Update comment and test

Author: DataCorrupted

lld/MachO/Arch/ARM64.cpp

@@ -173,7 +173,13 @@ void ARM64::populateThunk(InputSection *thunk, Symbol *funcSym) {
                              /*offset=*/0, /*addend=*/0,
                              /*referent=*/funcSym);
 }
-// Just a single direct branch to the target function.
+// Just a single direct branch to the target function and a nop.
+// The nop will improve the debuggability.
+//
+// Debug lines in DWARF does not recognize one-instruction sequence, and this
+// thunk will be merged into other sequences if it only had one instruction.
+// Adding a nop after the branch has no runtime cost, and little size impact,
+// but it would make DWARF linker and debuggers happier.
 static constexpr uint32_t icfSafeThunkCode[] = {
     0x14000000, // 08: b    target
     0xD503201F, // 0c: nop

lld/test/MachO/icf-safe-thunks.ll

@@ -39,12 +39,15 @@
 ;
 ; CHECK-ARM64:        _func_2identical_v2:
 ; CHECK-ARM64-NEXT:        b  _func_2identical_v1
+; CHECK-ARM64-NEXT:        nop
 ; CHECK-ARM64-NEXT:   _func_3identical_v2:
 ; CHECK-ARM64-NEXT:        b  _func_3identical_v1
+; CHECK-ARM64-NEXT:        nop
 ; CHECK-ARM64-NEXT:   _func_3identical_v3:
 ; CHECK-ARM64-NEXT:        b  _func_3identical_v1
+; CHECK-ARM64-NEXT:        nop
 
-
+; Check the size of each functions. Thunks' size will change from 4-> 8 because of added nop.
 ; CHECK-ARM64-MAP:      0x00000010 [  2] _func_unique_1
 ; CHECK-ARM64-MAP-NEXT: 0x00000010 [  2] _func_2identical_v1
 ; CHECK-ARM64-MAP-NEXT: 0x00000000 [  2] _func_unique_2_canmerge
@@ -57,9 +60,9 @@
 ; CHECK-ARM64-MAP-NEXT: 0x00000000 [  2] _func_call_thunked_2_merge
 ; CHECK-ARM64-MAP-NEXT: 0x00000034 [  2] _call_all_funcs
 ; CHECK-ARM64-MAP-NEXT: 0x00000050 [  2] _take_func_addr
-; CHECK-ARM64-MAP-NEXT: 0x00000004 [  2] _func_2identical_v2
-; CHECK-ARM64-MAP-NEXT: 0x00000004 [  2] _func_3identical_v2
-; CHECK-ARM64-MAP-NEXT: 0x00000004 [  2] _func_3identical_v3
+; CHECK-ARM64-MAP-NEXT: 0x00000008 [  2] _func_2identical_v2
+; CHECK-ARM64-MAP-NEXT: 0x00000008 [  2] _func_3identical_v2
+; CHECK-ARM64-MAP-NEXT: 0x00000008 [  2] _func_3identical_v3
 
 ;--- a.cpp
 #define ATTR __attribute__((noinline)) extern "C"