[Bug tree-optimization/77647] [6/7 Regression] Missed opportunity to use register value causes additional load

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77647

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
      Known to work|                            |5.4.0
   Target Milestone|---                         |6.3
            Summary|Missed opportunity to use   |[6/7 Regression] Missed
                   |register value causes       |opportunity to use register
                   |additional load             |value causes additional
                   |                            |load

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
DOM doesn't propagate the equivalence on the not unreachable edge, that is,
it doesn't copy-propagate because of PR71947

  if (ret_3 != _5)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  __builtin_unreachable ();

  <bb 4>:
  _4 = ret_3 + _5;

DOM does the following after Jeffs patch:

Optimizing block #4

1>>> STMT 1 = ret_3 le_expr _5
1>>> STMT 1 = ret_3 ge_expr _5
1>>> STMT 1 = ret_3 eq_expr _5
1>>> STMT 0 = ret_3 ne_expr _5
0>>> COPY ret_3 = _5
0>>> COPY _5 = ret_3
Optimizing statement _4 = ret_3 + _5;
  Replaced 'ret_3' with variable '_5'
  Replaced '_5' with variable 'ret_3'
  Folded to: _4 = ret_3 + _5;

which is of course a quite stupid replacement.  I still believe the change
should be reverted ... Jeff, are you still investigating this?

Works on the GCC 5 branch:

Optimizing block #4

0>>> COPY _5 = ret_3
1>>> STMT 1 = ret_3 le_expr _5
1>>> STMT 1 = ret_3 ge_expr _5
1>>> STMT 1 = ret_3 eq_expr _5
1>>> STMT 0 = ret_3 ne_expr _5
Optimizing statement _4 = ret_3 + _5;
  Replaced '_5' with variable 'ret_3'
LKUP STMT _4 = ret_3 plus_expr ret_3
2>>> STMT _4 = ret_3 plus_expr ret_3

with assembly

foo:
.LFB1:
        .cfi_startproc
#APP
# 4 "t.c" 1
        movq      (%rdi),%rax

# 0 "" 2
#NO_APP
        addq    %rax, %rax
        ret