X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FGarbageCollection.rst;h=40f5b1287b0a9ea147de208b26bd2b3c6f585279;hb=7ef23e62b2d97fec90de6ecf1a5dc609c4921f75;hp=323a6ea81659869a953c5d86de4f2354236ee867;hpb=9802a6cbdc2ddcb445fc8620fd756654de6d85b4;p=oota-llvm.git

diff --git a/docs/GarbageCollection.rst b/docs/GarbageCollection.rst
index 323a6ea8165..40f5b1287b0 100644
--- a/docs/GarbageCollection.rst
+++ b/docs/GarbageCollection.rst
@@ -633,7 +633,7 @@ Threaded
   Denotes a multithreaded mutator; the collector must still stop the mutator
   ("stop the world") before beginning reachability analysis.  Stopping a
   multithreaded mutator is a complicated problem.  It generally requires highly
-  platform specific code in the runtime, and the production of carefully
+  platform-specific code in the runtime, and the production of carefully
   designed machine code at safe points.
 
 Concurrent
@@ -721,8 +721,9 @@ this feature should be used by all GC plugins.  It is enabled by default.
 Custom lowering of intrinsics: ``CustomRoots``, ``CustomReadBarriers``, and ``CustomWriteBarriers``
 ---------------------------------------------------------------------------------------------------
 
-For GCs which use barriers or unusual treatment of stack roots, these flags
-allow the collector to perform arbitrary transformations of the LLVM IR:
+For GCs which use barriers or unusual treatment of stack roots, these 
+flags allow the collector to perform arbitrary transformations of the
+LLVM IR:
 
 .. code-block:: c++
 
@@ -733,70 +734,18 @@ allow the collector to perform arbitrary transformations of the LLVM IR:
       CustomReadBarriers = true;
       CustomWriteBarriers = true;
     }
-
-    virtual bool initializeCustomLowering(Module &M);
-    virtual bool performCustomLowering(Function &F);
   };
 
-If any of these flags are set, then LLVM suppresses its default lowering for the
-corresponding intrinsics and instead calls ``performCustomLowering``.
-
-LLVM's default action for each intrinsic is as follows:
-
-* ``llvm.gcroot``: Leave it alone.  The code generator must see it or the stack
-  map will not be computed.
-
-* ``llvm.gcread``: Substitute a ``load`` instruction.
-
-* ``llvm.gcwrite``: Substitute a ``store`` instruction.
-
-If ``CustomReadBarriers`` or ``CustomWriteBarriers`` are specified, then
-``performCustomLowering`` **must** eliminate the corresponding barriers.
-
-``performCustomLowering`` must comply with the same restrictions as
-:ref:`FunctionPass::runOnFunction <writing-an-llvm-pass-runOnFunction>`
-Likewise, ``initializeCustomLowering`` has the same semantics as
-:ref:`Pass::doInitialization(Module&)
-<writing-an-llvm-pass-doInitialization-mod>`
+If any of these flags are set, LLVM suppresses its default lowering for
+the corresponding intrinsics.  Instead, you must provide a custom Pass
+which lowers the intrinsics as desired.  If you have opted in to custom
+lowering of a particular intrinsic your pass **must** eliminate all 
+instances of the corresponding intrinsic in functions which opt in to
+your GC.  The best example of such a pass is the ShadowStackGC and it's 
+ShadowStackGCLowering pass.  
 
-The following can be used as a template:
-
-.. code-block:: c++
-
-  #include "llvm/IR/Module.h"
-  #include "llvm/IR/IntrinsicInst.h"
-
-  bool MyGC::initializeCustomLowering(Module &M) {
-    return false;
-  }
-
-  bool MyGC::performCustomLowering(Function &F) {
-    bool MadeChange = false;
-
-    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
-      for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; )
-        if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
-          if (Function *F = CI->getCalledFunction())
-            switch (F->getIntrinsicID()) {
-            case Intrinsic::gcwrite:
-              // Handle llvm.gcwrite.
-              CI->eraseFromParent();
-              MadeChange = true;
-              break;
-            case Intrinsic::gcread:
-              // Handle llvm.gcread.
-              CI->eraseFromParent();
-              MadeChange = true;
-              break;
-            case Intrinsic::gcroot:
-              // Handle llvm.gcroot.
-              CI->eraseFromParent();
-              MadeChange = true;
-              break;
-            }
-
-    return MadeChange;
-  }
+There is currently no way to register such a custom lowering pass 
+without building a custom copy of LLVM.
 
 .. _safe-points:
 
@@ -923,7 +872,7 @@ a realistic example:
 
   void MyGCPrinter::finishAssembly(AsmPrinter &AP) {
     MCStreamer &OS = AP.OutStreamer;
-    unsigned IntPtrSize = AP.TM.getDataLayout()->getPointerSize();
+    unsigned IntPtrSize = AP.TM.getSubtargetImpl()->getDataLayout()->getPointerSize();
 
     // Put this in the data section.
     OS.SwitchSection(AP.getObjFileLowering().getDataSection());