zygoloid wrote:
> Perhaps we need clarification on what GCC means by "may point to multiple
> objects" in this instance. To me that means either "get me the size of the
> largest of these multiple objects" or "size of the smallest." In my eyes,
> that means pointing to a union field.
Per @nikic's example, it seems reasonably clear to me that GCC's intended
semantics are to get either the best upper bound or the best lower bound that
the compiler is able to compute. I mean sure, we can ask, and it does no harm
to do so, but it is not reasonable to expect that a quantity like this coming
from the vagaries of GCC's implementation details will be exactly the same for
all examples when computed by a different implementation. (It's not even the
same across optimization levels in GCC.)
> I know that we lose precise struct information going to LLVM IR. If that's
> what's needed here, there are ways to pass this information along. We retain
> this information via DWARF. We could use similar metadata for this instance.
> Would that be acceptable?
In theory, yes, we could preserve enough information in metadata to compute
this in the middle-end. But we would need to emit a *lot* of metadata, just on
the off-chance that after optimization we happen to have a builtin_object_size
query that points to each object that we emit, so in practice I don't think
there's any chance we can do this. We can't use DWARF, because it won't
necessarily be available (and typically won't be available in the interesting
case where we find the object size only after optimization), and in any case,
the presence or absence of DWARF isn't supposed to affect the executable code.
Also, we'd need to annotate things that simply don't exist at all in the LLVM
IR:
```
typedef struct X { int a, b; } X;
int f(void) {
X *p = malloc(sizeof(X));
int *q = &p->a;
return __builtin_object_size(q, 1);
}
```
Here, `f` ideally would return 4, but at the LLVM IR level, `p` and `q` are
identical values and the `&p->a` operation is a no-op. In cases like this, the
best we can realistically do is to return 8.
https://github.com/llvm/llvm-project/pull/78526
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