It maybe the case that the way GCCJIT frontend generates the code will allow backend to think this as an "access to the inactive union field", which is undefined. Although I am uncertain about the semantic difference between GCCJIT and C.

P.S. The code on godbolt was in C++, which indeed regards this as an undefined behavior. One should switch to a C compiler instead. However, I believe there is no difference for clang/gcc in this particular case.

Pointed out by @Lancern

GNU C and C++ has this as defined code though (it is undefined in standard C/C++ due to the activeness of an union field). https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Optimize-Options.html#index-fstrict-aliasing

The GIMPLE smeantics are the same with respect to an union access is the same as the GNU C too.

Even after I making Float the same size as others by padding long to its tail and utilizing bitcast to to convert union to target struct, the problem still exists.

Even a simple program like:

struct Int;

struct Int
{
  int A:17;
  int B:5;
  int C:10;
};

static void
from_int_to_bitfield (int %arg0, int %arg1)
{
  int %0;
  int %1;
  struct Int %2;
  int %3;
  bool %4;

bb0:
  %0 = %arg0;
  %1 = %arg1;
  %2 = bitcast(%0, struct Int);
  %3 = %2.A:17;
  %4 = %3 == %1;
  if (%4) goto bb1; else goto bb2;

bb1:
  return;

bb2:
  (void)__builtin_trap ();
  goto bb2;
}

extern int
main ()
{
  int %0;
  int %1;
  int %2;

bb0:
  %0 = (int)-559038737;
  %1 = (int)-16657;
  (void)from_int_to_bitfield (%0, %1);
  %2 = (int)0;
  return %2;
}

will be folded into ud2, which exits normally when the function is not internal