[AArch64][ARM] Why fpack-struct=4 has a higher priority than attribute

[LukeSTM]

struct s1
{
  int __attribute__ ((aligned (8))) a;
};

struct
{
  char c;
  struct s1 m;
} v;

int
main (void)
{
  if ((int)&v.m & 7)
    abort ();
  exit (0);
}

compile with only one option：-fpack-struct=4

gcc could pass this testcase, but clang could not. https://godbolt.org/z/h5T5rdYWb

The code can be simplified as follows:

struct s1
{
  int __attribute__ ((aligned (8))) a;
}s;

https://godbolt.org/z/EbGsEz6eP

if compile without -fpack-struct=4, the alignment of struct is 8, as same as attribue. But if compile with -fpack-struct=4, the alignment of struct is 4. Why fpack-struct=4 has a higher priority than attribute? I think attribute should has a higher priority than Compilation options

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Luke (LukeSTM)

``` struct s1 { int __attribute__ ((aligned (8))) a; }; struct { char c; struct s1 m; } v; int main (void) { if ((int)&v.m & 7) abort (); exit (0); } ``` compile with only one option：-fpack-struct=4 gcc could pass this testcase, but clang could not. https://godbolt.org/z/h5T5rdYWb The code can be simplified as follows: ``` struct s1 { int __attribute__ ((aligned (8))) a; }s; ``` https://godbolt.org/z/EbGsEz6eP  if compile without -fpack-struct=4, the alignment of struct is 8, as same as attribue. But if compile with -fpack-struct=4, the alignment of struct is 4. Why fpack-struct=4 has a higher priority than attribute? I think attribute should has a higher priority than Compilation options

[LukeSTM]

attribute ((packed)) has a higher priority than fpack-struct=4

https://godbolt.org/z/j3x8P93z3

[LukeSTM]

It seem like llvm prefer a small alignment. If attribute ((align (4))) and fpack-struct=8, alignment = 4. If attribute ((align (4))) and fpack-struct, alignment = 1. https://godbolt.org/z/ddeMvzEGE

[hstk30-hw]

Is this an expected behavior? CC @AaronBallman @zygoloid

[LukeSTM]

[LukeSTM]

I understand that the alignment rules of the structures of gcc and llvm should be the same, but the default alignment rules are different. gcc arm struct alignment default 4byte，and could not less than 4byte。llvm does not have this restriction.