[Bug target/81356] __builtin_strcpy is not good for copying an empty string on aarch64

[qing.zhao at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81356

--- Comment #5 from Qing Zhao <qing.zhao at oracle dot com> ---
the following code in config/aarch64/aarch64.c cause such behavior:

14143 static bool
14144 aarch64_use_by_pieces_infrastructure_p (unsigned HOST_WIDE_INT size,
14145                                         unsigned int align,
14146                                         enum by_pieces_operation op,
14147                                         bool speed_p)
14148 {
14149   /* STORE_BY_PIECES can be used when copying a constant string, but
14150      in that case each 64-bit chunk takes 5 insns instead of 2 (LDR/STR).
14151      For now we always fail this and let the move_by_pieces code copy
14152      the string from read-only memory.  */
14153   if (op == STORE_BY_PIECES)
14154     return false;

when deleting line 14153 and 14154. and use this compiler to build the testing
case, I got:

f:
        mov     w1, 26952
        movk    w1, 0x21, lsl 16
        str     w1, [x0]
        ret

looks like exactly we want.