Optimiser Issue?

[RedDave]

I have a structure that is filled by one task which notifies another with a ready() call, which in turn calls get_data() to copy that structure.
This structure then gets copied into a section of memory in order to be transmitted as a UDP packet.
In addition to the structure there is a preceding message code and a CRC byte added.

I think the optimiser is stopping the entire structure from being copied.

Here is some notified code that demonstrates the problem.

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        case i_sync.ready():
            sync_data sync_pulse = i_sync.get_data();
            memcpy(&sync_msg[1], &sync_pulse, 5);
            printf("A: ");
            for(int i=0; i<sizeof(sync_msg); i++)
                printf("%02X ", sync_msg[i]);
            printf("\n");
            memset(sync_msg, 0x03, sizeof(sync_msg));
            //printf("-- %d\n", (int)sync_pulse.count);
            break;

The structure is trivial.

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typedef struct {
    unsigned int period;
    unsigned short count;
} sync_data;

If I run the code above, but with the printf line uncommented, I get the expected result.
The notifying task is printing SYNC: <count value>, which can be seen in position [5] of the array.

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SYNC: 5
A: 03 2A 42 0F 00 05 03 
-- 5
SYNC: 6
A: 03 2A 42 0F 00 06 03 
-- 6
SYNC: 7
A: 03 2A 42 0F 00 07 03 
-- 7
SYNC: 8
A: 03 2A 42 0F 00 08 03 
-- 8
SYNC: 9
A: 03 2A 42 0F 00 09 03 
-- 9
SYNC: 10
A: 03 2A 42 0F 00 0A 03 
-- 10
SYNC: 11
A: 03 2A 42 0F 00 0B 03 
-- 11
SYNC: 12
A: 03 2A 42 0F 00 0C 03 
-- 12

However, with the printf commented out, and so no reference to count, it seems that the memcpy of that part of the structure is optimised out in some way. The 03 from the previous memset is not replaced.

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SYNC: 5
A: 03 2A 42 0F 00 03 03 
SYNC: 6
A: 03 2A 42 0F 00 03 03 
SYNC: 7
A: 03 2A 42 0F 00 03 03 
SYNC: 8
A: 03 2A 42 0F 00 03 03 
SYNC: 9
A: 03 2A 42 0F 00 03 03 
SYNC: 10
A: 03 2A 42 0F 00 03 03 
SYNC: 11
A: 03 2A 42 0F 00 03 03 
SYNC: 12
A: 03 2A 42 0F 00 03 03

So, am I doing something stupid or is this the optimiser enbugging my code?