Re: Does TYPE_MAX_VALUE (size_type_node) overflow?

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Jan 20, 2011 at 9:50 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
> * Richard Guenther:
>
>> On Wed, Jan 19, 2011 at 10:53 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>> I get strange warnings when I do arithmetic involving TYPE_MAX_VALUE
>>> (size_type_node), in particular this code:
>>>
>>> /* Multiplies MUL1 with MUL2, and adds ADD. ?Returns (size_t)-1 if the
>>> ? result cannot be be represented as a size_t value. ?If ADD is
>>> ? null_tree, treat it as a zero constant.
>>> ?*/
>>> tree
>>> build_size_mult_saturated (tree mul1, tree mul2, tree add)
>>> {
>>> ?tree max_mul1, result;
>>> ?max_mul1 = TYPE_MAX_VALUE (size_type_node);
>>> ?if (add != NULL_TREE)
>>> ? ?max_mul1 = size_binop(MINUS_EXPR, max_mul1, add);
>>> ?max_mul1 = size_binop(TRUNC_DIV_EXPR, max_mul1, mul2);
>>> ?result = size_binop (MULT_EXPR, mul1, mul2);
>>> ?if (add != NULL_TREE)
>>> ? ?result = size_binop (PLUS_EXPR, result, add);
>>> ?return build3 (COND_EXPR, sizetype,
>>> ? ? ? ? ? ? ? ? build2 (EQ_EXPR, sizetype, mul2, size_zero_node),
>>> ? ? ? ? ? ? ? ? add == NULL_TREE ? size_zero_node : add,
>>> ? ? ? ? ? ? ? ? build3 (COND_EXPR, sizetype,
>>> ? ? ? ? ? ? ? ? ? ? ? ? build2 (LE_EXPR, sizetype, mul1, max_mul1),
>>> ? ? ? ? ? ? ? ? ? ? ? ? result, TYPE_MAX_VALUE (size_type_node)));
>>> }
>>>
>>> Is size_type_node really signed, and does TYPE_MAX_VALUE
>>> (size_type_node) lie outside the representable range? ?Is there an
>>> easy way to get a GCC type closely matching size_t in C++?
>>
>> The size_* functions are supposed to be used with sizetype,
>> not with size_type ;) ?sizetypes are strange beast.
>
> Thanks for the suggestion. ?TYPE_MAX_VALUE (sizetype) appears to be
> -1, as the result of this code in stor-layout.c:
>
> ?/* sizetype is unsigned but we need to fix TYPE_MAX_VALUE so that it is
> ? ? sign-extended in a way consistent with force_fit_type. ?*/
> ?max = TYPE_MAX_VALUE (sizetype);
> ?TYPE_MAX_VALUE (sizetype)
> ? ?= double_int_to_tree (sizetype, tree_to_double_int (max));
>
> Is there a way to obtain the actual maximum value?

It is the actual (sign-extended) maximum value (sizetype values are
always sign-extended).  zero-extending the value gives you the "real"
maximum
value.

Richard.