Comment and minor code cleanup for GCStrategy (NFC)

Updating comments to reflect the current state of the world after my recent changes to ownership structure and generally better describe what a GCStrategy is and how it works.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224086 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/GCStrategy.h

@@ -12,9 +12,14 @@
 // specified in a function's 'gc' attribute. Algorithms are enabled by setting
 // flags in a subclass's constructor, and some virtual methods can be
 // overridden.
+//
+// GCStrategy is relevant for implementations using either gc.root or
+// gc.statepoint based lowering strategies, but is currently focused mostly on
+// options for gc.root.  This will change over time.
 // 
-// When requested, the GCStrategy will be populated with data about each
-// function which uses it. Specifically:
+// When requested by a subclass of GCStrategy, the gc.root implementation will
+// populate GCModuleInfo and GCFunctionInfo with that about each Function in
+// the Module that opts in to garbage collection.  Specifically:
 // 
 // - Safe points
 //   Garbage collection is generally only possible at certain points in code.
@@ -31,6 +36,15 @@
 // This information can used to emit the metadata tables which are required by
 // the target garbage collector runtime.
 //
+// When used with gc.statepoint, information about safepoint and roots can be
+// found in the binary StackMap section after code generation.  Safepoint
+// placement is currently the responsibility of the frontend, though late
+// insertion support is planned.  gc.statepoint does not currently support
+// custom stack map formats; such can be generated by parsing the standard
+// stack map section if desired.
+// 
+// The read and write barrier support can be used with either implementation.
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CODEGEN_GCSTRATEGY_H
@@ -42,13 +56,6 @@
 #include <string>
 
 namespace llvm {
-  
-  class GCStrategy;
-  
-  /// The GC strategy registry uses all the defaults from Registry.
-  /// 
-  typedef Registry<GCStrategy> GCRegistry;
-  
   /// GCStrategy describes a garbage collector algorithm's code generation
   /// requirements, and provides overridable hooks for those needs which cannot
   /// be abstractly described.  GCStrategy objects currently must be looked up
@@ -56,8 +63,8 @@ namespace llvm {
   /// pass and recreated every time that pass is invalidated.
   class GCStrategy {
   private:
-    friend class GCModuleInfo;
     std::string Name;
+    friend class GCModuleInfo;
     
   protected:
     unsigned NeededSafePoints; ///< Bitmask of required safe points.
@@ -71,65 +78,96 @@ namespace llvm {
     
   public:
     GCStrategy();
-    
     virtual ~GCStrategy() {}
     
-    
-    /// getName - The name of the GC strategy, for debugging.
-    /// 
+    /// Return the name of the GC strategy.  This is the value of the collector
+    /// name string specified on functions which use this strategy.
     const std::string &getName() const { return Name; }
 
-    /// needsSafePoitns - True if safe points of any kind are required. By
-    //                    default, none are recorded.
+    /// By default, write barriers are replaced with simple store
+    /// instructions. If true, then performCustomLowering must instead lower
+    /// them. 
+    bool customWriteBarrier() const { return CustomWriteBarriers; }
+    
+    /// By default, read barriers are replaced with simple load
+    /// instructions. If true, then performCustomLowering must instead lower
+    /// them. 
+    bool customReadBarrier() const { return CustomReadBarriers; }
+
+    /** @name GCRoot Specific Properties
+     * These properties and overrides only apply to collector strategies using
+     * GCRoot. 
+     */
+    ///@{
+
+    /// True if safe points of any kind are required. By default, none are
+    /// recorded. 
     bool needsSafePoints() const {
       return CustomSafePoints || NeededSafePoints != 0;
     }
     
-    /// needsSafePoint(Kind) - True if the given kind of safe point is
-    //                          required. By default, none are recorded.
+    /// True if the given kind of safe point is required. By default, none are
+    /// recorded. 
     bool needsSafePoint(GC::PointKind Kind) const {
       return (NeededSafePoints & 1 << Kind) != 0;
     }
-    
-    /// customWriteBarrier - By default, write barriers are replaced with simple
-    ///                      store instructions. If true, then
-    ///                      performCustomLowering must instead lower them.
-    bool customWriteBarrier() const { return CustomWriteBarriers; }
-    
-    /// customReadBarrier - By default, read barriers are replaced with simple
-    ///                     load instructions. If true, then
-    ///                     performCustomLowering must instead lower them.
-    bool customReadBarrier() const { return CustomReadBarriers; }
-    
-    /// customRoots - By default, roots are left for the code generator so it
-    ///               can generate a stack map. If true, then
-    //                performCustomLowering must delete them.
+        
+    /// By default, roots are left for the code generator so it can generate a
+    /// stack map. If true, then performCustomLowering must delete them.
     bool customRoots() const { return CustomRoots; }
 
-    /// customSafePoints - By default, the GC analysis will find safe
-    ///                    points according to NeededSafePoints. If true,
-    ///                    then findCustomSafePoints must create them.
+    /// By default, the GC analysis will find safe points according to
+    /// NeededSafePoints. If true, then findCustomSafePoints must create them.
     bool customSafePoints() const { return CustomSafePoints; }
     
-    /// initializeRoots - If set, gcroot intrinsics should initialize their
-    //                    allocas to null before the first use. This is
-    //                    necessary for most GCs and is enabled by default.
+    /// If set, gcroot intrinsics should initialize their allocas to null
+    /// before the first use. This is necessary for most GCs and is enabled by
+    /// default. 
     bool initializeRoots() const { return InitRoots; }
     
-    /// usesMetadata - If set, appropriate metadata tables must be emitted by
-    ///                the back-end (assembler, JIT, or otherwise).
+    /// If set, appropriate metadata tables must be emitted by the back-end
+    /// (assembler, JIT, or otherwise). 
     bool usesMetadata() const { return UsesMetadata; }
+
+    ///@}
     
     /// initializeCustomLowering/performCustomLowering - If any of the actions
     /// are set to custom, performCustomLowering must be overriden to transform
     /// the corresponding actions to LLVM IR. initializeCustomLowering is
     /// optional to override. These are the only GCStrategy methods through
-    /// which the LLVM IR can be modified.
-    virtual bool initializeCustomLowering(Module &F);
-    virtual bool performCustomLowering(Function &F);
-    virtual bool findCustomSafePoints(GCFunctionInfo& FI, MachineFunction& MF);
+    /// which the LLVM IR can be modified.  These methods apply mostly to
+    /// gc.root based implementations, but can be overriden to provide custom
+    /// barrier lowerings with gc.statepoint as well.
+    ///@{
+    virtual bool initializeCustomLowering(Module &F) {
+      // No changes made
+      return false;
+    }
+    virtual bool performCustomLowering(Function &F) {
+      llvm_unreachable("GCStrategy subclass specified a configuration which"
+                       "requires a custom lowering without providing one");
+    }
+    ///@}
+    /// Called if customSafepoints returns true, used only by gc.root
+    /// implementations. 
+    virtual bool findCustomSafePoints(GCFunctionInfo& FI, MachineFunction& MF) {
+      llvm_unreachable("GCStrategy subclass specified a configuration which"
+                       "requests custom safepoint identification without"
+                       "providing an implementation for such");
+    }
   };
-  
+
+  /// Subclasses of GCStrategy are made available for use during compilation by
+  /// adding them to the global GCRegistry.  This can done either within the
+  /// LLVM source tree or via a loadable plugin.  An example registeration
+  /// would be:
+  /// static GCRegistry::Add<CustomGC> X("custom-name",
+  ///        "my custom supper fancy gc strategy");
+  /// 
+  /// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
+  /// register your GCMetadataPrinter subclass with the
+  /// GCMetadataPrinterRegistery as well. 
+  typedef Registry<GCStrategy> GCRegistry;
 }
 
 #endif

lib/CodeGen/GCStrategy.cpp

@@ -102,19 +102,6 @@ GCStrategy::GCStrategy() :
   UsesMetadata(false)
 {}
 
-bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
-
-bool GCStrategy::performCustomLowering(Function &F) {
-  dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
-  llvm_unreachable("must override performCustomLowering");
-}
-
-
-bool GCStrategy::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &F) {
-  dbgs() << "gc " << getName() << " must override findCustomSafePoints.\n";
-  llvm_unreachable(nullptr);
-}
-
 // -----------------------------------------------------------------------------
 
 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering",