Larry Hastings added the comment:
I must be missing something--because I thought Python *already* depended on
this apparently-undefined behavior. The small-block object allocator in
Objects/obmalloc.c determines whether a pointer belongs to a particular arena
using exactly this trick. I quote from the gigantic comment at the top of that
file:
Let B be the arena base address associated with the pool,
B = arenas[(POOL)->arenaindex].address. Then P belongs to
the arena if and only if
B <= P < B + ARENA_SIZE
Subtracting B throughout, this is true iff
0 <= P-B < ARENA_SIZE
This test is implemented as the following macro:
#define Py_ADDRESS_IN_RANGE(P, POOL) \
((arenaindex_temp = (POOL)->arenaindex) < maxarenas && \
(uptr)(P) - arenas[arenaindex_temp].address < (uptr)ARENA_SIZE && \
arenas[arenaindex_temp].address != 0)
Why is that permissible but _PyLong_IS_SMALL_INT is not?
----------
nosy: +larry
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