[Bug c++/17743] New: __alignof__ vs. typedefs

[Bug c++/17743] New: __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

Typedefs are stripping __alignof__ information. This is prohibiting work on
TR1's aligned_storage. Which libstdc++ would like to use, both within the
library and to implement a fixed-size, zero-allocation allocator for STL and
string containers.

Details here:
http://gcc.gnu.org/ml/libstdc++/2004-09/msg00255.html

-benjamin

-- 
           Summary: __alignof__ vs. typedefs
           Product: gcc
           Version: 3.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bkoz at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2004-09-30 04:28 -------
Created an attachment (id=7244)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=7244&action=view)
test case

test case

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gdr at cs dot tamu dot edu,
                   |                            |mark at codesourcery dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2004-09-30 04:30 -------
Add deps...

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |8670
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|3.4.0 3.4.1 3.4.2 4.0.0     |3.3.3 3.4.0 3.4.1 3.4.2
                   |                            |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |3.4.0 3.4.1 3.4.2 4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-09-30 04:45 -------
Actually this is the attribute aligned vs arrays.

I cannot decode what the documenation means:
Note that although you can ask the compiler to select a time-efficient alignment for a given type and 
then declare only individual stand-alone objects of that type, the compiler's ability to select a time-
efficient alignment is primarily useful only when you plan to create arrays of variables having the 
relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, 
then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts 
to the same thing) on pointers to the relevant type, and the code that the compiler generates for these 
pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other 
types.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|8670                        |
              nThis|                            |
      Known to fail|3.3.3 3.4.0 3.4.1 3.4.2     |
                   |4.0.0                       |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-09-30 04:47 -------
Revert back some changes that I did not mean to do.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |8670
              nThis|                            |
      Known to fail|                            |3.3.3 3.4.0 3.4.1 3.4.2
                   |                            |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[gdr at cs dot tamu dot edu]

------- Additional Comments From gdr at cs dot tamu dot edu  2004-09-30 11:12 -------
Subject: Re:  __alignof__ vs. typedefs

"pinskia at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| Actually this is the attribute aligned vs arrays.
| 
| I cannot decode what the documenation means:
| Note that although you can ask the compiler to select a
| time-efficient alignment for a given type and  
| then declare only individual stand-alone objects of that type, the
| compiler's ability to select a time- 
| efficient alignment is primarily useful only when you plan to create
| arrays of variables having the  
| relevant (efficiently aligned) type. If you declare or use arrays of
| variables of an efficiently-aligned type,  
| then it is likely that your program will also be doing pointer
| arithmetic (or subscripting, which amounts  
| to the same thing) on pointers to the relevant type, and the code
| that the compiler generates for these  
| pointer arithmetic operations will often be more efficient for
| efficiently-aligned types than for other types.

I don't see how much this is relevant to the different uses of the
aligned attribute in the library.  Right now, the compiler is just
ignoring the aligned attribute.  We're more intersted in the semantics
than in the speed.

What we want is to align a given *array object* at some alignment
boundaries.  We're not interested in pointer arithmetics on array
types or speed -- although we may be doing pointer arithmetic on
individual elements of the array, but that is a separate issue.
The primarily thing is the ability to align an array object on a given
boundary. 

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-09-30 12:27 -------
Benj, having a fix in time for 4.0 would help, or is it going to be 4.1 
material anyway?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giovannibajo at libero dot
                   |                            |it


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at redhat dot com]

------- Additional Comments From bkoz at redhat dot com  2004-09-30 14:48 -------
Subject: Re:  __alignof__ vs. typedefs


>Benj, having a fix in time for 4.0 would help, or is it going to be 4.1 
>material anyway?

Having a fix for this for 4.0.0 will definitely be useful. I'm kind of
surprised that this isn't working right now, actually: I thought it was.
See libstdc++-v3/src/globals*.cc.

If somebody wants to change the subjet to something better go for it.

-benjamin


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[giovannibajo at libero dot it]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |giovannibajo at libero dot
                   |dot org                     |it
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-15 01:49:31
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-16 02:00 -------
Patch submitted:
http://gcc.gnu.org/ml/gcc-patches/2004-10/msg01320.html

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[mark at codesourcery dot com]

------- Additional Comments From mark at codesourcery dot com  2004-10-16 03:37 -------
Subject: Re:  __alignof__ vs. typedefs

giovannibajo at libero dot it wrote:

>------- Additional Comments From giovannibajo at libero dot it  2004-10-16 02:00 -------
>Patch submitted:
>http://gcc.gnu.org/ml/gcc-patches/2004-10/msg01320.html
>  
>
OK.  But XFAIL the tr1 test cases so those do not show up as new FAILs.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[bkoz at redhat dot com]

------- Additional Comments From bkoz at redhat dot com  2004-10-16 13:52 -------
Subject: Re:  __alignof__ vs. typedefs


>OK.  But XFAIL the tr1 test cases so those do not show up as new FAILs.

I think I just took care of this. 

Giovanni, thanks!

-benjamin


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] __alignof__ vs. typedefs

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2004-10-17 04:39 -------
Subject: Bug 17743

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	giovannibajo@gcc.gnu.org	2004-10-17 04:39:46

Modified files:
	gcc/cp         : ChangeLog decl2.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/ext: attrib17.C 

Log message:
	PR c++/17743
	* decl2.c (grokfield): Apply attributes also to TYPE_DECLs.
	
	PR c++/17743
	* g++.dg/ext/attrib17.C: New test.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/ChangeLog.diff?cvsroot=gcc&r1=1.4440&r2=1.4441
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/decl2.c.diff?cvsroot=gcc&r1=1.753&r2=1.754
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.4463&r2=1.4464
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/ext/attrib17.C.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-17 04:47 -------
OK, I fixed part of this for 4.0. The missing part is when the expression in 
attributes is dependent, like:

template <class T>
struct A {
    char foo __attribute__((aligned(__alignof__(T))));
};

This is non-trivial to fix. Benjamin, if you want you can keep this bugreport 
open to track this. I do not plan to be able to fix this in the 4.0 timeframe, 
so I have unassigned the bug (I will attempt something though). If somebody 
more expert than me wants to help the v3 guys, feel free.

Otherwise, if you do not care about this, just close this bug.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|giovannibajo at libero dot  |unassigned at gcc dot gnu
                   |it                          |dot org
             Status|ASSIGNED                    |NEW
            Summary|__alignof__ vs. typedefs    |dependent expressions in
                   |                            |attributes


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[gdr at cs dot tamu dot edu]

------- Additional Comments From gdr at cs dot tamu dot edu  2004-10-17 07:50 -------
Subject: Re:  dependent expressions in attributes

On Sat, 17 Oct 2004, giovannibajo at libero dot it wrote:

| OK, I fixed part of this for 4.0. The missing part is when the expression in
| attributes is dependent, like:
|
| template <class T>
| struct A {
|     char foo __attribute__((aligned(__alignof__(T))));
| };
|
| This is non-trivial to fix. Benjamin, if you want you can keep this bugreport
| open to track this. I do not plan to be able to fix this in the 4.0 timeframe,
| so I have unassigned the bug (I will attempt something though). If somebody
| more expert than me wants to help the v3 guys, feel free.
|
| Otherwise, if you do not care about this, just close this bug.

I do care.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-18 03:51 -------
Subject: Re:  dependent expressions in attributes

>> template <class T>
>> struct A {
>>     char foo __attribute__((aligned(__alignof__(T))));
>> };
> 
> I do care.

Is this a showstopper for tr1 work?

Giovanni Bajo




-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[bkoz at redhat dot com]

------- Additional Comments From bkoz at redhat dot com  2004-10-18 05:23 -------
Subject: Re:  dependent expressions in attributes


>Is this a showstopper for tr1 work?
 
Not that I can see. From what I can tell, tr1::array is going to require
default-constructable types. 

I think the library hackers are interested in this kind of stuff for our
own selfish reasons.

I guess I don't see why/how alignof should be different than sizeof
here (which works):

 template <class T>
 struct A 
 {
   char foo[sizeof(T)];
 };

-benjamin


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-18 16:38 -------
(In reply to comment #15)

> I guess I don't see why/how alignof should be different than sizeof
> here (which works):
>  template <class T>
>  struct A 
>  {
>    char foo[sizeof(T)];
>  };

Yes, but how do you force the compiler to ensure that the alignment of char foo
[] is sufficient to placement-allocate an object of type T into it?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2004-10-18 18:57 -------
Giovanni, this is applicable to TR1 via 

4.5.5 Other transformations [tr.meta.trans.other] template <std :: size_t Len ,
std :: size_t Align > struct aligned_storage { typedef unspecified type ; }; 1
type: an implementation defined POD type with size Len and alignment Align, and
suitable for use as uninitialized storage for any object of a type whose size is
Len and whose alignment is Align.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[bkoz at redhat dot com]

------- Additional Comments From bkoz at redhat dot com  2004-10-19 06:23 -------
Subject: Re:  dependent expressions in attributes


>Yes, but how do you force the compiler to ensure that the alignment of char foo
>[] is sufficient to placement-allocate an object of type T into it?

...get __alignof__(T) working?

I don't see any other way, really. 

-benjamin


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[bkoz at gcc dot gnu dot org]

------- Additional Comments From bkoz at gcc dot gnu dot org  2004-10-27 17:19 -------

Jason, this is the bug I talked about briefly. Do you think the Redmond offsetof
work is also applicable here?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743

[Bug c++/17743] dependent expressions in attributes

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-28 03:38 -------
(In reply to comment #19)

> Jason, this is the bug I talked about briefly. Do you think
> the Redmond offsetof work is also applicable here?

Mind to share that?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17743