[ELF] Add comment to explain LinkerScript::CurAddressState [NFC]

r307367 via D34345 split out the temporary address state used within
processCommands() and assignAddresses(). Due to the way that getSymbolValue
is used by the ScriptParser there is no way of giving the current
OutputSection to getSymbolValue() without somehow accessing the created
addressState. The suggestion was that by making a pointer that would go out
of scope we would find out by ASAN/MSAN or a crash if someone had misused
currentAddressState.

Differential Revision: https://reviews.llvm.org/D34345

llvm-svn: 307637

lld/ELF/LinkerScript.cpp

@@ -374,6 +374,10 @@ void LinkerScript::processCommands(OutputSectionFactory &Factory) {
   Aether = make<OutputSection>("", 0, SHF_ALLOC);
   Aether->SectionIndex = 1;
   auto State = make_unique<AddressState>(Opt);
+  // CurAddressState captures the local AddressState and makes it accessible
+  // deliberately. This is needed as there are some cases where we cannot just
+  // thread the current state through to a lambda function created by the
+  // script parser.
   CurAddressState = State.get();
   CurAddressState->OutSec = Aether;
   Dot = 0;
@@ -437,6 +441,7 @@ void LinkerScript::processCommands(OutputSectionFactory &Factory) {
       }
     }
   }
+  CurAddressState = nullptr;
 }
 
 void LinkerScript::fabricateDefaultCommands() {
@@ -843,6 +848,10 @@ void LinkerScript::assignAddresses() {
   // Assign addresses as instructed by linker script SECTIONS sub-commands.
   Dot = 0;
   auto State = make_unique<AddressState>(Opt);
+  // CurAddressState captures the local AddressState and makes it accessible
+  // deliberately. This is needed as there are some cases where we cannot just
+  // thread the current state through to a lambda function created by the
+  // script parser.
   CurAddressState = State.get();
   ErrorOnMissingSection = true;
   switchTo(Aether);
@@ -861,6 +870,7 @@ void LinkerScript::assignAddresses() {
     auto *Cmd = cast<OutputSectionCommand>(Base);
     assignOffsets(Cmd);
   }
+  CurAddressState = nullptr;
 }
 
 // Creates program headers as instructed by PHDRS linker script command.