[Bug bootstrap/26998] bootstrap failure building libdecnumber, ICE in compare_values, tree-vrp.c:432

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #12 from law at redhat dot com  2006-06-08 16:38 -------
Subject: Re:  bootstrap failure building libdecnumber,
        ICE in compare_values, tree-vrp.c:432

On Thu, 2006-06-08 at 09:47 +0000, rguenth at gcc dot gnu dot org wrote:
> 
> ------- Comment #11 from rguenth at gcc dot gnu dot org  2006-06-08 09:47 -------
> This one:
> 
>   /* Apply the operation to each end of the range and see what we end
>      up with.  */
>   if (code == NEGATE_EXPR
>       && !TYPE_UNSIGNED (TREE_TYPE (expr)))
>     {
>       /* NEGATE_EXPR flips the range around.  */
>       min = (vr0.max == TYPE_MAX_VALUE (TREE_TYPE (expr)) && !flag_wrapv)
>              ? TYPE_MIN_VALUE (TREE_TYPE (expr))
>              : fold_unary_to_constant (code, TREE_TYPE (expr), vr0.max);
> 
>       max = (vr0.min == TYPE_MIN_VALUE (TREE_TYPE (expr)) && !flag_wrapv)
>              ? TYPE_MAX_VALUE (TREE_TYPE (expr))
>              : fold_unary_to_constant (code, TREE_TYPE (expr), vr0.min);
> 
>     }
> 
> is wrong for
> 
> (gdb) print vr0
> $3 = {type = VR_ANTI_RANGE, min = 0xb7d2b3a8, max = 0xb7d2b3a8, 
>   equiv = 0x896c298}
> (gdb) call debug_generic_expr (vr0.min)
> -2147483648
> (gdb) call debug_generic_expr (vr0.max)
> -2147483648
> 
> It needs to read:
> 
>       min = (vr0.max == TYPE_MIN_VALUE (TREE_TYPE (expr)) && !flag_wrapv)
>              ? TYPE_MAX_VALUE (TREE_TYPE (expr))
>              : fold_unary_to_constant (code, TREE_TYPE (expr), vr0.max);
> 
> values other than TYPE_MIN_VALUE are not special with inversion.
I'm not terribly familiar with the code in question or this PR.  But it
seems to me that negating a signed type's minimum value should give you
the same value with an overflow.

ie, for a 32bit signed type
- (-2147483648)  => -2147483648

If I read your proposed change, you'd instead return the type's max
value, in this case 2147483647.  Which seems rather wrong.

Diego is far more familiar with code than I am.

jeff


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26998