On Tue, Feb 12, 2013 at 9:14 AM, H. Peter Anvin <h...@zytor.com> wrote:
> On 02/12/2013 09:00 AM, Linus Torvalds wrote:
>> On Tue, Feb 12, 2013 at 8:38 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>
>>> Can you do something similar to what we did in glibc:
>>
>> No. Because we use macros to be type-independent (i e"get_user()"
>> works *regardless* of type), so casting to "uintptr_t" doesn't work.
>> It throws away the type information, and truncates 64-bit values on
>> 32-bit architectures.
>>
>> The whole point of the bitmask thing is that it doesn't have that
>> issue, and gets the size correct automatically. It's not pretty, but
>> it allows the rest of the sources to be readable.
>>
>
> No, I think what he is talking about it this bit:
>
> /* 1 if 'type' is a pointer type, 0 otherwise. */
> # define __pointer_type(type) (__builtin_classify_type ((type) 0) == 5)
>
> /* __intptr_t if P is true, or T if P is false. */
> # define __integer_if_pointer_type_sub(T, P) \
> __typeof__ (*(0 ? (__typeof__ (0 ? (T *) 0 : (void *) (P))) 0 \
> : (__typeof__ (0 ? (__intptr_t *) 0 : (void *)(!(P)))) 0))
>
> /* __intptr_t if EXPR has a pointer type, or the type of EXPR otherwise. */
> # define __integer_if_pointer_type(expr) \
> __integer_if_pointer_type_sub(__typeof__ ((__typeof__ (expr)) 0), \
> __pointer_type (__typeof__ (expr)))
>
> /* Cast an integer or a pointer VAL to integer with proper type. */
> # define cast_to_integer(val) ((__integer_if_pointer_type (val)) (val))
>
> Good grief, this makes the bitfield look like Mona Lisa. On the other
> hand, it relies on the *entirely* undocumented __builtin_classify_type()
> -- there appears to be absolutely no reference to it in gcc documentation.
>
> H.J., do you know what the bounds on the __builtin_classify_type() are
> (gcc versions available and so on)? Sadly I don't think one can use
> __builtin_types_compatible_p() instead.
>
> -hpa
>
__builtin_classify_type is documented in "info gccint". typeclass.h has
/* Values returned by __builtin_classify_type. */
enum type_class
{
no_type_class = -1,
void_type_class, integer_type_class, char_type_class,
enumeral_type_class, boolean_type_class,
pointer_type_class, reference_type_class, offset_type_class,
real_type_class, complex_type_class,
function_type_class, method_type_class,
record_type_class, union_type_class,
array_type_class, string_type_class,
lang_type_class
};
__builtin_classify_type ((type) 0) == 5 is compatible with all GCCs,
dating back as far as the first checkin when GCC was switched
to CVS from RCS in 1989, which predates the current Linux kernel:
commit 17cf341d65a3766f6451fe3bc20b9c010d38ba3b
Author: mycroft <mycroft@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Sun Aug 13 19:24:27 1989 +0000
entered into RCS
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@3
138bc75d-0d04-0410-961f-82ee72b054a4
diff --git a/gcc/typeclass.h b/gcc/typeclass.h
new file mode 100644
index 0000000..b166042
--- /dev/null
+++ b/gcc/typeclass.h
@@ -0,0 +1,14 @@
+/* Values returned by __builtin_classify_type. */
+
+enum type_class
+{
+ no_type_class = -1,
+ void_type_class, integer_type_class, char_type_class,
+ enumeral_type_class, boolean_type_class,
+ pointer_type_class, reference_type_class, offset_type_class,
+ real_type_class, complex_type_class,
+ function_type_class, method_type_class,
+ record_type_class, union_type_class,
+ array_type_class, string_type_class, set_type_class, file_type_class,
+ lang_type_class
+};
--
H.J.
--
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