[Bug target/85610] Unable to optimize away mov followed by compare into a cmpb in case of atomic_load

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85610

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|                            |x86_64-*-*, i?86-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-05-03
          Component|tree-optimization           |target
            Version|tree-ssa                    |7.3.1
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  RTL expansion looks ok so it looks like a combine/backend issue.

;; _5 = __atomic_load_1 (&MEM[(const struct __atomic_base *)&flag_atomic]._M_i,
0);

(insn 5 4 0 (set (reg:QI 87 [ _5 ])
        (mem/v:QI (symbol_ref:DI ("flag_atomic") [flags 0x2]  <var_decl
0x7ffff665d360 flag_atomic>) [-1  S1 A8]))
"/usr/include/c++/7/bits/atomic_base.h":396 -1
     (nil))

;; if (_5 == 0)

(insn 6 5 7 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:QI 87 [ _5 ])
            (const_int 0 [0]))) "t.C":9 -1
     (nil))

(jump_insn 7 6 0 (set (pc)
        (if_then_else (ne (reg:CCZ 17 flags)
                (const_int 0 [0]))
            (label_ref 0)
            (pc))) "t.C":9 -1
     (int_list:REG_BR_PROB 5400 (nil)))