https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82683
Wilco <wilco at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2017-10-24
CC| |segher at gcc dot gnu.org,
| |wilco at gcc dot gnu.org
Component|target |rtl-optimization
Summary|GCC generates bad code with |Combine: GCC generates bad
|-tune=thunderx2t99 |code with
| |-tune=thunderx2t99
Ever confirmed|0 |1
--- Comment #1 from Wilco <wilco at gcc dot gnu.org> ---
Combine seems to do the correct thing at first, creating a ldrsw but it then
invents a spurious REG_DEAD for r83:
Successfully matched this instruction:
(set (reg:DI 83 [ _26 ])
(sign_extend:DI (mem:SI (plus:DI (mult:DI (reg:DI 83 [ _26 ])
(const_int 4 [0x4]))
(reg/f:DI 76 [ _4 ])) [8 *_7+0 S4 A32])))
Successfully matched this instruction:
(set (reg/v:SI 82 [ pD.3023 ])
(subreg:SI (reg:DI 83 [ _26 ]) 0))
allowing combination of insns 19, 20 and 21
original costs 8 + 20 + 4 = 32
replacement costs 28 + 4 = 32
deferring rescan insn with uid = 21.
deferring deletion of insn with uid = 19.
modifying insn i2 20: r83:DI=sign_extend([r83:DI*0x4+r76:DI])
REG_DEAD r76:DI
deferring rescan insn with uid = 20.
modifying insn i3 21: r82:SI=r83:DI#0 ** all good so far, both r82/r83 are
live
REG_DEAD r83:DI *** OOPS
REG_DEAD r83:DI
REG_DEAD r83:DI
deferring rescan insn with uid = 21.
The REG_DEAD r83 incorrect as both r82 and r83 are still live. I think it may
be due to this earlier combine:
Trying 18 -> 19:
Successfully matched this instruction:
(set (reg/f:DI 78 [ _7 ])
(plus:DI (ashift:DI (reg:DI 83 [ _26 ])
(const_int 2 [0x2]))
(reg/f:DI 76 [ _4 ])))
allowing combination of insns 18 and 19
original costs 4 + 4 = 8
replacement cost 8
deferring deletion of insn with uid = 18.
modifying insn i3 19: r78:DI=r83:DI<<0x2+r76:DI
REG_DEAD r83:DI
REG_DEAD r76:DI
deferring rescan insn with uid = 19.
So I guess the bug is that when combining multiple instructions, dead notes
aren't removed if later instructions assign the the same register.
