* expr.h (store_by_pieces): Add prototype.
	(can_store_by_pieces): Likewise.
	* expr.c (struct store_by_pieces): Renamed from clear_by_pieces.
	(can_store_by_pieces): New.
	(store_by_pieces): New.
	(clear_by_pieces): New.
	(clear_by_pieces_1): New.
	(store_by_pieces_1): Renamed from clear_by_pieces, handle storing
	arbitrary compiler generated constants into memory block.
	(store_by_pieces_2): Renamed from clear_by_pieces_1, likewise.
	* builtins.c (c_readstr): New.
	(builtin_memcpy_read_str): New.
	(expand_builtin_memcpy): If src is string constant and
	emit_block_move would move it by pieces, compute integer constants
	from the string and store it into memory block instead.
	(builtin_strncpy_read_str): New.
	(expand_builtin_strncpy): If N is not constant zero and c_strlen does
	not return INTEGER_CST, don't optimize.
	If N is larger than strlen(src) + 1, try to copy the string
	including padding with store_by_pieces.
	(expand_builtin_strcmp): If both arguments have side effects, don't
	optimize.
	(expand_builtin_fputs): If STR has side effects, don't optimize.

	* gcc.c-torture/execute/string-opt-5.c: Add some strcmp and strncpy
	tests.
	* gcc.c-torture/execute/string-opt-6.c: New test.

From-SVN: r37851