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I'm pretty sure that the different semantics for negative signed ints (sign extension) are there to make sure it works as division. It's right-shifting for regular bit manipulation purposes that's unsafe with signed.

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Whether right-shift is arithmetic or logical is implementation-defined in C. The compiler (for obvious reasons) knows what hardware is available, and generally optimizes a divide by a power of two to a right-shift if arithmetic is available (as it is in most architectures), plus a check to see if the number is odd and negative (and to add 1 to the numerator if so). All of which is still faster than using the divide hardware.

One more instance where I just assumed that what GCC does was in the standard. Very interesting.

For the curious who don't want to duplicate my look-up - right shift is by definition division by two except negative signed numbers, for which it's implementation defined. GCC on every architecture I've checked uses arithmetic shift for signed, probably because that extends the base definition to negatives.

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