Fall back to the selection dag isel to select tail calls.

This shouldn't affect codegen for -O0 compiles as tail call markers are not
emitted in unoptimized compiles.  Testing with the external/internal nightly
test suite reveals no change in compile time performance.  Testing with -O1,
-O2 and -O3 with fast-isel enabled did not cause any compile-time or
execution-time failures.  All tests were performed on my x86 machine.
I'll monitor our arm testers to ensure no regressions occur there.

In an upcoming clang patch I will be marking the objc_autoreleaseReturnValue
and objc_retainAutoreleaseReturnValue as tail calls unconditionally.  While
it's theoretically true that this is just an optimization, it's an
optimization that we very much want to happen even at -O0, or else ARC
applications become substantially harder to debug.

Part of rdar://12553082

llvm-svn: 169796

2 files changed, 7 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 97d975ecb72..ee8f272093a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5273,11 +5273,6 @@ void SelectionDAGBuilder::LowerCallTo(ImmutableCallSite CS, SDValue Callee,
       !isInTailCallPosition(CS, CS.getAttributes().getRetAttributes(), TLI))
     isTailCall = false;
 
-  // If there's a possibility that fast-isel has already selected some amount
-  // of the current basic block, don't emit a tail call.
-  if (isTailCall && TM.Options.EnableFastISel)
-    isTailCall = false;
-
   TargetLowering::
   CallLoweringInfo CLI(getRoot(), RetTy, FTy, isTailCall, Callee, Args, DAG,
                        getCurDebugLoc(), CS);
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index ca252743d8d..c755d729d54 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -1113,19 +1113,21 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
           }
 
           bool HadTailCall = false;
+          MachineBasicBlock::iterator SavedInsertPt = FuncInfo->InsertPt;
           SelectBasicBlock(Inst, BI, HadTailCall);
 
-          // Recompute NumFastIselRemaining as Selection DAG instruction
-          // selection may have handled the call, input args, etc.
-          unsigned RemainingNow = std::distance(Begin, BI);
-          NumFastIselFailures += NumFastIselRemaining - RemainingNow;
-
           // If the call was emitted as a tail call, we're done with the block.
+          // We also need to delete any previously emitted instructions.
           if (HadTailCall) {
+            FastIS->removeDeadCode(SavedInsertPt, FuncInfo->MBB->end());
             --BI;
             break;
           }
 
+          // Recompute NumFastIselRemaining as Selection DAG instruction
+          // selection may have handled the call, input args, etc.
+          unsigned RemainingNow = std::distance(Begin, BI);
+          NumFastIselFailures += NumFastIselRemaining - RemainingNow;
           NumFastIselRemaining = RemainingNow;
           continue;
         }