[Bug target/23071] New: Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[Bug target/23071] New: Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[sabre at nondot dot org]

Darwin ignores the alignment/packed attributes on this structure, forcing the
structure to have 8-byte alignment and 16-byte size:

---
struct Test {
  double D __attribute__((packed,aligned(4)));
  short X;
};
---

Ian Lance Taylor did a great analysis of the history of this, tracking it back
to a patch by David Edelsohn in 1996.  The analysis is here:
http://gcc.gnu.org/ml/gcc-help/2005-07/msg00280.html

-Chris

-- 
           Summary: Darwin alignment ignores "attribute packed" for first
                    'double' element of a struct
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: dje at watson dot ibm dot com
        ReportedBy: sabre at nondot dot org
                CC: gcc-bugs at gcc dot gnu dot org,ian at airs dot com
  GCC host triplet: powerpc-darwin-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-26 13:21 -------
I still don't believe this is a bug.
As the alignment of whole struct is still 8 as double is first, even if the alignment of that double is 4.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
   GCC host triplet|powerpc-darwin-*            |
 GCC target triplet|                            |powerpc-darwin, powerpc-aix


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2005-07-26 14:08 -------
The ABI specifies the alignment of the entire record is doubleword if the 
first field is an FP double, regardless of the alignment of the type itself.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[sabre at nondot dot org]

------- Additional Comments From sabre at nondot dot org  2005-07-26 16:25 -------
Okay, in that case, how does one change the alignment of the structure?  The only way I'm aware of to 
do this is with attribute packed, but that will also modify the inter-field padding of the structure.  For 
example, in this case:

struct Test {
  double D __attribute__((packed,aligned(4)));
  short X;
  int Y;
  short Z;
} __attribute__((packed,aligned(4)));

Not only is the alignment of the structure reduced to 4 bytes, the structure is also packed, resulting in 
the 'int' being unaligned and the size of the structure being 16 bytes (instead of 20).

"it sure would be nice if you could use 'attribute aligned' to reduce alignment without attribute packed"

-Chris

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[ian at airs dot com]

------- Additional Comments From ian at airs dot com  2005-07-26 16:55 -------
The whole point of attribute ((packed)) is to change the ABI.  If it can't
change the ABI, it is meaningless.  So I don't find the argument based on the
ABI to be particularly convincing.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[dje at watson dot ibm dot com]

------- Additional Comments From dje at watson dot ibm dot com  2005-07-26 16:58 -------
Subject: Re:  Darwin alignment ignores "attribute packed" for first 'double' element of a struct 

	If Chris and Apple want to change the behavior for Darwin, be my
guest. 

David


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2005-07-26 17:02 -------
reopen

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|WONTFIX                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[dje at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|dje at watson dot ibm dot   |unassigned at gcc dot gnu
                   |com                         |dot org
             Status|UNCONFIRMED                 |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[mrs at gcc dot gnu dot org]

-- 

mrs at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[mrs at gcc dot gnu dot org]

------- Comment #9 from mrs at gcc dot gnu dot org  2010-03-22 23:21 -------
This has now been fixed.


-- 

mrs at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[mikestump at comcast dot net]

------- Comment #8 from mikestump at comcast dot net  2010-03-22 23:18 -------
The previous behavior was fairly broken, adding packed, _increased_ the
alignment.  A user that adds packing, never wants more alignment:

struct Test {
  double D  __attribute__((packed,aligned(4)));
  short X;
} x;

struct {
  char x;
  struct Test t;
} b = { 1, { 2, 3}};

compared to:

struct Test {
  double D /*  __attribute__((packed,aligned(4))) */;
  short X;
} x;

struct {
  char x;
  struct Test t;
} b = { 1, { 2, 3}};


-- 

mikestump at comcast dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikestump at comcast dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071

[Bug target/23071] Darwin alignment ignores "attribute packed" for first 'double' element of a struct

[mrs at gcc dot gnu dot org]

------- Comment #7 from mrs at gcc dot gnu dot org  2010-03-22 23:13 -------
Subject: Bug 23071

Author: mrs
Date: Mon Mar 22 23:13:10 2010
New Revision: 157654

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=157654
Log:
        PR target/23071
        * config/rs6000/rs6000.c (darwin_rs6000_special_round_type_align):
        Don't overly align based upon packed packed fields.

Added:
    trunk/gcc/testsuite/gcc.target/powerpc/darwin-abi-12.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/rs6000/rs6000.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23071