[Bug target/64215] -Os misses an opportunity to merge two ret instructions

["law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64215

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #5 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Andrew, the reason the patch you referenced doesn't help this case is because
we don't have an unconditional jump to a return only block.

To optimize this case we'd have to detect that we have a return only block that
is immediately preceded by another return block after bbro.

ie:

(note 48 23 59 6 [bb 6] NOTE_INSN_BASIC_BLOCK)
(insn 59 48 49 6 (use (reg/i:SI 10 a0)) -1
     (nil))
(jump_insn 49 59 37 6 (simple_return) 346 {simple_return}
     (nil)
 -> simple_return)
;; lr  out       1 [ra] 2 [sp] 10 [a0]
;; live  out     1 [ra] 2 [sp] 10 [a0]

;;  succ:       EXIT [always]  count:52738306 (estimated locally, freq 0.4591)

;; basic block 7, loop depth 0, count 6317494 (estimated locally, freq 0.0550),
maybe hot
;;  prev block 6, next block 1, flags: (REACHABLE, RTL)
;;  pred:       2 [5.5% (guessed)]  count:6317494 (estimated locally, freq
0.0550) (CAN_FALLTHRU)
;; bb 7 artificial_defs: { }
;; bb 7 artificial_uses: { u-1(2){ }}
;; lr  in        1 [ra] 2 [sp] 10 [a0]
;; lr  use       2 [sp] 10 [a0]
;; lr  def
;; live  in      1 [ra] 2 [sp] 10 [a0]
;; live  gen
;; live  kill

(code_label 37 49 36 7 4 (nil) [1 uses])
(note 36 37 60 7 [bb 7] NOTE_INSN_BASIC_BLOCK)
(insn 60 36 51 7 (use (reg/i:SI 10 a0)) -1
     (nil))
(jump_insn 51 60 41 7 (simple_return) 346 {simple_return}
     (nil)
 -> simple_return)