Index: bolt/lib/Passes/MCF.cpp
===================================================================
--- bolt/lib/Passes/MCF.cpp
+++ bolt/lib/Passes/MCF.cpp
@@ -264,6 +264,7 @@
       continue;
 
     Pred->getBranchInfo(*BB).Count = Guessed;
+    GuessedArcs.insert(std::make_pair(Pred, BB));
     return true;
   }
   llvm_unreachable("Expected unguessed arc");
Index: bolt/test/Inputs/guess.c
===================================================================
--- /dev/null
+++ bolt/test/Inputs/guess.c
@@ -0,0 +1,28 @@
+// Verify that bolt will not fall in the endless loop when using  
+// the fdata file in case of enabling iterative-guess option.
+
+// RUN: %clang %cflags %p/Inputs/guess.c -o %t.exe
+// RUN: llvm-bolt %t.exe -o %t -data=%p/Inputs/guess.fdata --iterative-guess
+
+// The problem arises in the following situation 
+//    BB0 -- BB1        BB0 -> BB1
+//     \     |          BB1 -> BB2
+//      \    |          BB0 -> BB2
+//       \   |
+//         BB2
+
+int test(int a, int b) {
+    if (b == 1) {
+       a = b + 1;
+    }
+    return a;
+}
+
+int main() {
+    long long i = 5000000000;
+    while (i > 0) {
+        test(2, 1);
+        i--;
+    }
+    return 0;
+}
Index: bolt/test/Inputs/guess.fdata
===================================================================
--- /dev/null
+++ bolt/test/Inputs/guess.fdata
@@ -0,0 +1,8 @@
+no_lbr cycles:u:
+1 test 20 12
+1 test c 5778
+1 test 30 5494
+1 test 8 1
+1 main 34 15379
+1 main 44 12401
+1 main 38 16433
Index: bolt/test/iterative-guess.test
===================================================================
--- /dev/null
+++ bolt/test/iterative-guess.test
@@ -0,0 +1,7 @@
+# Check that bolt will not fall in the endless loop when using  
+# the fdata file in case of enabling iterative-guess option.
+# The fdata file is generated based on aarch64, this test need to 
+# be run in aarch64 to make sense
+
+RUN: %clang %cflags %S/Inputs/guess.c -o %t.exe
+RUN: llvm-bolt %t.exe -o %t -data=%S/Inputs/guess.fdata --iterative-guess