https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64081
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, the change allows DOM to CSE pos = 0 in
<bb 2>:
pos = 0;
dir = 1;
_45 = (long unsigned int) argc_9(D);
if (_45 != 0)
goto <bb 3>;
else
goto <bb 21>;
<bb 3>:
pretmp_75 = data;
pretmp_77 = token;
arr1.5_23 = arr1;
arr2.7_27 = arr2;
pos_lsm.17_22 = pos;
which in turn allows jump threading to do its work.
Registering jump thread: (11, 12) incoming edge; (12, 5) joiner; (5, 23)
normal; (23, 21) nocopy;
...
Threaded jump 11 --> 12 to 25
this changes the loop to have two latches (thus it becomes a loop nest), and
it adds one exit (now the loop has three). Further down the road the extra
loop is no longer there but the three exits remain.
I don't see what is wrong with what DOM does here.
We do miss some interesting kind of optimization opportunities like
transforming
if (prephitmp_87 == 1)
goto <bb 9>;
else
goto <bb 10>;
<bb 9>:
_24 = arr1.5_23 + _62;
pos.6_25 = *_24;
goto <bb 11>;
<bb 10>:
_28 = arr2.7_27 + _62;
pos.8_29 = *_28;
<bb 11>:
# prephitmp_89 = PHI <pos.6_25(9), pos.8_29(10)>
to
if (prephitmp_87 == 1)
goto <bb 9>;
else
goto <bb 11>;
<bb 9>:
goto <bb 11>;
<bb 11>:
# _24 = PHI <arr1.5_23, arr2.7_27>
_28 = _24 + _62;
prephitmp_89 = *_28;
sinking common computations through a PHI.
With the followup optimization in out-of-SSA to coalesce arr1.5_23 and
arr2.7_27 which means we can drop the conditional entirely.
Heh. Fun idea.
Anyway - for this PR it is RTL unroll IV analysis that needs to be improved.
