gcc creates 'strd' instructions which are accessing non-double-word
aligned addresses. This is forbidden accordingly ARM Reference Manual
and causes an alignment exception:
| 10.6.14 STRD
| However, the address of the first of the two words is required to be
| doubleword-aligned (that is, the address must be divisible by 8).
Example:
------------
$ cat foo.c
struct A {
unsigned int a;
unsigned long long l;
};
int main()
{
extern struct A volatile *a;
a->l = 0;
}
$ arm-xscale-linux-gnu-gcc -c -Os -mabi=aapcs -march=armv5te ./foo.c
$ objdump -d foo.o
foo.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <main>:
0: e59f3010 ldr r3, [pc, #16] ; 18 <.text+0x18>
4: e3a00000 mov r0, #0 ; 0x0
8: e5933000 ldr r3, [r3]
c: e3a01000 mov r1, #0 ; 0x0
10: e1c300f8 strd r0, [r3, #8]
14: e12fff1e bx lr
18: 00000000 andeq r0, r0, r0
------------
AAPCS requires only a 4-Byte alignment for data pointers, so the
external 'a' might point to a non-8 divisible address. The resulting
address at 10: is non-8 divisible too and will cause an alignment
exception.
-------------
$ $C-gcc -v
Using built-in specs.
Target: arm-xscale-linux-gnu
Configured with: ../configure --prefix=/usr --build=i686-redhat-linux-gnu
--host=i686-redhat-linux-gnu --target=arm-xscale-linux-gnu
--with-sysroot=/usr/arm-xscale-linux-gnu/sys-root --disable-__cxa_atexit
--enable-target-optspace --with-gnu-ld --disable-nls --infodir=/usr/share/info
--mandir=/usr/share/man --enable-version-specific-runtime-libs
--enable-languages=c,c++,java,objc --enable-shared --enable-threads
--disable-multilib --with-cpu=xscale --enable-cxx-flags=-mcpu=xscale
-fomit-frame-pointer
Thread model: posix
gcc version 4.1.0
--
Summary: ARM: creates 'strd' instructions for unaligned addresses
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: enrico dot scholz at informatik dot tu-chemnitz dot de
GCC build triplet: i686-redhat-linux-gnu
GCC host triplet: i686-redhat-linux-gnu
GCC target triplet: arm-xscale-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26800
