Re: How do I stop gcc from loading data into registers when that's not needed?

[Paul Koning <paulkoning at comcast dot net>]

> On May 22, 2018, at 3:26 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> 
> -fdump-rtl-combine-all (or just -da or -dap), and then look at the dump
> file.  Does combine try this combination?  If so, it will tell you what
> the resulting costs are.  If not, why does it not try it?
> 
>> Sorry, I'm not very familiar with this area of GCC either.  Did you confirm
>> that combine at least tries to merge the memory ops into the instruction?
> 
> It should, it's a simple reg dependency.  In many cases it will even do
> it if it is not single-use (via a 3->2 combination).

I examined what gcc does with two simple functions:

void c2(void)
{
    if (x < y)
        z = 1;
    else if (x != y)
        z = 42;
    else
        z = 9;
}

void c3(void)
{
    if (x < y)
        z = 1;
    else
        z = 9;
}

Two things popped out.

1. The original RTL (from the expand phase) has a memory->register move for x and y in c2, but it doesn't for c3 (it simply generates a memory/memory compare there).  What triggers the different choice in that phase?

2. The reported costs for the various insns are
	r22:HI=['x']		6
	cmp(r22:HI,r23:HI)	4
	cmp(['x'],['y'])	16
   so the added cost for the memory argument in the cmp is 6 -- the same as the whole cost for the mov.  That certainly explains the behavior.  It isn't what I want it to be.  Which target hook(s) are involved in these numbers?  I don't see them in my rtx_costs hook.

	paul