Messages in this thread Patch in this message | | | Date | Tue, 12 Feb 2013 09:30:48 -0800 | Subject | Re: [tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get_user() warnings | From | "H.J. Lu" <> |
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On Tue, Feb 12, 2013 at 9:14 AM, H. Peter Anvin <hpa@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.tools@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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