[no_unique_address] Member not stored in previous member's padding

[zoecarver]

Bugzilla Link	51424
Version	trunk
OS	All
CC	@dwblaikie,@zygoloid

Extended Description

It seems like a missed optimization not to store members in a previous member's padding when the previous member is marked as [[no_unique_address]].

GCC implements this. The difference in codegen can be seen here: https://godbolt.org/z/zGs97xKG7

[zygoloid]

Slightly further reduced:

struct A {
int *p;
[[no_unique_address]] char a;
};

struct B {
[[no_unique_address]] A cache;
char b;
};

static_assert(sizeof(B) == 16);
static_assert(alignof(B) == 8);

It looks like Clang doesn't properly implement the rule that a class with a potentially-overlapping member is not POD for the purpose of layout. (If the attribute is removed from A::a, then struct A is POD for the purpose of layout and we're not allowed to reuse its tail padding per the ABI rule.)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Zoe Carver (zoecarver)

| | | | --- | --- | | Bugzilla Link | [51424](https://llvm.org/bz51424) | | Version | trunk | | OS | All | | CC | @dwblaikie,@zygoloid |

Extended Description

It seems like a missed optimization not to store members in a previous member's padding when the previous member is marked as [[no_unique_address]].

GCC implements this. The difference in codegen can be seen here: https://godbolt.org/z/zGs97xKG7