http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819
Summary: [4.5 Regression] unexpected unaligned access to
volatile int
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: an...@mba.ocn.ne.jp
An unexpected unaligned access instructions for volatile int are
generated on gcc 4.5 or 4.6 with this test case.
struct st {
int ptr;
} __attribute__ ((packed));
int foo(struct st *st)
{
int v = *(volatile int *)&st->ptr;
return v;
}
This test case is derived from something like this in ARM linux kernel.
struct st {
unsigned int ptr;
} __attribute__ ((packed));
unsigned int foo(struct st *st)
{
return readl(&st->ptr);
}
mipsel-linux-gcc-4.5.1 -O2 foo.c -S output:
lwl $2,3($4)
nop
lwr $2,0($4)
j $31
nop
arm-linux-gnueabi-gcc-4.5.1 -O2 foo.c -S output:
ldrb r3, [r0, #0] @ zero_extendqisi2
ldrb r1, [r0, #1] @ zero_extendqisi2
ldrb r2, [r0, #2] @ zero_extendqisi2
orr r3, r3, r1, asl #8
ldrb r0, [r0, #3] @ zero_extendqisi2
orr r3, r3, r2, asl #16
orr r0, r3, r0, asl #24
bx lr
It seems the cast is ignored and unaligned access is assumed.
gcc 4.4.4 works fine.
mipsel-linux-gcc-4.4.4 -O2 foo.c -S output:
lw $2,0($4)
j $31
nop
arm-linux-gnueabi-gcc-4.4.4 -O2 foo.c -S output:
ldr r0, [r0, #0]
bx lr
The similar problem was found as PR 45704.
git-bisect tell me same commit
0d9f1189f3df5ce5c0efc3ecadc7c0a4f75b202d is the first bad commit.
Like PR 45704, reverting this change fixes this problme too.
- STRIP_USELESS_TYPE_CONVERSION (sub);
+ STRIP_NOPS (sub);
But the final fix for PR 45704 does not fix this problem.
mipsel-linux-gcc (4.6.0 20100927):
lbu $5,0($4)
lbu $6,1($4)
lbu $3,2($4)
andi $6,$6,0x00ff
lbu $2,3($4)
andi $5,$5,0x00ff
sll $6,$6,8
andi $3,$3,0x00ff
or $4,$6,$5
sll $3,$3,16
or $3,$3,$4
sll $2,$2,24
j $31
or $2,$2,$3
arm-linux-gnueabi-gcc (4.6.0 20100927):
ldrb r3, [r0, #0] @ zero_extendqisi2
ldrb r1, [r0, #1] @ zero_extendqisi2
ldrb r2, [r0, #2] @ zero_extendqisi2
orr r3, r3, r1, asl #8
ldrb r0, [r0, #3] @ zero_extendqisi2
orr r3, r3, r2, asl #16
orr r0, r3, r0, asl #24
bx lr
