[Statepoints][NFC] Add Statepoint::operator bool()

[sanjoy]

Summary

This allows the "if (Statepoint SP = Statepoint(I))" idiom.

(I don't think this change needs review, this was uploaded to
phabricator to provide context for later dependent changes.)

[reames]

LGTM

include/llvm/IR/Statepoint.h

 bool isGCResult(const ImmutableCallSite &CS);
 
 /// Analogous to CallSiteBase, this provides most of the actual
 /// functionality for Statepoint and ImmutableStatepoint.  It is
 /// templatized to allow easily specializing of const and non-const
 /// concrete subtypes.  This is structured analogous to CallSite
 /// rather than the IntrinsicInst.h helpers since we want to support
 /// invokable statepoints in the near future.
-/// TODO: This does not currently allow the if(Statepoint S = ...)
-///   idiom used with CallSites.  Consider refactoring to support.
 template <typename InstructionTy, typename ValueTy, typename CallSiteTy>
 class StatepointBase {
   CallSiteTy StatepointCS;
   void *operator new(size_t, unsigned) = delete;
   void *operator new(size_t s) = delete;
 
 protected:
-  explicit StatepointBase(InstructionTy *I) : StatepointCS(I) {
-    assert(isStatepoint(I));
+  explicit StatepointBase(InstructionTy *I) {
+    if (isStatepoint(I)) {
+      StatepointCS = CallSiteTy(I);
+      assert(StatepointCS && "isStatepoint implies CallSite");
     }
-  explicit StatepointBase(CallSiteTy CS) : StatepointCS(CS) {
-    assert(isStatepoint(CS));
+  }
+  explicit StatepointBase(CallSiteTy CS) {
+    if (isStatepoint(CS))
+      StatepointCS = CS;
   }
 
 public:
   typedef typename CallSiteTy::arg_iterator arg_iterator;
 
   enum {
     IDPos = 0,
     NumPatchBytesPos = 1,
     ActualCalleePos = 2,
     NumCallArgsPos = 3,
     FlagsPos = 4,
     CallArgsBeginPos = 5,
   };
 
+  operator bool() const {
+    // We do not assign non-statepoint CallSites to StatepointCS.
+    return (bool)StatepointCS;
+  }
+
   /// Return the underlying CallSite.
-  CallSiteTy getCallSite() { return StatepointCS; }
+  CallSiteTy getCallSite() const {
+    assert(*this && "check validity first!");
+    return StatepointCS;
+  }
 
   uint64_t getFlags() const {
-    return cast<ConstantInt>(StatepointCS.getArgument(FlagsPos))
+    return cast<ConstantInt>(getCallSite().getArgument(FlagsPos))
         ->getZExtValue();
   }
 
   /// Return the ID associated with this statepoint.
   uint64_t getID() {
-    const Value *IDVal = StatepointCS.getArgument(IDPos);
+    const Value *IDVal = getCallSite().getArgument(IDPos);
     return cast<ConstantInt>(IDVal)->getZExtValue();
   }
 
   /// Return the number of patchable bytes associated with this statepoint.
   uint32_t getNumPatchBytes() {
-    const Value *NumPatchBytesVal = StatepointCS.getArgument(NumPatchBytesPos);
+    const Value *NumPatchBytesVal = getCallSite().getArgument(NumPatchBytesPos);
     uint64_t NumPatchBytes =
       cast<ConstantInt>(NumPatchBytesVal)->getZExtValue();
     assert(isInt<32>(NumPatchBytes) && "should fit in 32 bits!");
     return NumPatchBytes;
   }
 
   /// Return the value actually being called or invoked.
   ValueTy *getActualCallee() {
-    return StatepointCS.getArgument(ActualCalleePos);
+    return getCallSite().getArgument(ActualCalleePos);
   }
 
   /// Return the type of the value returned by the call underlying the
   /// statepoint.
   Type *getActualReturnType() {
     auto *FTy = cast<FunctionType>(
         cast<PointerType>(getActualCallee()->getType())->getElementType());
     return FTy->getReturnType();
   }
 
   /// Number of arguments to be passed to the actual callee.
   int getNumCallArgs() {
-    const Value *NumCallArgsVal = StatepointCS.getArgument(NumCallArgsPos);
+    const Value *NumCallArgsVal = getCallSite().getArgument(NumCallArgsPos);
     return cast<ConstantInt>(NumCallArgsVal)->getZExtValue();
   }
 
   typename CallSiteTy::arg_iterator call_args_begin() {
-    assert(CallArgsBeginPos <= (int)StatepointCS.arg_size());
-    return StatepointCS.arg_begin() + CallArgsBeginPos;
+    assert(CallArgsBeginPos <= (int)getCallSite().arg_size());
+    return getCallSite().arg_begin() + CallArgsBeginPos;
   }
   typename CallSiteTy::arg_iterator call_args_end() {
     auto I = call_args_begin() + getNumCallArgs();
-    assert((StatepointCS.arg_end() - I) >= 0);
+    assert((getCallSite().arg_end() - I) >= 0);
     return I;
   }
 
   /// range adapter for call arguments
   iterator_range<arg_iterator> call_args() {
     return iterator_range<arg_iterator>(call_args_begin(), call_args_end());
   }
 
   /// Number of GC transition args.
   int getNumTotalGCTransitionArgs() {
     const Value *NumGCTransitionArgs = *call_args_end();
     return cast<ConstantInt>(NumGCTransitionArgs)->getZExtValue();
   }
   typename CallSiteTy::arg_iterator gc_transition_args_begin() {
     auto I = call_args_end() + 1;
-    assert((StatepointCS.arg_end() - I) >= 0);
+    assert((getCallSite().arg_end() - I) >= 0);
     return I;
   }
   typename CallSiteTy::arg_iterator gc_transition_args_end() {
     auto I = gc_transition_args_begin() + getNumTotalGCTransitionArgs();
-    assert((StatepointCS.arg_end() - I) >= 0);
+    assert((getCallSite().arg_end() - I) >= 0);
     return I;
   }
 
   /// range adapter for GC transition arguments
   iterator_range<arg_iterator> gc_transition_args() {
     return iterator_range<arg_iterator>(gc_transition_args_begin(),
                                         gc_transition_args_end());
   }
 
   /// Number of additional arguments excluding those intended
   /// for garbage collection.
   int getNumTotalVMSArgs() {
     const Value *NumVMSArgs = *gc_transition_args_end();
     return cast<ConstantInt>(NumVMSArgs)->getZExtValue();
   }
 
   typename CallSiteTy::arg_iterator vm_state_begin() {
     auto I = gc_transition_args_end() + 1;
-    assert((StatepointCS.arg_end() - I) >= 0);
+    assert((getCallSite().arg_end() - I) >= 0);
     return I;
   }
   typename CallSiteTy::arg_iterator vm_state_end() {
     auto I = vm_state_begin() + getNumTotalVMSArgs();
-    assert((StatepointCS.arg_end() - I) >= 0);
+    assert((getCallSite().arg_end() - I) >= 0);
     return I;
   }
 
   /// range adapter for vm state arguments
   iterator_range<arg_iterator> vm_state_args() {
     return iterator_range<arg_iterator>(vm_state_begin(), vm_state_end());
   }
 
   typename CallSiteTy::arg_iterator gc_args_begin() { return vm_state_end(); }
   typename CallSiteTy::arg_iterator gc_args_end() {
-    return StatepointCS.arg_end();
+    return getCallSite().arg_end();
   }
 
   /// range adapter for gc arguments
   iterator_range<arg_iterator> gc_args() {
     return iterator_range<arg_iterator>(gc_args_begin(), gc_args_end());
   }
 
   /// Get list of all gc reloactes linked to this statepoint