new auto-inc-dec pass

[Roman Zippel <zippel at linux-m68k dot org>]

Hi,

I'mm currently looking at the dataflow branch for m68k mainly because of 
the new auto-inc-dec pass, I worked a bit on the old code in flow.c, but 
considering the new pass, I think it doesn't make much sense to continue 
it.

I'm attaching my current patch and some test cases, but before delving 
deeper into the code, I have a few questions and I'd like to hear some 
opinions. Currently I only looked at the output and it misses some 
opportunities (not that my patch can do everything either :) ).

One case is about multiple increments, the tree optimizer merges them and 
increments the register only once, so if one only looks at the size of the
pointer value one misses them, e.g. something like this:

	(set (mem (reg)) (x))
	(set (mem (plus (reg) (const_int 4))) (x))
	(set (reg) (plus (reg) (const_int 8))

Another case are multiple uses of the register within an instruction, the 
old code didn't do this at all, the new code seems to do a bit better, but 
there as a rather tricky case I looked into, e.g.

	(set (mem) (plus (mem) (reg)))

could be turned into:

	(set (post_inc (mem)) (plus (mem) (reg)))

or

	(set (mem) (plus (pre_dec (mem)) (reg)))

This is at least what operands_match_p() accepts, but hasn't been 
generated in a long time and has the potential to trigger bugs in the 
back end (e.g. m68k only looks only at one of the operands).
A question would be now if there is a general interest in reactivating 
this, so gcc could generate a instruction like "add.l %d0,(%a0)+" for 
m68k.

The more general question is whether someone is already working on further 
improvements, so I won't duplicate anything (which wouldn't be before I 
looked into the remaining m68k dataflow problems anyway. :) ).

The dataflow porting document mentions other possible, but doesn't mention 
any examples. Anything I might have to look out for regardings the m68k 
post_inc/pre_dec addressing modes?

bye, Roman

void f1(int *p, int i)
{
	*--p = i;
}
int f1b(int *p, int i)
{
	return *--p;
}
void f2(int *p, int i)
{
	*--p = i++;
	*--p = i;
}
int *f3(int *p, int i)
{
	*p++ = i;
	return p;
}
int *f4(int *p, int i)
{
	*p++ = 1;
	*p++ = 2;
	return p;
}
int *f5(int *p, int i)
{
	*p++ = i++;
	*p++ = i;
	return p;
}
void f6(int *p, int i)
{
	while (--i) {
		*p++ = 1;
		*p++ = 2;
	}
}
void f7(int *p, int i)
{
	while (--i) {
		*--p = 1;
		*--p = 2;
	}
}
void f8(int *p, int i)
{
	*p++ = 1;
	*p = 2;
}
void f9(int *p, int i)
{
	*p++ += 1;
	*p = 2;
}
void f10(int *p, int i)
{
	*p++ = 1;
	*p += 2;
}

Attachment: autoinc.diff
Description: Text document