[LLVMdev] Folding cast of a global address to boolean

[Morten Ofstad]

I noticed that folding a cast of a global address to boolean doesn't 
work like I expected - there is a comment in the code that casts of 
external symbols should not happen, but I don't understand the rationale 
for this.

// FIXME: When we support 'external weak' references, we have to prevent
// this transformation from happening.  In the meantime we avoid folding
// any cast of an external symbol.
     if (!GV->isExternal())
       return ConstantBool::True;

This particular transformation is quite important for me because I have 
some symbols which are either ConstantPointerNull or a GlovalVariable 
depending on which context the program is compiled in. This is used to 
implement a kind of conditional compilation, and I'm relying on the 
constant folding to remove branches in my code. I tried to change the 
linkage type of the GlobalValue, but that's not what the isExternal() 
function checks so it did not have any effect.

If I understood better why this is disabled, maybe I can fix it. 
Otherwise I will have to implement the conditional compilation as part 
of the code generation and that will be quite a bit more complicated.

m.