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Date: Tue, 06 Jul 2004 09:11:00 -0000
From: "zhangjie at magima dot com dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20040706091123.16381.zhangjie@magima.com.cn>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug c++/16381] New: Structure layout bug
X-Bugzilla-Reason: CC
X-SW-Source: 2004-07/txt/msg00461.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

GCC incorrectly calculated the structure layout in the following case.

#include <stdio.h>

typedef int __m128 __attribute__ ((__mode__(__V4SF__)));

struct Quad
{
	Quad() { }
	float x __attribute__((aligned(16)));
	float y;
	float z;
	float w;
};

struct Base1
{
	Quad q;
	int i;
};

struct Derived1 : public Base1
{
	int i2;
};

struct Base2
{
	__m128 m;
	int i;
};

struct Derived2 : public Base2
{
	int i2;
};

#define offset(C,M) (int((&((C*)(16))->M))-16)

int main()
{
	printf("__m128 size %i, align %i\n", sizeof(__m128), __alignof__(__m128));
	printf("Quad   size %i, align %i\n", sizeof(Quad), __alignof__(Quad));
	printf("Base1 %i Derived1 %i Offset %i\n", sizeof(Base1), sizeof(Derived1),
offset(Derived1, i2) );
	printf("Base2 %i Derived2 %i Offset %i\n", sizeof(Base2), sizeof(Derived2),
offset(Derived2, i2) );
	return 0;
}

The results on cygwin:

g++ (GCC) 3.3.1 (cygming special)

__m128 size 16, align 16
Quad   size 16, align 16
Base1 24 Derived1 24 Offset 20
Base2 24 Derived2 24 Offset 20


g++ (GCC) 3.5.0 20040613 (experimental)

__m128 size 16, align 16
Quad   size 16, align 16
Base1 32 Derived1 32 Offset 20
Base2 24 Derived2 32 Offset 24

Neither looks correct. 

This test case was given in http://gcc.gnu.org/ml/gcc/2004-06/msg01490.html.
Discussion of this bug began from this mail
http://gcc.gnu.org/ml/gcc/2004-06/msg00622.html.

-- 
           Summary: Structure layout bug
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zhangjie at magima dot com dot cn
                CC: gcc-bugs at gcc dot gnu dot org,nathan at codesourcery
                    dot com,stephen dot kennedy at havok dot com
  GCC host triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16381