http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47006
Summary: ARM: GCC generates faulty assembly code for pre ARMv6
CPUs on unaligned word access
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: peterbjorgen...@gmail.com
arm-elf-gcc produces wrong assembly code for ARM CPU's not supporting unaligned
word access. Prior to ARMv6 word access to non-aligned memory will truncate the
address. Refer to pdf page 76 of
http://www.scss.tcd.ie/~waldroj/3d1/arm_arm.pdf.
Here is the example code:
// compile with:
// arm-elf-gcc -O1 -Wall -g -mcpu=arm7tdmi bug.c
// see disassembly with:
// arm-elf-objdump a.out -d |less
#include <stdio.h>
int main () {
char buf[10];
*((unsigned int*)(buf + 3)) = 0;
printf(buf);
return 0;
}
And the disassembly:
00008218 <main>:
8218: e92d4010 push {r4, lr}
821c: e24dd00c sub sp, sp, #12
8220: e3a04000 mov r4, #0
8224: e58d4003 str r4, [sp, #3]
8228: e1a0000d mov r0, sp
822c: eb00008e bl 846c <printf>
8230: e1a00004 mov r0, r4
8234: e28dd00c add sp, sp, #12
8238: e8bd8010 pop {r4, pc}
The instruction "str r4, [sp, #3]" is equivalent to "str r4, [sp]" on the
ARM7TDMI (ARMv4t architecture), as the sp is word aligned. This is not the
intended behaviour of the C program, and no warning is given.
When doing non-aligned word access by the use of a 'packed' struct, GCC
correctly produces multiple byte access instructions.
