Fix a bug that would cause subzero to fail when --threads=0.

src/IceGlobalContext.h

Index: src/IceGlobalContext.h
diff --git a/src/IceGlobalContext.h b/src/IceGlobalContext.h
index 763a75b487ced3554f3bf53127106f792eabdf31..4be18a652e7cecbbb3950062ebbf3acd4f1ae35f 100644
--- a/src/IceGlobalContext.h
+++ b/src/IceGlobalContext.h
@@ -303,6 +303,8 @@ public:
   // Emit file header for output file.
   void emitFileHeader();
 
+  void lowerConstants();
+
   void emitQueueBlockingPush(EmitterWorkItem *Item);
   EmitterWorkItem *emitQueueBlockingPop();
   void emitQueueNotifyEnd() { EmitQ.notifyEnd(); }
@@ -380,6 +382,13 @@ public:
   // until the queue is empty.
   void emitItems();
 
+  // Uses DataLowering to lower Globals. As a side effect, clears the Globals
+  // array.
+  void lowerGlobals(const IceString &SectionSuffix);
+
+  // Lowers the profile information.
+  void lowerProfileData();
+
   // Utility function to match a symbol name against a match string.
   // This is used in a few cases where we want to take some action on
   // a particular function or symbol based on a command-line argument,
@@ -435,6 +444,12 @@ private:
   std::unique_ptr<ELFObjectWriter> ObjectWriter;
   BoundedProducerConsumerQueue<Cfg> OptQ;
   BoundedProducerConsumerQueue<EmitterWorkItem> EmitQ;
+  std::unique_ptr<TargetDataLowering> DataLowering;

[Jim Stichnoth, 2015/06/16 17:03:24]

Non-const fields in the GlobalContext object generally need to be accessed while
holding a lock.  (The intention is that anything that might need a lock is part
of GlobalContext.)

[John, 2015/06/16 17:30:55]

The DataLowering is only ever used in
(1) emitItems; or
(2) IceCompiler::run when emitItems is done emitting items.

In both those contexts the use is sequential (in either IceCompilerServer and
IceBrowserCompileServer the context is created in a function and a compile
thread is started.) I added a comment, but happy to add a lock.

[Jim Stichnoth, 2015/06/17 16:04:56]

I do think the fields should use a lock, to protect against future misuse. 
Maybe bundle those 3 fields into a class and wrap that object in LockedPtr<> ?

+  // If !HasEmittedCode, SubZero will accumulate all Globals (which are "true"
+  // program global variables) until the first code WorkItem is seen.
+  bool HasSeenCode;
+  VariableDeclarationList Globals;
+  std::unique_ptr<VariableDeclaration> ProfileBlockInfoVarDecl;
 
   LockedPtr<ArenaAllocator<>> getAllocator() {
     return LockedPtr<ArenaAllocator<>>(&Allocator, &AllocLock);
@@ -449,6 +464,19 @@ private:
     return LockedPtr<TimerList>(&Timers, &TimerLock);
   }
 
+  void accumulateGlobals(std::unique_ptr<VariableDeclarationList> Globls) {
+    if (Globls != nullptr)
+      Globals.insert(Globals.end(), Globls->begin(), Globls->end());
+  }
+
+  void lowerGlobalsIfNoCodeHasBeenSeen() {
+    if (HasSeenCode)
+      return;
+    constexpr char NoSuffix[] = "";
+    lowerGlobals(NoSuffix);
+    HasSeenCode = true;
+  }
+
   llvm::SmallVector<ThreadContext *, 128> AllThreadContexts;
   llvm::SmallVector<std::thread, 128> TranslationThreads;
   llvm::SmallVector<std::thread, 128> EmitterThreads;