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gcc should do the right thing if you add a special case for zero. exponent will be negative in that case. For larger integers you might end up with some false p
by oldcigarette 15y ago
gcc should do the right thing if you add a special case for zero. exponent will be negative in that case. For larger integers you might end up with some false positives with floats though for say 2^30+1.
- imurray 15y agoThanks, I had confused myself. You're right: shifting with a negative value (or far too big) gives undefined behaviour in C. (The type punning stuff is formally undefined too, a memcpy would be more portable, but gcc promises to make the widely-used union trick work.) Regarding the second point, the posted code works fine with 1073741825U (the literal for 2^30+1). The algorithm doesn't need the float to keep full precision, because only the exponent is consulted.