Georg-Johann Lay wrote:
> Richard Guenther wrote:
>> On Tue, Apr 24, 2012 at 7:24 PM, Georg-Johann Lay <a...@gjlay.de> wrote:
>>> Richard Guenther wrote:
>>>
>>>>> I've run into another issue supporting a 20-bit integer for which I'd
>>>>> appreciate a hint. With this code:
>>>>>
>>>>> typedef long int __attribute__((__a20__)) int20_t;
>>>>> int20_t xi;
>>>>> int20_t addit () { xi += 0x54321L; }
>>>>>
>>>>> xi ends up in mode PSImode, which is a MODE_PARTIAL_INT with 20 bits of
>>>>> precision and 32-bit width.
>>>>>
>>>>> convert() notices that, because the constant in the add expression is
>>>>> SImode, there's an SImode add being truncated to a PSImode result, and
>>>>> pushes the truncation down into the operands.
>>>>>
>>>>> The problem is this ends up in convert_to_integer, which detects that the
>>>>> signed operation might overflow so calls unsigned_type_for() to get the
>>>>> unsigned variant.
>>>>>
>>>>> Unfortunately, this ends up in c_common_type_for_size(), which knows
>>>>> nothing
>>>>> about PSImode, and returns an unsigned type with 32 bits of precision when
>>>>> asked for one with 20 bits of precision.
>>>> That's expected - this function returns a type that is suitable for
>>>> holding all
>>>> values, not a type that has necessarily matching precision. If the caller
>>>> wants such type it needs to verify what the function returned. Which seems
>>>> to me to be the correct fix for this (premature) optimization in
>>>> convert_to_integer.
>>>>
>>>> Richard.
>>>>
>>>>> The expression is rewritten with
>>>>> the 32-bit constant integer recast to the 32-bit unsigned type (instead
>>>>> of the 20-bit one it might have used), and infinite recursion results.
>>> This is already filed as
>>>
>>> http://gcc.gnu.org/PR51527
>>>
>>> It works with 4.8 trunk but crashes with 4.7.
>>> Did not yet track what changes made it work with 4.8, though.
>>> Unfortunately, noone remembers :-(
>>>
>>> http://gcc.gnu.org/ml/gcc/2012-03/msg00440.html
>> I have done changes in this area, trying to remove the type_for_size
>> langhook.
>>
>> Richard.
>
> One apparent change is tree.c:signed_or_unsigned_type_for
>
> that changed from 4.7:
>
> tree
> signed_or_unsigned_type_for (int unsignedp, tree type)
> {
> tree t = type;
> if (POINTER_TYPE_P (type))
> {
> /* If the pointer points to the normal address space, use the
> size_type_node. Otherwise use an appropriate size for the pointer
> based on the named address space it points to. */
> if (!TYPE_ADDR_SPACE (TREE_TYPE (t)))
> t = size_type_node;
> else
> return lang_hooks.types.type_for_size (TYPE_PRECISION (t), unsignedp);
> }
>
> if (!INTEGRAL_TYPE_P (t) || TYPE_UNSIGNED (t) == unsignedp)
> return t;
>
> return lang_hooks.types.type_for_size (TYPE_PRECISION (t), unsignedp);
> }
>
> to 4.8:
>
> tree
> signed_or_unsigned_type_for (int unsignedp, tree type)
> {
> if (TREE_CODE (type) == INTEGER_TYPE && TYPE_UNSIGNED (type) == unsignedp)
> return type;
>
> if (!INTEGRAL_TYPE_P (type)
> && !POINTER_TYPE_P (type))
> return NULL_TREE;
>
> return build_nonstandard_integer_type (TYPE_PRECISION (type), unsignedp);
> }
>
> at 2012-03-12
>
> http://gcc.gnu.org/viewcvs?view=revision&revision=185226
>
> Is this appropriate to backport?
>
> Or is the preferred solution to override lang_hooks.types.type_for_size in the
> backend, if applicable?
Now I have this piece of code that works for 4.7.
I don't like it much, but if there are no plans to otherwise fix PR51527,
I'd make a patch and propose it as target-specific work-around.
/* Some auxiliary stuff for the AVR-specific built-in 24-bit scalar integer
types __int24 and __uint24 to be used in `avr_init_builtins'. */
static GTY(()) tree avr_int24_type;
static GTY(()) tree avr_uint24_type;
/* Implement `LANG_HOOKS_TYPE_FOR_SIZE'. */
/* To avoid infinite recursion in `convert_to_integer' as
reported in PR c/51527. */
static tree (*avr_default_type_for_size)(unsigned int, int);
static tree
avr_type_for_size (unsigned int bits, int unsignedp)
{
if (bits == 24)
return unsignedp ? avr_uint24_type : avr_int24_type;
return avr_default_type_for_size (bits, unsignedp);
}
static void
avr_init_builtin_int24 (void)
{
avr_int24_type = make_signed_type (GET_MODE_BITSIZE (PSImode));
avr_uint24_type = make_unsigned_type (GET_MODE_BITSIZE (PSImode));
lang_hooks.types.register_builtin_type (avr_int24_type, "__int24");
lang_hooks.types.register_builtin_type (avr_uint24_type, "__uint24");
/* Hook-in to avoid infinite recursion in `convert_to_integer' as
reported in PR c/51527. */
avr_default_type_for_size = lang_hooks.types.type_for_size;
lang_hooks.types.type_for_size = avr_type_for_size;
}
A proper, target-independent fix would be great because it is the right place
and because Peter has the same problem.
Johann
>>>>> Is it proper for c_common_type_for_size() to know about partial int modes
>>>>> and return the best one available? Is there a hook that would allow me to
>>>>> do this customized in my back-end? Or is there another way to get
>>>>> unsigned_type_for() to return the "right" type for an unusual integer
>>>>> precision?
>>>>>
>>>>> Thanks.
>>>>>
>>>>> Peter
