[PATCH] Introduce an example statepoint GC strategy

[David Majnemer]

================
Comment at: include/llvm/CodeGen/GCStrategy.h:71-73
@@ -69,1 +70,5 @@
   protected:
+    bool UseStatepoints;       /// Uses gc.statepoints as opposed to gc.roots,
+                               /// if set, none of the other options can be
+                               /// anything but their default values.
+
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The other comments are formatted using ///<

Also, the layout of GCStrategy might be a little more compact if you shifted this field down.

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Comment at: lib/CodeGen/SelectionDAG/StatepointLowering.cpp:429
@@ +428,3 @@
+    GCStrategy &S = Builder.GFI->getStrategy();
+    for(const Value *V : Bases) {
+      auto Opt = S.isGCManagedPointer(V);
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There should be a space between the for and the (

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Comment at: lib/CodeGen/StatepointExampleGC.cpp:29
@@ +28,3 @@
+  StatepointGC() {
+    UseStatepoints=true;
+    // These options are all gc.root specific, we specify them so that the
----------------
Space before and after =

================
Comment at: lib/CodeGen/StatepointExampleGC.cpp:39-40
@@ +38,4 @@
+  Optional<bool> isGCManagedPointer(const Value *V) const override {
+    PointerType *PT = dyn_cast<PointerType>(V->getType());
+    assert(PT && "method is only valid on pointer typed values");
+    // For the sake of this example GC, we arbitrarily pick addrspace(1) as our
----------------
If this is the case, why not just cast<PointerType> ?

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Comment at: lib/CodeGen/StatepointExampleGC.cpp:44
@@ +43,3 @@
+    // updated and that no other pointer does.
+    return (1 == PT->getAddressSpace());
+  }
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Address space 1 has a special meaning in LLVM, it's identical to address space 0 except for the fact that "null" may be dereferenced.  You might want to consider a different address space.

http://reviews.llvm.org/D6808

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