[Bug rtl-optimization/78579] redundant instruction of the form cmp r0, r0 generated in assembly with -O2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78579

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-11-29
                 CC|                            |bernds at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, it's really the inability to optimize away the comparison post-reload. 
Only the register allocator figures that the return value is equal to the arg.

I'd have expected postreload-cse to handle this but it doesn't (it doesn't
copy-propagate).  jump2 faces

(insn 14 29 15 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/f:DI 0 ax [orig:87 _1 ] [87])
            (reg:DI 0 ax [92]))) "t.c":3 8 {*cmpdi_1}
     (expr_list:REG_DEAD (reg:DI 0 ax [92])
        (nil)))

but it doesn't seem to be able to do anything about it (maybe the pass name
puts up hope too high...).

That said, pass_postreload_cse should have done its job here.