This:

typedef union {
    struct {
        unsigned sign     : 1;
        unsigned exponent :11;
        U64      mantissa :52;
    };
    double d;
    U64 u;
} DFields;
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Should be this:

typedef union {
    struct {
        U64 sign     : 1;
        U64 exponent :11;
        U64 mantissa :52;
    };
    double d;
    U64 u;
} DFields;
[download]

Turns out that unless all the bit fields are defined to be of the same unsigned type, they will not be mapped to the same unit of memory.

I originally had them all defined as just unsigned; but the compiler complained that the mantissa was too large for a 32-bit uint. So, I switch it to U64 and it compiled.

The problem is that produces a struct that occupies more than 64-bits. The first two bitfields are merged into a U32, but because the third is a different underlying type, it gets mapped to a different chunk of uninitialised memory. Hence, the apparently random nature of the output.

Making all three fields the same type means they get amalgamated into a single 64-bit piece of ram. (But it took some searching to locate this rather arcane piece of knowledge.)