<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/55151>55151</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [aarch64] clang miscompilation of struct initializer in inline function (not properly zeroed)
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          rsms
      </td>
    </tr>
</table>

<pre>
    Initializing a struct with certain union fields inside an inlined function does not properly zero out non-named fields in the initializer.

Minimal repro case attached:
[clang-init-bug.c.zip](https://github.com/llvm/llvm-project/files/8576446/clang-init-bug.c.zip)

Here's a summary demonstration:

```c
typedef struct Attr {
  union { u8 a; u8 b[3]; }; u8 c[3];
} Attr;
typedef struct Foo {
  Attr a;
} Foo;
inline static Foo make_foo(Attr a) { return (Foo){a}; }
int fail() {
  Foo foo = make_foo( ((Attr){ .a = 7 }) );
  return foo.a.b[2] != 0; // bug: foo.a.b[2] is not zero initialized
}
int pass() {
  Foo foo = (Foo){ ((Attr){ .a = 7 }) };
  return foo.a.b[2] != 0;
}
```

Compiling this with `-std=c11`, the `fail` function which uses the `make_foo` helper will not fully initialize foo, while the `pass` function which "manually inlines" the same code from `make_foo` correctly and fully initializes foo.

Note that compiling with optimizations yields correct code. Looking at the IR produced _with_ optimizations the `pass` and `fail` functions ends up doing the exact same thing, but not when disabling (`-O0`) optimizations.

Versions:
- LLVM: 14.0.0 (329fda39c507e8740978d10458451dcdb21563be)
- Target: arm64-apple-darwin21.3.0
- Host: `uname -srm => Darwin 21.3.0 arm64` macOS 12.2.1 (21D62)
</pre>
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