[Bug target/110136] After optimization, the $r1 register will be broken when jumping to the jump table, resulting in a significant increase in the false prediction rate of branch prediction.

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110136

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> >In the regrename passover optimization
> 
> I am trying to understand the issue.
> 
> 5912         ldx.d   $r20,$r16,$r19
>  5913         add.d   $r1,$r16,$r20
>  5914         jr      $r1
> 
> Is the issue is jr does not like r1 register or some other kind of
> performance issue?

If it is just r1 that is the issue, you could change the pattern in
loongarch.md to discourage r1 by changing the constraints there.
Because right now it assumes all registers are similar in cost:

(define_insn "@indirect_jump<mode>"
  [(set (pc) (match_operand:P 0 "register_operand" "r"))]
  ""
  "jr\t%0"
  [(set_attr "type" "jump")
   (set_attr "mode" "none")])