xtensa: Eliminate unused stack frame allocation/freeing

In the example below, 'x' is once placed on the stack frame and then read
into registers as the argument value of bar():

    /* example */
    struct foo {
      int a, b;
    };
    extern struct foo bar(struct foo);
    struct foo test(void) {
      struct foo x = { 0, 1 };
      return bar(x);
    }

Thanks to the dead store elimination, the initialization of 'x' turns into
merely loading the immediates to registers, but corresponding stack frame
growth is not rolled back.  As a result:

    ;; prereq: the CALL0 ABI
    ;; before
    test:
	addi	sp, sp, -16	// unused stack frame allocation/freeing
	movi.n	a2, 0
	movi.n	a3, 1
	addi	sp, sp, 16	// because no instructions that refer to
	j.l	bar, a9		// the stack pointer between the two

This patch eliminates such unused stack frame allocation/freeing:

    ;; after
    test:
	movi.n	a2, 0
	movi.n	a3, 1
	j.l	bar, a9

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (machine_function): New boolean member as
	a flag that controls whether to emit the insns for stack pointer
	adjustment inside of the pro/epilogue.
	(xtensa_emit_adjust_stack_ptr): New function to share the common
	codes and to emit insns if not inhibited.
	(xtensa_expand_epilogue): Change to use the function mentioned
	above when using the CALL0 ABI.
	(xtensa_expand_prologue): Ditto.
	And also change to set the inhibit flag used by
	xtensa_emit_adjust_stack_ptr() to true if the stack pointer is only
	used for its own adjustment.

0 files changed, 0 insertions, 0 deletions