[LLVMdev] A working garbage collector - finally :)

[Rafael Ávila de Espíndola]

Since you are using a copying collector, may I ask how do you handle 
registers holding pointers to intermediate values?

The example I am considering is something like

void foo(void);
void f(long long *v, long long n) {
   long long int i;
   for (i = 0; i < n; ++i) {
     v[i] = i;
     foo();
   }
}

If *v points to gc memory, we can start the function with

%v.addr = alloca i64*, align 8
%foobar = bitcast i64** %v.addr to i8**
call void @llvm.gcroot(i8** %foobar, i8* null)
store i64* %v, i64** %v.addr, align 8

but nothing prevents llvm from putting the call to foo in between the 
computation of &v[i] and the store by using a callee saved register. If 
GC moves moves v during the call to foo, the next store will be wrong.

> -- Talin
>

Cheers,
Rafael