From c82667c1488de69609ee1597cd29ccc160f758e8 Mon Sep 17 00:00:00 2001
From: David Rose <david@ddrose.com>
Date: Tue, 19 Mar 2002 01:53:14 +0000
Subject: [PATCH] implement SIMPLE_STRUCT_POINTERS

---
 panda/src/putil/pipelineCycler.h     |  8 +++++++-
 panda/src/putil/pipelineCyclerBase.I | 20 ++++++++++++++++++++
 panda/src/putil/pipelineCyclerBase.h | 18 +++++++++++++++++-
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/panda/src/putil/pipelineCycler.h b/panda/src/putil/pipelineCycler.h
index 75e7bb3cb1..3d69f3ec65 100644
--- a/panda/src/putil/pipelineCycler.h
+++ b/panda/src/putil/pipelineCycler.h
@@ -48,9 +48,15 @@
 //               same interface but with minimal runtime overhead.
 //               (Actually, this isn't true yet, but it will be one
 //               day.)
+//
+//               We define this as a struct instead of a class to
+//               guarantee byte placement within the object, so that
+//               (particularly for the trivial implementation) the
+//               inherited struct's data is likely to be placed by the
+//               compiler at the "this" pointer.
 ////////////////////////////////////////////////////////////////////
 template<class CycleDataType>
-class PipelineCycler : public PipelineCyclerBase {
+struct PipelineCycler : public PipelineCyclerBase {
 public:
   INLINE PipelineCycler(Pipeline *pipeline = NULL);
   INLINE PipelineCycler(const PipelineCycler<CycleDataType> &copy);
diff --git a/panda/src/putil/pipelineCyclerBase.I b/panda/src/putil/pipelineCyclerBase.I
index c09b9ab6a8..4b3108128e 100644
--- a/panda/src/putil/pipelineCyclerBase.I
+++ b/panda/src/putil/pipelineCyclerBase.I
@@ -299,7 +299,11 @@ PipelineCyclerBase(CycleData *initial_data, Pipeline *) {
   // If this turns out not to be true on a particular platform, we
   // will have to store the pointer in this class, for a little bit of
   // extra overhead.
+#ifdef SIMPLE_STRUCT_POINTERS
   nassertv(initial_data == (CycleData *)this);
+#else
+  _data = initial_data;
+#endif  // SIMPLE_STRUCT_POINTERS
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -339,7 +343,11 @@ INLINE PipelineCyclerBase::
 ////////////////////////////////////////////////////////////////////
 INLINE const CycleData *PipelineCyclerBase::
 read() const {
+#ifdef SIMPLE_STRUCT_POINTERS
   return (const CycleData *)this;
+#else
+  return _data;
+#endif  // SIMPLE_STRUCT_POINTERS
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -381,7 +389,11 @@ release_read(const CycleData *) const {
 ////////////////////////////////////////////////////////////////////
 INLINE CycleData *PipelineCyclerBase::
 write() {
+#ifdef SIMPLE_STRUCT_POINTERS
   return (CycleData *)this;
+#else
+  return _data;
+#endif  // SIMPLE_STRUCT_POINTERS
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -395,7 +407,11 @@ write() {
 ////////////////////////////////////////////////////////////////////
 INLINE CycleData *PipelineCyclerBase::
 elevate_read(const CycleData *) {
+#ifdef SIMPLE_STRUCT_POINTERS
   return (CycleData *)this;
+#else
+  return _data;
+#endif  // SIMPLE_STRUCT_POINTERS
 }
 
 ////////////////////////////////////////////////////////////////////
@@ -467,7 +483,11 @@ release_write_stage(int, CycleData *) {
 ////////////////////////////////////////////////////////////////////
 INLINE CycleData *PipelineCyclerBase::
 cheat() const {
+#ifdef SIMPLE_STRUCT_POINTERS
   return (CycleData *)this;
+#else
+  return _data;
+#endif  // SIMPLE_STRUCT_POINTERS
 }
 
 ////////////////////////////////////////////////////////////////////
diff --git a/panda/src/putil/pipelineCyclerBase.h b/panda/src/putil/pipelineCyclerBase.h
index eb28f8a281..5abd7c957e 100644
--- a/panda/src/putil/pipelineCyclerBase.h
+++ b/panda/src/putil/pipelineCyclerBase.h
@@ -29,8 +29,14 @@
 //       Class : PipelineCyclerBase
 // Description : This is the non-template part of the implementation
 //               of PipelineCycler.  See PipelineCycler.
+//
+//               We define this as a struct instead of a class to
+//               guarantee byte placement within the object, so that
+//               (particularly for the trivial implementation) the
+//               inherited struct's data is likely to be placed by the
+//               compiler at the "this" pointer.
 ////////////////////////////////////////////////////////////////////
-class EXPCL_PANDA PipelineCyclerBase {
+struct EXPCL_PANDA PipelineCyclerBase {
 public:
   INLINE PipelineCyclerBase(CycleData *initial_data, Pipeline *pipeline = NULL);
   INLINE PipelineCyclerBase(const PipelineCyclerBase &copy);
@@ -63,6 +69,16 @@ private:
   PT(CycleData) _data;
   Pipeline *_pipeline;
   short _read_count, _write_count;
+
+#else  // !DO_PIPELINING
+  // In a trivial implementation, we only need to store the CycleData
+  // pointer.  Actually, we don't even need to do that, if we're lucky
+  // and the compiler doesn't do anything funny with the struct
+  // layout.
+  #ifndef SIMPLE_STRUCT_POINTERS
+  CycleData *_data;
+  #endif  // SIMPLE_STRUCT_POINTERS
+
 #endif  // DO_PIPELINING
 };