In guaranteed tailcall mode, don't decline the tailcall optimization

[oota-llvm.git] / lib / CodeGen / SelectionDAG / SelectionDAGBuilder.cpp

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 284b984f0a1708934b54ccc69648e462dcfe62b2..93ae043dbc14435c5f3c203b971d7481c2372b78 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -4197,15 +4197,24 @@ SelectionDAGBuilder::visitIntrinsicCall(CallInst &I, unsigned Intrinsic) {
 ///
 /// This function only tests target-independent requirements.
 static bool
-isInTailCallPosition(const Instruction *I, Attributes CalleeRetAttr,
+isInTailCallPosition(CallSite CS, Attributes CalleeRetAttr,
                      const TargetLowering &TLI) {
+  const Instruction *I = CS.getInstruction();
   const BasicBlock *ExitBB = I->getParent();
   const TerminatorInst *Term = ExitBB->getTerminator();
   const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);
   const Function *F = ExitBB->getParent();
 
-  // The block must end in a return statement or an unreachable.
-  if (!Ret && !isa<UnreachableInst>(Term)) return false;
+  // The block must end in a return statement or unreachable.
+  //
+  // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
+  // an unreachable, for now. The way tailcall optimization is currently
+  // implemented means it will add an epilogue followed by a jump. That is
+  // not profitable. Also, if the callee is a special function (e.g.
+  // longjmp on x86), it can end up causing miscompilation that has not
+  // been fully understood.
+  if (!Ret &&
+      (!GuaranteedTailCallOpt || !isa<UnreachableInst>(Term))) return false;
 
   // If I will have a chain, make sure no other instruction that will have a
   // chain interposes between I and the return.
@@ -4234,6 +4243,10 @@ isInTailCallPosition(const Instruction *I, Attributes CalleeRetAttr,
   if ((CalleeRetAttr ^ CallerRetAttr) & ~Attribute::NoAlias)
     return false;
 
+  // It's not safe to eliminate the sign / zero extension of the return value.
+  if ((CallerRetAttr & Attribute::ZExt) || (CallerRetAttr & Attribute::SExt))
+    return false;
+
   // Otherwise, make sure the unmodified return value of I is the return value.
   for (const Instruction *U = dyn_cast<Instruction>(Ret->getOperand(0)); ;
        U = dyn_cast<Instruction>(U->getOperand(0))) {
@@ -4348,9 +4361,7 @@ void SelectionDAGBuilder::LowerCallTo(CallSite CS, SDValue Callee,
   // Check if target-independent constraints permit a tail call here.
   // Target-dependent constraints are checked within TLI.LowerCallTo.
   if (isTailCall &&
-      !isInTailCallPosition(CS.getInstruction(),
-                            CS.getAttributes().getRetAttributes(),
-                            TLI))
+      !isInTailCallPosition(CS, CS.getAttributes().getRetAttributes(), TLI))
     isTailCall = false;
 
   std::pair<SDValue,SDValue> Result =