, Richard Biener wrote:
On Sat, Nov 9, 2013 at 3:23 PM, Kenneth Zadeck
On 11/08/2013 05:30 AM, Richard Sandiford wrote:
    From tree-vrp.c:

@@ -1893,6 +1884,10 @@ vrp_int_const_binop (enum tree_code code
         /* If the singed operation wraps then int_const_binop has done
            everything we want.  */
+  /* Signed division of -1/0 overflows and by the time it gets here
+     returns NULL_TREE.  */
+  else if (!res)
+    return NULL_TREE;
       else if ((TREE_OVERFLOW (res)
               && !TREE_OVERFLOW (val1)
               && !TREE_OVERFLOW (val2))

Why is this case different from trunk?  Or is it a bug-fix independent
of wide-int?
the api for division is different for wide-int than it was for
But this is using a tree API (int_const_binop) that didn't change
(it returned NULL for / 0 previously).  So what makes us arrive here
now?  (I agree there is a bug in VRP, but it shouldn't manifest itself
only on wide-int)

My reading of the code is that is that i changed int_const_binop to
null_tree for this case.
Trunk has:

      case TRUNC_DIV_EXPR:
      case FLOOR_DIV_EXPR: case CEIL_DIV_EXPR:
      case EXACT_DIV_EXPR:
        /* This is a shortcut for a common special case.  */
        if (op2.high == 0 && (HOST_WIDE_INT) op2.low > 0
            && !TREE_OVERFLOW (arg1)
            && !TREE_OVERFLOW (arg2)
            && op1.high == 0 && (HOST_WIDE_INT) op1.low >= 0)
            if (code == CEIL_DIV_EXPR)
              op1.low += op2.low - 1;

            res.low = op1.low / op2.low, res.high = 0;

        /* ... fall through ...  */

      case ROUND_DIV_EXPR:
        if (op2.is_zero ())
          return NULL_TREE;

so it already returns NULL_TREE on divide by zero.
I found the reason!!!!  This is one of the many "tree-vrp was not properly
tested for TImode bugs."

on the trunk, the case 0/(smallest negative number) case will only trigger
overflow in TImode.     On the wide-int branch, tree-vrp works at the
precision of the operands so overflow is triggered properly for this case.
So for HImode, the trunk produces the a result for 0/0x80 and then force_fit
code at the bottom of int_const_binop_1 turns this into an overflow tree
value rather than a null tree.

on the wide-int branch, this case causes the overflow bit to be returned
from the wide-int divide because the overflow case is properly handled for
all types and that overflow is turned into null_tree by the wide-int version
of int_const_binop_1.

apparently there are no test cases that exercise the true divide by 0 case
but there are test cases that hit the 0/ largest negative number case for
modes smaller than TImode.
You probably mean <largest negative number> / -1?  I don't see where
NULL_TREE is returned for any overflow case in int_const_binop_1.

Ah, you made it so.  That looks like a bogus change.  What's the
reason?  int_const_binop_1 is supposed to return a value with
TREE_OVERFLOW set in these cases, also required for frontend
constant folding.  Try compiling

const int i = (-__INT_MAX__ - 1) / -1;

which should say

./cc1 -quiet t.c
t.c:1:34: warning: integer overflow in expression [-Woverflow]
  const int i = (-__INT_MAX__ - 1) / -1;

but not error or ICE.  Seems to work on the branch, but only
because the expression is still folded by

12357         /* X / -1 is -X.  */
12358         if (!TYPE_UNSIGNED (type)
12359             && TREE_CODE (arg1) == INTEGER_CST
12360             && wi::eq_p (arg1, -1))
12361           return fold_convert_loc (loc, type, negate_expr (arg0));


     case TRUNC_DIV_EXPR:
     case EXACT_DIV_EXPR:
       res = wi::div_trunc (arg1, arg2, sign, &overflow);
       if (overflow)
         return NULL_TREE;

should retain the arg2 == 0 checks (and return NULL_TREE) but
otherwise keep overflow handling the same.
I see your point. i did not think that it was necessary to distinguish the two types of behavior by division.

i will make this fix today and test and install it.



On the trunk, only rem returns null_tree for divide by 0, on the wide int
branch, both div and rem return null tree.

I know that this is going to bring on a string of questions that i do not
remember the answers to as to why i made that change. but
fold-const.c:int_const_binop_1 now returns null_tree and this is just
fallout from that change.


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