__builtin_object_size does not match structure size when flexible array members are statically initialized

[kees]

Given a structure ending in a flexible array, that has a static initializer:

struct foo {
    size_t count;
    int some[];
};

static struct foo instance = {
    .count = 3,
    .some = { 5, 10, 15, },
};

Clang reports the incorrect size with __builtin_object_size().

GCC correctly reports 20:

__builtin_object_size(&instance, 1) == 20

Clang misses the flexible array members and reports only 8 (the sizeof()):

__builtin_object_size(&instance, 1) == 8

https://godbolt.org/z/W5soYdrdK

cc @nickdesaulniers @isanbard

[ryao]

Nice find.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[nickdesaulniers]

1. https://reviews.llvm.org/D150891
2. https://reviews.llvm.org/D150892

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[nickdesaulniers]

(Fix will ship in clang-17)

[nickdesaulniers]

change got backed out https://reviews.llvm.org/rGf5af7d2d987b0b16d7a641b7aa9086a75d533944