Hi,
David Woodhouse wrote:
On Thu, 2007-05-17 at 20:30 +0000, Matthieu CASTET wrote:
On arch that don't support aligned access, packed struct access will be
done byte per byte (but it could be the expected behavior if there
unaligned access).
When I tested this on ARM, the output for je32_to_cpu et al was fine.
For _other_ structures where I'd used __attribute__((packed)) to be
safe, gcc would emit code to handle unaligned loads. But not in the
simple case where the struct has only one member.
Are you suggesting that this has changed since I did my testing?
here a small example I made and try with a sourcery 2006q3-27
/tmp/arm-2006q3/bin/arm-none-linux-gnueabi-gcc -S test.c -O3 -o
test.s.packed
/tmp/arm-2006q3/bin/arm-none-linux-gnueabi-gcc -S test.c -DNOP -O3 -o test.s
$diff test.s test.s.packed 19,23c19,34
< @ link register save eliminated.
< ldrh r2, [r1, #0]
< ldrh r1, [r0, #2]
< ldr r0, [r0, #4]
< @ lr needed for prologue
---
> stmfd sp!, {r4, r5, r6, lr}
> ldrb r2, [r0, #5] @ zero_extendqisi2
> ldrb r3, [r0, #4] @ zero_extendqisi2
> mov lr, r1
> ldrb r4, [r0, #6] @ zero_extendqisi2
> ldrb r5, [r0, #3] @ zero_extendqisi2
> ldrb r6, [r1, #1] @ zero_extendqisi2
> orr r3, r3, r2, asl #8
> ldrb r1, [r0, #2] @ zero_extendqisi2
> ldrb ip, [r0, #7] @ zero_extendqisi2
> ldrb r2, [lr, #0] @ zero_extendqisi2
> orr r3, r3, r4, asl #16
> orr r0, r3, ip, asl #24
> orr r1, r1, r5, asl #8
> orr r2, r2, r6, asl #8
> ldmfd sp!, {r4, r5, r6, lr}
Matthieu
#include <stdint.h>
#ifdef NOP
#define jint32_t uint32_t
#define jint16_t uint16_t
#define je16_to_cpu(x) ((x))
#define je32_to_cpu(x) ((x))
#else
typedef struct {
uint32_t v32;
} __attribute__((packed)) jint32_t;
typedef struct {
uint16_t v16;
} __attribute__((packed)) jint16_t;
#define je16_to_cpu(x) ((x).v16)
#define je32_to_cpu(x) ((x).v32)
#endif
struct jffs2_sum_unknown_flash
{
jint16_t nodetype; /* node type */
};
struct jffs2_unknown_node
{
/* All start like this */
jint16_t magic;
jint16_t nodetype;
jint32_t totlen; /* So we can skip over nodes we don't grok */
jint32_t hdr_crc;
};
int boo(struct jffs2_unknown_node *node, void *sp)
{
foo(je32_to_cpu(node->totlen), je16_to_cpu(node->nodetype), je16_to_cpu(((struct jffs2_sum_unknown_flash *)sp)->nodetype));
}
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