xtensa: Optimize stack pointer updates in function pro/epilogue under certain conditions

This patch enforces the use of "addmi" machine instruction instead of
addition/subtraction with two source registers for adjusting the stack
pointer, if the adjustment fits into a signed 16-bit and is also a multiple
of 256.

    /* example */
    void test(void) {
      char buffer[4096];
      __asm__(""::"m"(buffer));
    }

    ;; before
    test:
	movi.n	a9, 1
	slli	a9, a9, 12
	sub	sp, sp, a9
	movi.n	a9, 1
	slli	a9, a9, 12
	add.n	sp, sp, a9
	addi	sp, sp, 0
	ret.n

    ;; after
    test:
	addmi	sp, sp, -0x1000
	addmi	sp, sp, 0x1000
	ret.n

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_expand_prologue):
	Use an "addmi" machine instruction for updating the stack pointer
	rather than addition/subtraction via hard register A9, if the amount
	of change satisfies the literal value conditions of that instruction
	when the CALL0 ABI is used.
	(xtensa_expand_epilogue): Ditto.
	And also inhibit the stack pointer addition of constant zero.

0 files changed, 0 insertions, 0 deletions