if-conv: Small improvement for expansion of complex PHIs [PR109154]

if-conv: Small improvement for expansion of complex PHIs [PR109154]

The following patch is just a dumb improvement, gets rid of 2 unnecessary
instructions on both the PR's original testcase and on the two reduced ones,
both on -mcpu=neoverse-v1 and -mavx512f.

The thing is, if we have args_len (args_len >= 2) unique PHI arguments,
we need only args_len - 1 COND_EXPRs to expand the PHI, because first
COND_EXPR can merge 2 unique arguments and all the following ones merge
another unique argument with the previously merged arguments,
while the code for mysterious reasons was always emitting args_len
COND_EXPRs, where the first COND_EXPR merged the first and second unique
arguments, the second COND_EXPR merged the second unique argument with
result of merging the first and second unique arguments and the rest was
already expectable, nth COND_EXPR for n > 2 merged the nth unique argument
with result of merging the previous unique arguments.
Now, in my understanding, the bb_predicate for bb's predecessor need to
form a disjunct set which together creates the successor's bb_predicate,
so I don't see why we'd need to check all the bb_predicates, if we check
all but one then when all those other ones are false the last bb_predicate
is necessarily true.  Given that the code attempts to sort argument with
most occurrences (so likely most complex combined predicate) last, I chose
not to test that last argument's predicate.
So e.g. on the testcase from comment 47 in the PR:
void
foo (int *f, int d, int e)
{
  for (int i = 0; i < 1024; i++)
    {
      int a = f[i];
      int t;
      if (a < 0)
        t = 1;
      else if (a < e)
        t = 1 - a * d;
      else
        t = 0;
      f[i] = t;
    }
}
we used to emit:
  _7 = a_10 < 0;
  _21 = a_10 >= 0;
  _22 = a_10 < e_11(D);
  _23 = _21 & _22;
  _26 = a_10 >= e_11(D);
  _27 = _21 & _26;
  _ifc__42 = _7 ? 1 : t_13;
  _ifc__43 = _23 ? t_13 : _ifc__42;
  t_6 = _27 ? 0 : _ifc__43;
while the following patch changes it to:
  _7 = a_10 < 0;
  _21 = a_10 >= 0;
  _22 = a_10 < e_11(D);
  _23 = _21 & _22;
  _ifc__42 = _23 ? t_13 : 0;
  t_6 = _7 ? 1 : _ifc__42;
which I believe should be sufficient for a PHI <1, t_13, 0>.

I've gathered some statistics and on x86_64-linux and i686-linux
bootstraps/regtests, this code triggers:
     92 4 4
    112 2 4
    141 3 4
   4046 3 3
(where 2nd number is args_len and 3rd argument EDGE_COUNT (bb->preds)
and first argument count of those from sort | uniq -c | sort -n).
In all these cases the patch should squeze one extra COND_EXPR and
its associated predicate (the latter only if it wasn't used elsewhere).

Incrementally, I think we should try to perform some analysis on which
predicates depend on inverses of other predicates and if possible try
to sort the arguments better and omit testing unnecessary predicates.
So essentially for the above testcase deconstruct it back to:
  _7 = a_10 < 0;
  _22 = a_10 < e_11(D);
  _ifc__42 = _22 ? t_13 : 0;
  t_6 = _7 ? 1 : _ifc__42;
which is like what this patch produces, but with the & a_10 >= 0 part
removed, because the last predicate is a_10 < 0 and so testing a_10 >= 0
on what appears on the false branch doesn't make sense.
But I'm afraid that will take more work than is doable in stage4 right now.

2023-04-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/109154
	* tree-if-conv.cc (predicate_scalar_phi): For complex PHIs, emit just
	args_len - 1 COND_EXPRs rather than args_len.  Formatting fix.