Re: How to type-pun and why

[Andrew Haley <aph at redhat dot com>]

Mattias Engdegård writes:
 > Some code I had needed to do integer operations on (the bits of)
 > floating-point values. The naive
 > 
 >   double d = something;
 >   uint64_t x = *(uint64_t *)&d;
 > 
 > gives a type aliasing diagnostic in 4.1, and rightly so. However,
 > attempting to "fix" this by adding an alias-anything intermediate
 > pointer to void or char,
 > 
 >   uint64_t x = *(uint64_t *)(void *)&d;
 > 
 > suppresses the diagnostic but still won't work properly - gcc does
 > not admit that x depends on d, so the assignment of d sometimes
 > ends up after that of x.  Using a union intermediate instead,
 > 
 >   union { double d; uint64_t x; } u;
 >   u.d = something;
 >   uint64_t x = u.x;
 > 
 > works as expected, but I would like to know why

It's a gcc extension to ISO C.  

 > - if it is a matter of luck with the current implementation it
 > might cease working when the compiler changes or becomes more
 > aggressive in the future.

I doubt it.  We have to keep it working becasue so much library code
requires it.

 > As far as I can tell from the Standard, it could be argued that
 > none of the above versions are allowed at all, so it's not obvious
 > to me why one should be permitted but not another. I would be
 > grateful for any help with references.

Andrew.