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    <p>Casting pointers like you're doing violates the aliasing rules of the C language (pointers of different types may be assumed by the compiler not to point to the same data, except in certain very restricted cases). A better approach might be:</p> <pre><code>#define REP(x) ((union { double v; uint64_t r; }){ x }).r #define HI(x) (uint32_t)(REP(x) &gt;&gt; 32) #define LO(x) (uint32_t)(REP(x)) </code></pre> <p>Note that this also fixed the endian dependency (assuming the floating point and integer endianness are the same) and the illegal <code>_</code>-prefix on the macro names.</p> <p>An even better way might be not breaking it into high/low portions at all, and using the <code>uint64_t</code> representation <code>REP(x)</code> directly.</p> <p>From a standards perspective, this use of unions is a little bit suspect, but better than the pointer casts. Using a cast to <code>unsigned char *</code> and accessing the data byte-by-byte would be better in some ways, but worse in that you have to worry about endian considerations, and probably a lot slower..</p>
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    1. POHow to get the upper-/lower machine-word of a double according to IEEE 754 (ansi-c)?
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    1. COOkay, not that i would understand that construct at the moment, but it is working. But it is C99, isn't it. Maybe i should mention that i tried to get a C90 version. I replaced these uint64_t and uint32_t types with unsigned long long int and unsigned int. That is surely not that portable, but working for me. Thanks for your answer.
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    2. COOne more thing: the code posted by me is the original one from fdlibm from the netlib software repository. Maybe you are right with the violation of aliasing rules. The question which arises is then: why don't they (the developers) care about that too? This library has been developed for many years, also a few years after the standard and it is used a lot (example: base for the java math functions). Maybe they had good reasons to do it like they did it. But i'm not able to differentiate and evaluate these implementation methods. For what it's worth :-).
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    3. COPrior to C99, *all* casts to the wrong pointer types and union tricks were undefined behavior, which was so extreme that everyone just ignored the standard and pretended it was well-defined, and compilers didn't complain. Then compilers started to get smarter and realized that good optimization depends on ruling out the possibility of pointers aliasing one another, so C99 specified this all carefully, and compilers started to take advantage of the cases that were left undefined for optimization. Some people with old broken code are too lazy or stubborn to fix it...
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