[PlaceSafeopints] Extract out `callsGCLeafFunction`, NFC

[sanjoy]

Summary

This will be used later in RewriteStatepointsForGC.

[sanjoy]

• edit comment

[sanjoy]

Actually, I'm not sure Transform/Utils is the right place to put callsGCLeafFunction, I'll look for a better place.

[reames]

LGTM either in it's current location or somewhere in Analysis. I would suggest updating the documentation to reflect that most passes can and should ignore this information and that's effectively only used during lowering by the GC infrastructure. (a.k.a. the property doesn't effect optimization.)

include/llvm/Transforms/Utils/Local.h

 /// \brief Replace each use of 'From' with 'To' if that use is dominated by 
 /// the given edge.  Returns the number of replacements made.
 unsigned replaceDominatedUsesWith(Value *From, Value *To, DominatorTree &DT,
                                   const BasicBlockEdge &Edge);
 /// \brief Replace each use of 'From' with 'To' if that use is dominated by
 /// the given BasicBlock. Returns the number of replacements made.
 unsigned replaceDominatedUsesWith(Value *From, Value *To, DominatorTree &DT,
                                   const BasicBlock *BB);
+
+/// \brief Return true if the CallSite CS calls a gc leaf function.
+///
+/// A leaf function is a function that does not safepoint the thread during its
+/// execution.  During a call or invoke to such a function, the callers stack
+/// does not have to be made parseable.
+bool callsGCLeafFunction(ImmutableCallSite CS);
+
 } // End llvm namespace
 
 #endif

lib/Transforms/Scalar/PlaceSafepoints.cpp

 
 // Insert a safepoint poll immediately before the given instruction.  Does
 // not handle the parsability of state at the runtime call, that's the
 // callers job.
 static void
 InsertSafepointPoll(Instruction *InsertBefore,
                     std::vector<CallSite> &ParsePointsNeeded /*rval*/);
 
-static bool isGCLeafFunction(const CallSite &CS);
-
 static bool needsStatepoint(const CallSite &CS) {
-  if (isGCLeafFunction(CS))
+  if (callsGCLeafFunction(CS))
     return false;
   if (CS.isCall()) {
     CallInst *call = cast<CallInst>(CS.getInstruction());
     if (call->isInlineAsm())
       return false;
   }
   if (isStatepoint(CS) || isGCRelocate(CS) || isGCResult(CS)) {
     return false;
INITIALIZE_PASS_END(PlaceBackedgeSafepointsImpl,
                     "place-backedge-safepoints-impl",
                     "Place Backedge Safepoints", false, false)
 
 INITIALIZE_PASS_BEGIN(PlaceSafepoints, "place-safepoints", "Place Safepoints",
                       false, false)
 INITIALIZE_PASS_END(PlaceSafepoints, "place-safepoints", "Place Safepoints",
                     false, false)
 
-static bool isGCLeafFunction(const CallSite &CS) {
-  Instruction *inst = CS.getInstruction();
-  if (isa<IntrinsicInst>(inst)) {
-    // Most LLVM intrinsics are things which can never take a safepoint.
-    // As a result, we don't need to have the stack parsable at the
-    // callsite.  This is a highly useful optimization since intrinsic
-    // calls are fairly prevalent, particularly in debug builds.
-    return true;
-  }
-
-  // If this function is marked explicitly as a leaf call, we don't need to
-  // place a safepoint of it.  In fact, for correctness we *can't* in many
-  // cases.  Note: Indirect calls return Null for the called function,
-  // these obviously aren't runtime functions with attributes
-  // TODO: Support attributes on the call site as well.
-  const Function *F = CS.getCalledFunction();
-  bool isLeaf =
-      F &&
-      F->getFnAttribute("gc-leaf-function").getValueAsString().equals("true");
-  if (isLeaf) {
-    return true;
-  }
-  return false;
-}
-
 static void
 InsertSafepointPoll(Instruction *InsertBefore,
                     std::vector<CallSite> &ParsePointsNeeded /*rval*/) {
   BasicBlock *OrigBB = InsertBefore->getParent();
   Module *M = InsertBefore->getModule();
   assert(M && "must be part of a module");
 
   // Inline the safepoint poll implementation - this will get all the branch,

lib/Transforms/Utils/Local.cpp

if (DT.dominates(BB, I->getParent())) {
       U.set(To);
       DEBUG(dbgs() << "Replace dominated use of '" << From->getName() << "' as "
                    << *To << " in " << *U << "\n");
       ++Count;
     }
   }
   return Count;
 }
+
+bool llvm::callsGCLeafFunction(ImmutableCallSite CS) {
+  if (isa<IntrinsicInst>(CS.getInstruction()))
+    // Most LLVM intrinsics are things which can never take a safepoint.
+    // As a result, we don't need to have the stack parsable at the
+    // callsite.  This is a highly useful optimization since intrinsic
+    // calls are fairly prevalent, particularly in debug builds.
+    return true;
+
+  // Check if the function is specifically marked as a gc leaf function.
+  //
+  // TODO: we should be checking the attributes on the call site as well.
+  if (const Function *F = CS.getCalledFunction())
+    return F->hasFnAttribute("gc-leaf-function");
+
+  return false;
+}