Re: [PATCH] reduce large do_mount stack usage with noinlines

[Andi Kleen <andi@firstfloor.org>]

Andrew Morton <akpm@linux-foundation.org> writes:
>>   */
>> -static int do_change_type(struct nameidata *nd, int flag)
>> +static noinline int do_change_type(struct nameidata *nd, int flag)
>
> What we could do here is defined a new noinline_because_of_stack_suckiness
> and use that.  Reasons:
>
> - self-documenting, so we don't need to comment each site
>
> - can be made a no-op for suitable __GNUC__ values if gcc ever fixes this

In theory it should be already fixed; iirc Richard H. (cc'ed) added
code for this somewhere in 4.x. Don't quite remember which x, likely 
either 1 or 2.

e.g. if I do a quick test here on gcc 4.2 then it definitely
reuses stack slots between inlines. As you can see only ~100 bytes
are allocated, not ~200.

-Andi

% cat ts.c
static inline a(void)
{
	char x[100];
	extf(x);
}

static inline b(void)
{
	char y[100];
	extf(y);
}

f()
{
	a();
	b();		
}
% gcc -O2 -S ts.c
% cat ts.s
  ...
f:
.LFB4:
	pushq	%rbx
.LCFI0:
	xorl	%eax, %eax
	subq	$112, %rsp
.LCFI1:
	movq	%rsp, %rdi
	call	extf
	movq	%rsp, %rdi
	xorl	%eax, %eax
	call	extf
	addq	$112, %rsp
	popq	%rbx
	ret
...
%