On Thu, Apr 19, 2018 at 04:00:47PM +0100, Mark Rutland wrote:
> On Thu, Apr 19, 2018 at 12:33:10AM +0100, Michael Brown wrote:
> > On 18/04/18 20:21, Heinrich Schuchardt wrote:
> For better or worse, I don't think that it is sufficient to pass the
> compiler -mno-unaligned-accesses, even if it happens to mask the problem
> in this specific case. The compiler can validly assume structures have
> their usual alignment, and hence the LDRD should not be misaligned.
I've just verified that this is the case:
----
struct foo {
int a;
int b;
};
int foo_cmp (struct foo *f1)
{
return (f1->a == f1->b);
};
----
... when compiled with:
arm-linux-gnueabihf-gcc -mcpu=cortex-a15 -mabi=aapcs -mno-unaligned-access
-O2 -c test.c
... yields:
00000000 <foo_cmp>:
0: e9d0 2000 ldrd r2, r0, [r0]
4: eba2 0000 sub.w r0, r2, r0
8: fab0 f080 clz r0, r0
c: ea4f 1050 mov.w r0, r0, lsr #5
10: 4770 bx lr
12: bf00 nop
... whereas using packed (rather than aligned(1), which I was wrong
about):
----
struct foo {
int a;
int b;
} __attribute__ ((packed));
int foo_cmp (struct foo *f1)
{
return (f1->a == f1->b);
};
----
... when compiled allowing unaligned accesses with:
arm-linux-gnueabihf-gcc -mcpu=cortex-a15 -mabi=aapcs -O2 -c test.c
... yields:
00000000 <foo_cmp>:
0: 6802 ldr r2, [r0, #0]
2: 6840 ldr r0, [r0, #4]
4: eba2 0000 sub.w r0, r2, r0
8: fab0 f080 clz r0, r0
c: ea4f 1050 mov.w r0, r0, lsr #5
10: 4770 bx lr
12: bf00 nop
... which should avoid alignment traps so long as SCTLR.A is clear to
permit unaligned LDR.
Thanks,
Mark.
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