Revert 94937 and move the noreturn check to codegen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95198 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -4205,8 +4205,13 @@ isInTailCallPosition(CallSite CS, Attributes CalleeRetAttr,
   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.
+  // FIXME: Disallow tailcall 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) return false;
 
   // Unless we are explicitly forcing tailcall optimization do not tailcall if
   // the called function is bitcast'ed. The analysis may not be entirely

lib/Transforms/Scalar/TailRecursionElimination.cpp

@@ -184,11 +184,10 @@ bool TailCallElim::runOnFunction(Function &F) {
   if (!FunctionContainsEscapingAllocas)
     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-        if (CallInst *CI = dyn_cast<CallInst>(I))
-          if (!CI->doesNotReturn()) {
-            CI->setTailCall();
-            MadeChange = true;
-          }
+        if (CallInst *CI = dyn_cast<CallInst>(I)) {
+          CI->setTailCall();
+          MadeChange = true;
+        }
 
   return MadeChange;
 }

test/CodeGen/X86/tailcall1.ll

@@ -1,4 +1,4 @@
-; RUN: llc < %s -march=x86 -tailcallopt | grep TAILCALL | count 5
+; RUN: llc < %s -march=x86 -tailcallopt | grep TAILCALL | count 4
 
 declare fastcc i32 @tailcallee(i32 %a1, i32 %a2, i32 %a3, i32 %a4)
 

test/CodeGen/X86/tailcall2.ll

@@ -127,3 +127,16 @@ entry:
   %1 = tail call signext i16 %0(i32 0) nounwind
   ret i16 %1
 }
+
+define void @t10() nounwind ssp {
+entry:
+; 32: t10:
+; 32: call
+
+; 64: t10:
+; 64: callq
+  %0 = tail call i32 @foo4() noreturn nounwind
+  unreachable
+}
+
+declare i32 @foo4()

test/Transforms/TailCallElim/no-return-calls.ll

@@ -1,12 +0,0 @@
-; RUN: opt < %s -tailcallelim -S | FileCheck %s
-
-define void @t() nounwind ssp {
-entry:
-; CHECK: entry:
-; CHECK: %0 = call i32 @foo()
-; CHECK: ret void
-  %0 = call i32 @foo() nounwind noreturn
-  ret void
-}
-
-declare i32 @foo()