> On May 22, 2018, at 3:26 PM, Segher Boessenkool <seg...@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
