Hi John,
On Mon, Aug 12, 2019 at 08:47:43AM +0200, John Darrington wrote:
> On Sat, Aug 10, 2019 at 11:12:18AM -0500, Segher Boessenkool wrote:
> On Sat, Aug 10, 2019 at 08:05:53AM +0200, John Darrington wrote:
> > Choosing alt 5 in insn 14: (0) m (1) m {*movsi}
> > 14: [r40:PSI+0x20]=[r41:PSI]
> > Inserting insn reload before:
> > 48: r40:PSI=r34:PSI
> > 49: r41:PSI=[y:PSI+0x2f]
>
> insn 14 is a mem-to-mem move (another feature not many more modern /
> more RISCy CPUs have). That requires both of your address registers.
> So far, so good. The reloads (insn 48 and 49) require address
> registers themselves; that isn't necessarily a problem either.
>
> So far as I can see, insn 48 is completely redundant. It's copying a
> pseudo reg (74) into another pseudo reg (40).
> This is pointless and a waste, since insn 14 does not modify 74.
> I don't understand why lra feels the need to do it.
LRA always does this, I think... it reloads all inputs to all insns
that may need reloading. It later optimises most of that away again,
but this gives it a lot of freedom to move things around.
Or that is what it always looked like to me. I haven't looked at the
code to see if that is the real reason, blush.
> If lra knew about (mem (mem ...)) style addressing, then insn 49 would
> also be redundant (which is why I raised the topic).
Yes. But it probably should be able to deal with things like this, too,
or some other testcases will die a horrible death.
> In summary, what we have is:
>
> (insn 48 84 49 2 (set (reg/f:PSI 40 [34])
> (reg/f:PSI 74 [34]))
> (nil))
> (insn 49 48 14 2 (set (reg:PSI 41)
> (mem/f/c:PSI (plus:PSI (reg/f:PSI 9 y)
> (const_int 47 [0x2f])) [3 p+0 S4 A8]))
> (nil))
> (insn 14 49 15 2 (set (mem:SI (plus:PSI (reg/f:PSI 40 [34])
> (const_int 32 [0x20])) [2 S4 A64])
> (mem:SI (reg:PSI 41) [2 *p_5(D)+0 S4 A8]))
>
> where, like you say, insns 48 and 49 are reloads. But these two reloads
> are unnecessary and cause the machine to run out of PSImode registers.
Anyway, please have patience, and see what Vladimir comes up with. These
things take time.
Segher
