efriedma-quic wrote:
> ```c
> struct x {
> int a;
> char foo[2][40];
> int b;
> int c;
> };
>
> size_t f(struct x *p, int idx) {
> return __builtin_dynamic_object_size(&p->foo[idx], 1);
> }
> ```
If I'm following correctly, the return here is 0, 40, or 80, depending on the
value of idx? That's not a constant, but the computation is entirely
syntactic; it doesn't matter what "p" actually points to. So clang can lower
the builtin itself. Currently it doesn't, I think, because all the relevant
code is in ExprConstant, but the code could be adapted.
The problem, really, is that we can't easily extend that approach to stuff like
the following:
```c
size_t f(struct x *p, int idx) {
char *c = &p->foo[idx];
return __builtin_dynamic_object_size(c, 1);
}
```
https://github.com/llvm/llvm-project/pull/78526
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