[Bug tree-optimization/51964] Missed tail merging opportunity

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51964

--- Comment #1 from vries at gcc dot gnu.org 2012-01-23 13:43:33 UTC ---
I have a still rather vague idea that we might value number the uses rather
than the defs: assign the same number to uses which use a value in the same
way. I don't know how that would work exactly, but the idea is something like
this:
...
syntax:
a -> b      : a is used to copy to b
a -> <b,c,d>: a is used as b to define c and d.

  # .MEMD.1719_7 = PHI <.MEMD.1719_10(5), .MEMD.1719_11(6)>
.MEMD.1719_10(5) -> .MEMD.1719_7
.MEMD.1719_11(6) -> .MEMD.1719_7

  # aD.1712_1 = PHI <aD.1712_4(5), aD.1712_5(6)>
aD.1712_4(5) -> aD.1712_1
aD.1712_5(6) -> aD.1712_1

  # .MEMD.1719_10 = VDEF <.MEMD.1719_6>
  # USE = nonlocal 
  # CLB = nonlocal 
  aD.1712_4 = barD.1703 (7);
bar          -> <call, .MEMD.1719_7, aD.1712_1>
.MEMD.1719_6 -> <vuse, .MEMD.1719_7, aD.1712_1>
7            -> <callarg0, .MEMD.1719_7, aD.1712_1>

  # .MEMD.1719_11 = VDEF <.MEMD.1719_8(D)>
  # USE = nonlocal 
  # CLB = nonlocal 
  aD.1712_5 = barD.1703 (7);
bar             -> <call, .MEMD.1719_7, aD.1712_1>
.MEMD.1719_8(D) -> <vuse, .MEMD.1719_7, aD.1712_1>
7               -> <callarg0, .MEMD.1719_7, aD.1712_1>
...

By comparing the value numbers of the uses of the 2 calls, we can conclude that
the calls use values in the same way, which means we can merge them. And in
order to merge them we need to insert phis to merge the actual values that are
used. In the example above, we would only need a phi for .MEMD.1719_6 and
.MEMD.1719_8(D).

And if we had f.i. 'bar (7)' and 'bar (8)' in the example, this still would
compare equal, and we would have to insert a phi (7,8) and use that as argument
of the tail-merged call to bar.