tree-if-conv.c: fix ICE seen with -fno-tree-forwprop (PR tree-optimization/84178)

PR tree-optimization/84178 reports a couple of source files that ICE inside
ifcvt when compiled with -03 -fno-tree-forwprop (trunk and gcc 7).

Both cases involve problems with ifcvt's per-BB gimplified predicates.

Testcase 1 fails this assertion within release_bb_predicate during cleanup:

283	      if (flag_checking)
284		for (gimple_stmt_iterator i = gsi_start (stmts);
285		     !gsi_end_p (i); gsi_next (&i))
286		  gcc_assert (! gimple_use_ops (gsi_stmt (i)));

The testcase contains a division in the loop, which leads to
if_convertible_loop_p returning false (due to gimple_could_trap_p being true
for the division).  This happens *after* the per-BB gimplified predicates
have been created in predicate_bbs (loop).
Hence tree_if_conversion bails out to "cleanup", but the gimplified predicates
exist and make use of SSA names; for example this conjunction for two BB
conditions:

  _4 = h4.1_112 != 0;
  _175 = (signed char) _117;
  _176 = _175 >= 0;
  _174 = _4 & _176;

is using SSA names.

This assertion was added in r236498 (aka c3deca2519d97c55876869c57cf11ae1e5c6cf8b):

    2016-05-20  Richard Biener  <rguenther@suse.de>

        * tree-if-conv.c (add_bb_predicate_gimplified_stmts): Use
        gimple_seq_add_seq_without_update.
        (release_bb_predicate): Assert we have no operands to free.
        (if_convertible_loop_p_1): Calculate post dominators later.
        Do not free BB predicates here.
        (combine_blocks): Do not recompute BB predicates.
        (version_loop_for_if_conversion): Save BB predicates around
        loop versioning.

        * gcc.dg/tree-ssa/ifc-cd.c: Adjust.

The following patch fixes this by adding a call to gimple_seq_discard
to release_bb_predicate.  It also updates the assertion, so that
instead of asserting the stmts have no imm uses, instead assert that
they weren't added to a bb before discarding them (otherwise discarding
them would be a bug).  We know this is the case because
insert_gimplified_predicates has:

	  /* Once the sequence is code generated, set it to NULL.  */
	  set_bb_predicate_gimplified_stmts (bb, NULL);

but asserting it seems appropriate as a double-check.

The patch doesn't address the 2nd issue within PR tree-optimization/84178.

gcc/ChangeLog:
	PR tree-optimization/84178
	* tree-if-conv.c (release_bb_predicate): Remove the
	the assertion that the stmts have NULL use_ops.
	Discard the statements, asserting that they haven't
	yet been added to a BB.

gcc/testsuite/ChangeLog:
	PR tree-optimization/84178
	* gcc.c-torture/compile/pr84178-1.c: New test.

From-SVN: r258363

gcc/ChangeLog

+2018-03-08  David Malcolm  <dmalcolm@redhat.com>
+
+	PR tree-optimization/84178
+	* tree-if-conv.c (release_bb_predicate): Remove the
+	the assertion that the stmts have NULL use_ops.
+	Discard the statements, asserting that they haven't
+	yet been added to a BB.
+
 2018-03-08  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/84746

gcc/testsuite/ChangeLog

+2018-03-08  David Malcolm  <dmalcolm@redhat.com>
+
+	PR tree-optimization/84178
+	* gcc.c-torture/compile/pr84178-1.c: New test.
+
 2018-03-08  Thomas Schwinge  <thomas@codesourcery.com>
 
 	* lib/target-supports.exp: Do not return zero from a ifunc

gcc/testsuite/gcc.c-torture/compile/pr84178-1.c

+/* { dg-options "-fno-tree-forwprop" } */
+
+int zy, h4;
+
+void
+r8 (long int mu, int *jr, int *fi, short int dv)
+{
+  do
+    {
+      int tx;
+
+      tx = !!h4 ? (zy / h4) : 1;
+      mu = tx;
+      *jr = (((unsigned char) mu > (254 >> dv)) ? 0 : (unsigned char) tx) + *fi;
+    } while (*jr == 0);
+
+  r8 (mu, jr, fi, 1);
+}

gcc/tree-if-conv.c

@@ -271,8 +271,7 @@ init_bb_predicate (basic_block bb)
   set_bb_predicate (bb, boolean_true_node);
 }
 
-/* Release the SSA_NAMEs associated with the predicate of basic block BB,
-   but don't actually free it.  */
+/* Release the SSA_NAMEs associated with the predicate of basic block BB.  */
 
 static inline void
 release_bb_predicate (basic_block bb)
@@ -280,11 +279,14 @@ release_bb_predicate (basic_block bb)
   gimple_seq stmts = bb_predicate_gimplified_stmts (bb);
   if (stmts)
     {
+      /* Ensure that these stmts haven't yet been added to a bb.  */
       if (flag_checking)
 	for (gimple_stmt_iterator i = gsi_start (stmts);
 	     !gsi_end_p (i); gsi_next (&i))
-	  gcc_assert (! gimple_use_ops (gsi_stmt (i)));
+	  gcc_assert (! gimple_bb (gsi_stmt (i)));
 
+      /* Discard them.  */
+      gimple_seq_discard (stmts);
       set_bb_predicate_gimplified_stmts (bb, NULL);
     }
 }