8291775: C2: assert(r != __null && r->is_Region()) failed: this phi must have a region

src/hotspot/share/opto/stringopts.cpp

@@ -294,10 +294,12 @@ void StringConcat::eliminate_unneeded_control() {
       Node* cmp = bol->in(1);
       assert(cmp->is_Cmp(), "unexpected if shape");
       if (cmp->in(1)->is_top() || cmp->in(2)->is_top()) {
-        // This region should lose its Phis and be optimized out by igvn but there's a chance the if folds to top first
-        // which then causes a reachable part of the graph to become dead.
+        // This region should lose its Phis. They are removed either in PhaseRemoveUseless (for data phis) or in IGVN
+        // (for memory phis). During IGVN, there is a chance that the If folds to top before the Region is processed
+        // which then causes a reachable part of the graph to become dead. To prevent this, set the boolean input of
+        // the If to a constant to nicely let the diamond Region/If fold away.
         Compile* C = _stringopts->C;
-        C->gvn_replace_by(n, iff->in(0));
+        C->gvn_replace_by(iff->in(1), _stringopts->gvn()->intcon(0));
       }
     }
   }

test/hotspot/jtreg/compiler/c2/Test7179138_1.java

@@ -30,6 +30,9 @@
  *      compiler.c2.Test7179138_1
  * @run main/othervm -Xbatch -XX:+IgnoreUnrecognizedVMOptions -XX:-TieredCompilation
  *      -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN compiler.c2.Test7179138_1
+ * @run main/othervm -Xbatch -XX:+IgnoreUnrecognizedVMOptions -XX:-TieredCompilation
+ *      -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN -XX:+AlwaysIncrementalInline
+ *      compiler.c2.Test7179138_1
  *
  * @author Skip Balk
  */