[llvm-bugs] [Bug 163899] Unnecessary copying of struct field when the same field is later overwritten

[LLVM Bugs via llvm-bugs]

Issue	163899
Summary	Unnecessary copying of struct field when the same field is later overwritten
Labels	backend:AArch64, backend:X86, missed-optimization
Assignees
Reporter	Kmeakin

    These functions should all produce the same assembly

https://godbolt.org/z/4c674fKxE

```c
typedef unsigned long u64;

typedef struct {
    u64 x;
    u64 y;
    u64 z;
} Big;

// Not optimized

Big assign(Big b) {
    b.x = 0;
    return b;
}

Big copy_then_assign(Big b) {
    Big c = b;
    c.x = 0;
    return c;
}

Big memcpy_then_assign(Big b) {
    Big c;
 __builtin_memcpy(&c, &b, sizeof(b));
    c.x = 0;
    return c;
}

// Optimized

Big c99_initializer(Big b) { return (Big){0, b.y, b.z}; }

Big c99_initializer2(Big b) {
    Big c = {b.x, b.y, b.z};
    c.x = 0;
 return c;
}

Big copy_individually(Big b) {
    Big c;
    c.x = 0;
 c.y = b.y;
    c.z = b.z;
    return c;
}

Big copy_individually_then_assign(Big b) {
    Big c;
    c.x = b.x;
    c.y = b.y;
    c.z = b.z;
    c.x = 0;
    return c;
}
```

Unoptimized assembly:
```asm
assign:
        str     xzr, [x0]
        ldr     x9, [x0, #16]
        ldr     q0, [x0]
        str     x9, [x8, #16]
 str     q0, [x8]
        ret

```

Optimized assembly:
```asm
c99_initializer:
        ldur    q0, [x0, #8]
 str     xzr, [x8]
        stur    q0, [x8, #8]
        ret
```

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