https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71976
--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Bugzille does not allow me to attach the .combine dump (for reference).
...anyway the relevant part of the dump is:
In .242r.ud_dce there is the following right shift insn:
(insn 51 50 52 2 (set (reg:QI 16 r16)
(const_int 40 [0x28])) bug-combin.c:29 71 {movqi_insn}
(nil))
(insn 52 51 61 2 (set (reg:DI 18 r18)
(ashiftrt:DI (reg:DI 18 r18)
(reg:QI 16 r16))) bug-combin.c:29 1416 {ashrdi3_insn}
(expr_list:REG_DEAD (reg:QI 16 r16)
(nil)))
Insn combine tries to macht the combination of these two insns:
Trying 51 -> 52:
Failed to match this instruction:
(set (reg:DI 18 r18)
(reg:DI 18 r18))
allowing combination of insns 51 and 52
original costs 4 + 32 = 36
replacement cost 4
deferring deletion of insn with uid = 51.
modifying insn i3 52: r18:DI=r18:DI
deferring rescan insn with uid = 52.
So the combination is wrong; for some reason the shift is transformed to a
no-op move and then removed...
