c++/6546: static const members optimization failure

[Gabriel Dos Reis]

Nathan Myers <ncm-nospam@cantrip.org> writes:

| I thought folks on this list should know about this.  In short, 
| the "will-not-fix" below means that on gcc-3.1, as in 3.0, and 
| maybe in 3.2 and beyond, in
| 
|   struct foo { static const int bar = 3; };
|   int f(int i) { return std::min(foo::bar, i); }
| 
| the compiler actually generates code to load the static value 
| for foo::bar from memory each time (twice, in the code above), 
| instead of using the manifest constant 3 in its place.  

I don't know why loading twice.  But I can understand loading once,
and I'm not sure I would qualify the behaviour as a bug.

std::min() expect an lvalue, foo::bar is such a thing, therefore its
the designated object's address is taken; thyen its definition is
required.  Anything after that is optimization, optimization that
can't change the semantics.  Therefore the definition is required.

| Furthermore, it insists on having a int foo::bar defined 
| someplace, creating a porting difficulty from other compilers 
| that don't.

Well, I think it is assuming the compiler will not require the
definition that is the non-portable assumption.

| 
| Better dust off the old "member enum" trick if you hope to get 
| compile-time-constant performance with static-const members.
| As an alternative, casting the value: 
| 
|   int g(int i) { return std::min(int(foo::bar), i); }
| 
| seems to work around the problem.

Indeed, that use doesn't need the address of the object, so the
compiler is free to apply any relevant optimization.

-- Gaby