compiler/codeGen/StgCmmMonad.hs¶
Note [Virtual and real heap pointers]¶
The code generator can allocate one or more objects contiguously, performing one heap check to cover allocation of all the objects at once. Let’s call this little chunk of heap space an “allocation chunk”. The code generator will emit code to
- Perform a heap-exhaustion check
- Move the heap pointer to the end of the allocation chunk
- Allocate multiple objects within the chunk
The code generator uses VirtualHpOffsets to address words within a single allocation chunk; these start at one and increase positively. The first word of the chunk has VirtualHpOffset=1, the second has VirtualHpOffset=2, and so on.
- The field realHp tracks (the VirtualHpOffset) where the real Hp register is pointing. Typically it’ll be pointing to the end of the allocation chunk.
- The field virtHp gives the VirtualHpOffset of the highest-allocated word so far. It starts at zero (meaning no word has been allocated), and increases whenever an object is allocated.
The difference between realHp and virtHp gives the offset from the real Hp register of a particular word in the allocation chunk. This is what getHpRelOffset does. Since the returned offset is relative to the real Hp register, it is valid only until you change the real Hp register. (Changing virtHp doesn’t matter.)
