https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89226
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That is because in copy1 it is a normal memcpy expansion.
And, the generic move_by_pieces case is done in preference to target specific
one. In i386.h we have:
/* Max number of bytes we can move from memory to memory
in one reasonably fast instruction. */
#define MOVE_MAX 16
/* MOVE_MAX_PIECES is the number of bytes at a time which we can
move efficiently, as opposed to MOVE_MAX which is the maximum
number of bytes we can move with a single instruction.
??? We should use TImode in 32-bit mode and use OImode or XImode
if they are available. But since by_pieces_ninsns determines the
widest mode with MAX_FIXED_MODE_SIZE, we can only use TImode in
64-bit mode. */
#define MOVE_MAX_PIECES \
((TARGET_64BIT \
&& TARGET_SSE2 \
&& TARGET_SSE_UNALIGNED_LOAD_OPTIMAL \
&& TARGET_SSE_UNALIGNED_STORE_OPTIMAL) \
? GET_MODE_SIZE (TImode) : UNITS_PER_WORD)
and in defaults.h
/* MOVE_MAX_PIECES is the number of bytes at a time which we can
move efficiently, as opposed to MOVE_MAX which is the maximum
number of bytes we can move with a single instruction. */
#ifndef MOVE_MAX_PIECES
#define MOVE_MAX_PIECES MOVE_MAX
#endif
/* STORE_MAX_PIECES is the number of bytes at a time that we can
store efficiently. Due to internal GCC limitations, this is
MOVE_MAX_PIECES limited by the number of bytes GCC can represent
for an immediate constant. */
#ifndef STORE_MAX_PIECES
#define STORE_MAX_PIECES MIN (MOVE_MAX_PIECES, 2 * sizeof (HOST_WIDE_INT))
#endif
/* Likewise for block comparisons. */
#ifndef COMPARE_MAX_PIECES
#define COMPARE_MAX_PIECES MOVE_MAX_PIECES
#endif
#ifndef MAX_MOVE_MAX
#define MAX_MOVE_MAX MOVE_MAX
#endif
