Hey Simon, I was chatting with Sergio Benitez about GHC's HEAP_ALLOCED woes and he suggested an alternate fix which I'm not sure we have considered. The idea is simple: pre-assign some portion of the virtual address space for the dynamic heap, and then have HEAP_ALLOCED check if it's inside this space. Now, *obviously* this doesn't work for 32-bit (and I assume this is why we didn't go this route), but that's fine: the bitmap we use for 32-bit works pretty great and isn't a bottleneck. For 64-bit, we have a lot more address space to play with. Certainly we have to make sure the system linker never puts segments inside our pre-assigned space, but this seems far more manageable. I'm not particularly wedded to the indirections patchset, so if we can make this work for 64-bit, it seems good enough to me. Edward