[BOLT] Add constant island check in scanExternalRefs() (#165577)

The [previous patch](#163418)
has added a check to prevent adding an entry point into a constant
island, but only for successfully disassembled functions.

Because scanExternalRefs() is also called when a function fails to be
disassembled or is skipped, it can still attempt to add an entry point
at constant islands. The same issue may occur if without a check for it

So, this patch complements the 'constant island' check in
scanExternalRefs().

Author: Jinjie-Huang

bolt/lib/Core/BinaryFunction.cpp

@@ -1699,9 +1699,19 @@ bool BinaryFunction::scanExternalRefs() {
 
       const uint64_t FunctionOffset =
           TargetAddress - TargetFunction->getAddress();
-      BranchTargetSymbol =
-          FunctionOffset ? TargetFunction->addEntryPointAtOffset(FunctionOffset)
-                         : TargetFunction->getSymbol();
+      if (!TargetFunction->isInConstantIsland(TargetAddress)) {
+        BranchTargetSymbol =
+            FunctionOffset
+                ? TargetFunction->addEntryPointAtOffset(FunctionOffset)
+                : TargetFunction->getSymbol();
+      } else {
+        TargetFunction->setIgnored();
+        BC.outs() << "BOLT-WARNING: Ignoring entry point at address 0x"
+                  << Twine::utohexstr(Address)
+                  << " in constant island of function " << *TargetFunction
+                  << '\n';
+        continue;
+      }
     }
 
     // Can't find more references. Not creating relocations since we are not

bolt/test/AArch64/constant-island-entry.s

@@ -1,10 +1,15 @@
-// This test checks that we ignore functions which add an entry point that
-// is in a constant island.
+## This test checks that we ignore functions which add an entry point that
+## is in a constant island.
 
 # RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o
 # RUN: %clang %cflags %t.o -pie -Wl,-q -o %t.exe
+
+## Check when the caller is successfully disassembled.
 # RUN: llvm-bolt %t.exe -o %t.bolt 2>&1 | FileCheck %s
 
+## Skip caller to check the identical warning is triggered from ScanExternalRefs().
+# RUN: llvm-bolt %t.exe -o %t.bolt -skip-funcs=caller 2>&1 | FileCheck %s
+
 # CHECK: BOLT-WARNING: Ignoring entry point at address 0x{{[0-9a-f]+}} in constant island of function func
 
 .globl func