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I'm reverse engineering an application which commonly adds 4.294967296e9 to a double if the number is negative. Part of the disassembly:

mov     eax, label_to_memory
fild    label_to_memory
test    eax, eax
jge     short some_label
fadd    ds:some_double_value
some_label:

If I'm correct, it takes the jump if the highest bit isn't set, i.e. its a positive number. some_double_value points to the constant 4.294967296e9.

To understand what the piece of code is doing I need to know why it adds this specific number, the only piece of Information I found was a line of code in http://web.mit.edu/~mkgray/afs/bar/afs/net/project/attic/quipu/isode-8.0/others/ntp/ntpsubs.c which says

#ifdef  VAX_COMPILER_FLT_BUG
    if (b < 0.0) b += 4.294967296e9;
#endif

So, what is the significance of that number?

Additional Info: The number can not be negative, so this can actually never happen.

10

I actually found the answer while writing this question!

This number is exactly UINT_MAX + 1, stored as a double. So it seems this code converts a unsigned integer to a double. The fild instruction loads the 32 bit value as signed value, after adding the max possible unsigned value + 1, the double contains the same value as the unsigned integer.

So the compiler produced this probably automatically from something like this:

unsigned int num; // some arbitrary 32 bit unsigned number
double d = num;

Since it took me some time to figure it out, I thought it may help someone else find the solution faster than me.

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