This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Some wide-int review comments

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 return
null_tree for this case.
Trunk has:

     case TRUNC_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.


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.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]