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.

If lra knew about (mem (mem ...)) style addressing, then insn 49 would
also be redundant (which is why I raised the topic).

In summary, what we have is:

(insn 48 84 49 2 (set (reg/f:PSI 40 [34])
        (reg/f:PSI 74 [34]))
(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]))
(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.
The above could be easier and more efficiently done simply as:

(insn 14 11 15 2 (set 
        (mem:SI (plus:PSI (reg/f:PSI 74 [34]) (const_int 32 [0x20])) [2  S4 
        (mem/f/c:PSI (mem:PSI (plus:PSI (reg/f:PSI 9 y)
                (const_int 47 [0x2f])) [3 p+0 S4 A8])))

This is exactly what we had before lra messed with things.  It can be
represented in the ISA with one assembler instruction: 
  mov.p (32, x), [47, y]
and if I'm not mistaken, alternative 5 of my "movpsi" pattern should do
this just fine.

     this requires careful juggling.  Maybe you will need some backend code

Could you give a hint into which set of hooks/constraints/predicates
this backend code should go?

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