[Bug tree-optimization/71815] SLSR misses several PHI candidate cases

["wschmidt at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71815

--- Comment #5 from Bill Schmidt <wschmidt at gcc dot gnu.org> ---
I'll note that in the case where the stride is known (slsr-35.c), SLSR is
making at least a somewhat rational decision based on cost not to
strength-reduce the phi candidate.  In this case the stride is a power of 2, so
the benefit of replacing a multiply is pretty low.  SLSR would have to replace
the multiply with an add, and introduce two adds for the PHI arguments.  There
is a somewhat simplistic calculation of dead code savings for removal of the
PHI itself, in which only PHI arguments that have a single use are considered
removable.  In this case that is overly conservative because the only other use
of _17 is in the definition of _18, which is judged to go dead.  If we
aggressively assume we would remove the definition of _17 as well, SLSR would
make the transformation because the savings would be zero, and we currently do
optimize when the result looks neutral.  So there is opportunity here to be
more intelligent about dead code savings estimation.

Processing dependency tree rooted at 1.
  Conditional candidate 7:
    add_costs = 12                 # Replace mult and phi args
    mult_savings = 4               # It's a shift, so low cost
    dead_savings = 4               # Really should be 8
    cost = 4                       # Really should be 0
  Not replaced.                    # Could replace, but it's on the bubble.

However, for the case of an unknown stride as in slsr-36.c, this seems unlikely
to be a close decision, so I suspect something worse is wrong there.  Will
analyze that next.