[llvm-bugs] [Bug 50623] New: Overly optimized simultaneous copy of struct fields corrupts destination struct

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50623

            Bug ID: 50623
           Summary: Overly optimized simultaneous copy of struct fields
                    corrupts destination struct
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: samuel.sadok at bluewin.ch
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Clang 12.0.0 modifies a field of a struct that should not be modified. This is
a regression from clang 11.0.1.

Input File:
```
struct S { char val1; char padding; short val2; };

void func(struct S src, struct S* dest) {
    dest->val1 = src.val1;
    dest->val2 = src.val2;
}
```

Output from clang 12.0.0 (clang -c test.c -o test.o -O1 && objdump -S test.o -M
intel):
```
0000000000000000 <func>:
   0:   89 3e                   mov    DWORD PTR [rsi],edi
   2:   c3                      ret    
```

Output from clang 11.0.1:
```
0000000000000000 <func>:
   0:   40 88 3e                mov    BYTE PTR [rsi],dil
   3:   c1 ef 10                shr    edi,0x10
   6:   66 89 7e 02             mov    WORD PTR [rsi+0x2],di
   a:   c3                      ret    
```

The output from clang 12.0.0 is incorrect because it modifies the field
`dest->padding`.

See also here: https://godbolt.org/z/dsr9soqbf
"armv8-a clang (trunk)" exhibits the same problem.

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