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LDRD instruction confused me, please help

Frank Sai

I used keil mdk v5.1 for arm to develop my project.
I encounterd a serious problem about LDRD instruction.
Look at c codes below:

#include <stdint.h>

char b[2];
char c[4];
char d[2];

typedef struct {
        uint32_t                m_front;                        /* ¶ÓÍ·         */
        uint32_t                m_rear;                         /* ¶Óβ        */
        uint16_t                m_maxData;                      /* ¶ÓÁÐÖÐÔÊÐí´æ´¢µÄÊý¾Ý¸öÊý */

        uint8_t         (* ReadEmpty)();                /* ¶Á¿Õ´¦Àíº¯Êý             */
        uint8_t         (* WriteFull)();                /* дÂú´¦Àíº¯Êý             */
} DataQueue;

void SystemInit()
{
}

void main()
{
    DataQueue* p = (DataQueue*)c;

    char* q = c;
    char* xx = b;
    char* yy = d;

    if (p->m_front == p->m_rear)
    {
        int i = 0;
    }
}

disassembly of part of c codes is below

    28:     if (p->m_front == p->m_rear)
    29:     {
0x08000194 E9D04500  LDRD     r4,r5,[r0,#0]
0x08000198 42AC      CMP      r4,r5
0x0800019A D101      BNE      0x080001A0
    30:         int i = 0;
0x0800019C 2400      MOVS     r4,#0x00
    31:     }

I started a debug session.
When I stepped over sentance "if (p->m_front == p->m_rear)",
I found that program crashed(Hard fault error).

i thought that LDRD need value of r0 to a multiple of 4,
but r0 value was 0x20000002 at this time.

so i have to add __align(4) keyword to make sure that address of "c" is multiple of 4 and everything is fine.
you can see the codes below

#include <stdint.h>

char b[2];
__align(4) char c[4];
char d[2];

typedef struct {
        uint32_t                m_front;                        /* ¶ÓÍ·         */
        uint32_t                m_rear;                         /* ¶Óβ        */
        uint16_t                m_maxData;                      /* ¶ÓÁÐÖÐÔÊÐí´æ´¢µÄÊý¾Ý¸öÊý */

        uint8_t         (* ReadEmpty)();                /* ¶Á¿Õ´¦Àíº¯Êý             */
        uint8_t         (* WriteFull)();                /* дÂú´¦Àíº¯Êý             */
} DataQueue;

void SystemInit()
{
}

void main()
{
        DataQueue* p = (DataQueue*)c;

    char* q = c;
    char* xx = b;
    char* yy = d;

    if (p->m_front == p->m_rear)
    {
        int i = 0;
    }
}

i was confused if this was an bug of compiler?
Please help me

Parents
  • 0 in reply to HansBernhard Broeker

    Since you already have a fixed data type for the elements, you could do:

    enum {
    NUM_ENTRIES = 20,
    NUM_OTHER_ENTRIES = 40,
};
Queue my_queue;
QueueEntry my_entries[NUM_ENTRIES];

Queue my_other_queue;
QueueEntry my_other_entries[NUM_OTHER_ENTRIES];

void init_queue(Queue *q,QueueEntry entries[],unsigned num_entries) {
    QueueEntries *p;
    unsigned i;

    memset(q,0,sizeof(*q));
    q->first = NULL;
    q->last = NULL;
    q->num_entries = num_entries;
    // Chain together all entries in a list of unused.
    q->free = p = entries;
    for (i = 1; i < num_entries; i++,p++) {
        p->next = p+1;
    }
    p->next = NULL;
    ...
}

void main() {
    init_queue(&my_queue,my_entries,NUM_ENTRIES);
    init_queue(&my_other_queue,my_other_entries,NUM_OTHER_ENTRIES);
    ...
}


    Two statically allocated queues of different length but with correct align.

Reply
  • 0 in reply to HansBernhard Broeker

    Since you already have a fixed data type for the elements, you could do:

    enum {
    NUM_ENTRIES = 20,
    NUM_OTHER_ENTRIES = 40,
};
Queue my_queue;
QueueEntry my_entries[NUM_ENTRIES];

Queue my_other_queue;
QueueEntry my_other_entries[NUM_OTHER_ENTRIES];

void init_queue(Queue *q,QueueEntry entries[],unsigned num_entries) {
    QueueEntries *p;
    unsigned i;

    memset(q,0,sizeof(*q));
    q->first = NULL;
    q->last = NULL;
    q->num_entries = num_entries;
    // Chain together all entries in a list of unused.
    q->free = p = entries;
    for (i = 1; i < num_entries; i++,p++) {
        p->next = p+1;
    }
    p->next = NULL;
    ...
}

void main() {
    init_queue(&my_queue,my_entries,NUM_ENTRIES);
    init_queue(&my_other_queue,my_other_entries,NUM_OTHER_ENTRIES);
    ...
}


    Two statically allocated queues of different length but with correct align.

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