An excerpt from GCC's reload.c

gistfile1.c

static int
find_reusable_reload (rtx *p_in, rtx out, enum reg_class rclass,
		      enum reload_type type, int opnum, int dont_share)
{
  rtx in = *p_in;
  int i;
  /* We can't merge two reloads if the output of either one is
     earlyclobbered.  */

  if (earlyclobber_operand_p (out))
    return n_reloads;

  /* We can use an existing reload if the class is right
     and at least one of IN and OUT is a match
     and the other is at worst neutral.
     (A zero compared against anything is neutral.)

     For targets with small register classes, don't use existing reloads
     unless they are for the same thing since that can cause us to need
     more reload registers than we otherwise would.  */

  for (i = 0; i < n_reloads; i++)
    if ((reg_class_subset_p (rclass, rld[i].rclass)
	 || reg_class_subset_p (rld[i].rclass, rclass))
	/* If the existing reload has a register, it must fit our class.  */
	&& (rld[i].reg_rtx == 0
	    || TEST_HARD_REG_BIT (reg_class_contents[(int) rclass],
				  true_regnum (rld[i].reg_rtx)))
	&& ((in != 0 && MATCHES (rld[i].in, in) && ! dont_share
	     && (out == 0 || rld[i].out == 0 || MATCHES (rld[i].out, out)))
	    || (out != 0 && MATCHES (rld[i].out, out)
		&& (in == 0 || rld[i].in == 0 || MATCHES (rld[i].in, in))))
	&& (rld[i].out == 0 || ! earlyclobber_operand_p (rld[i].out))
	&& (small_register_class_p (rclass)
	    || targetm.small_register_classes_for_mode_p (VOIDmode))
	&& MERGABLE_RELOADS (type, rld[i].when_needed, opnum, rld[i].opnum))
      return i;

  /* Reloading a plain reg for input can match a reload to postincrement
     that reg, since the postincrement's value is the right value.
     Likewise, it can match a preincrement reload, since we regard
     the preincrementation as happening before any ref in this insn
     to that register.  */
  for (i = 0; i < n_reloads; i++)
    if ((reg_class_subset_p (rclass, rld[i].rclass)
	 || reg_class_subset_p (rld[i].rclass, rclass))
	/* If the existing reload has a register, it must fit our
	   class.  */
	&& (rld[i].reg_rtx == 0
	    || TEST_HARD_REG_BIT (reg_class_contents[(int) rclass],
				  true_regnum (rld[i].reg_rtx)))
	&& out == 0 && rld[i].out == 0 && rld[i].in != 0
	&& ((REG_P (in)
	     && GET_RTX_CLASS (GET_CODE (rld[i].in)) == RTX_AUTOINC
	     && MATCHES (XEXP (rld[i].in, 0), in))
	    || (REG_P (rld[i].in)
		&& GET_RTX_CLASS (GET_CODE (in)) == RTX_AUTOINC
		&& MATCHES (XEXP (in, 0), rld[i].in)))
	&& (rld[i].out == 0 || ! earlyclobber_operand_p (rld[i].out))
	&& (small_register_class_p (rclass)
	    || targetm.small_register_classes_for_mode_p (VOIDmode))
	&& MERGABLE_RELOADS (type, rld[i].when_needed,
			     opnum, rld[i].opnum))
      {
	/* Make sure reload_in ultimately has the increment,
	   not the plain register.  */
	if (REG_P (in))
	  *p_in = rld[i].in;
	return i;
      }
  return n_reloads;
}

WHAT THE FUCK

Ah yes

https://gcc.gnu.org/wiki/reload

Reload does everything, and probably no one exactly knows how much that is.