Guillaume Melquiond wrote:
bool signbit(float x) { const float one = 1; const float neg_one = -1; const int sign_mask = reinterpret_cast<const int&>(one) ^ reinterpret_cast<const int&>(neg_one); return reinterpret_cast<int&>(x) & sign_mask; }
You are invoking undefined behavior here: you are accessing a float through a reference to an int. As a consequence, GCC produces code that accesses uninitialized memory (since one and neg_one are optimized away) and returns random values. You have to use char* pointers and memcopies so that there is no aliasing issue.
Best regards,
Guillaume
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Thanks for the information. All I need to do is to get my hands on the bit patterns of 1.0f, -1.0f and x. Do I really have to copy the memory to do that? That seems like overkill. Can't I just form pointers to the float constants, cast the pointers to int pointers, and dereference them? Something like
bool signbit(float x) { const float one = 1; const float neg_one = -1; const int sign_mask = *static_cast<const int*>(&one) ^ *static_cast<const int*>(&neg_one); return *static_cast<int*>(&x) & sign_mask; }
--Johan