[Bug middle-end/126118] [SH] Redundant movt tst sequence

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126118

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |sh*-*-*
   Last reconfirmed|                            |2026-07-05
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
          Component|target                      |middle-end

--- Comment #1 from Oleg Endo <olegendo at gcc dot gnu.org> ---
Initial expansion on GCC 17 and of GCC 15 (last known working):

(insn 6 5 7 (set (reg:SI 165)
        (const_int 0 [0])) "sh_tmp.cpp":6:5 -1
     (nil))

(insn 7 6 0 (parallel [
            (asm_operands/v ("cmp/str %0,%1") ("") 0 [
                    (reg:SI 165)
                    (reg/v:SI 164 [ val ])
                ]
                 [
                    (asm_input:SI ("r") sh_tmp.cpp:6)
                    (asm_input:SI ("r") sh_tmp.cpp:6)
                ]
                 [] sh_tmp.cpp:6)
            (clobber (reg:SI 147 t))
        ]) "sh_tmp.cpp":6:5 -1
     (nil))

;; t.0_1 = t;

(insn 8 7 0 (set (reg:SI 162 [ t.0_1 ])
        (reg/v:SI 147 t [ t ])) "sh_tmp.cpp":8:5 -1
     (nil))

;; return _4;

(insn 9 8 10 (set (reg:SI 147 t)
        (eq:SI (reg:SI 162 [ t.0_1 ])
            (const_int 0 [0]))) "sh_tmp.cpp":8:12 -1
     (nil))

(insn 10 9 11 (set (reg:SI 168)
        (const_int -1 [0xffffffffffffffff])) "sh_tmp.cpp":8:12 -1
     (nil))

(insn 11 10 12 (parallel [
            (set (reg:SI 167)
                (xor:SI (reg:SI 147 t)
                    (const_int 1 [0x1])))
            (set (reg:SI 147 t)
                (const_int 1 [0x1]))
            (use (reg:SI 168))
        ]) "sh_tmp.cpp":8:12 -1
     (nil))

(insn 12 11 13 (set (reg:QI 166 [ _4 ])
        (subreg:QI (reg:SI 167) 0)) "sh_tmp.cpp":8:12 -1
     (nil))

(insn 13 12 14 (set (reg:SI 169)
        (zero_extend:SI (reg:QI 166 [ _4 ]))) "sh_tmp.cpp":8:12 discrim 1 -1
     (nil))


However, looking at the first diff points in the combine log...

GCC 15:

Trying 11 -> 13:
   11: {r167:SI=t:SI^0x1;t:SI=0x1;use r168:SI;}
      REG_DEAD r168:SI
      REG_UNUSED t:SI
   13: r169:SI=zero_extend(r167:SI#0)
      REG_DEAD r167:SI
Successfully matched this instruction:
(set (reg:SI 169 [ _4 ])
    (xor:SI (zero_extend:SI (subreg:QI (reg:SI 147 t) 0))
        (const_int 1 [0x1])))


GCC 17:

Trying 11 -> 13:
   11: {r167:SI=t:SI^0x1;t:SI=0x1;use r168:SI;}
      REG_DEAD r168:SI
      REG_UNUSED t:SI
   13: r169:SI=zero_extend(r167:SI#0)
      REG_DEAD r167:SI
Failed to match this instruction:
(set (reg:SI 169 [ _4 ])
    (xor:SI (zero_extend:SI (clobber:QI (const_int 0 [0])))
        (const_int 1 [0x1])))



For whatever reason, combine replaces 

     (subreg:QI (reg:SI 147 t) 0)

with 

     (clobber:QI (const_int 0 [0]))


This doesn't make any sense to me.