Re: [boost] Portable signbit macro/function?

From: Johan Råde (rade_at_[hidden])
Date: 2006-08-03 11:49:32

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