[AArch64, fast-isel] Fall back to SelectionDAG to select tail calls.

Certain functions such as objc_autoreleaseReturnValue have to be called as
tail-calls even at -O0. Since normal fast-isel doesn't emit calls as tail calls,
we have to fall back to SelectionDAG to select calls that are marked as tail.

<rdar://problem/17991614>

llvm-svn: 215600

lib/Target/AArch64/AArch64FastISel.cpp

@@ -1687,10 +1687,15 @@ bool AArch64FastISel::FinishCall(CallLoweringInfo &CLI, MVT RetVT,
 
 bool AArch64FastISel::FastLowerCall(CallLoweringInfo &CLI) {
   CallingConv::ID CC  = CLI.CallConv;
+  bool IsTailCall     = CLI.IsTailCall;
   bool IsVarArg       = CLI.IsVarArg;
   const Value *Callee = CLI.Callee;
   const char *SymName = CLI.SymName;
 
+  // Allow SelectionDAG isel to handle tail calls.
+  if (IsTailCall)
+    return false;
+
   CodeModel::Model CM = TM.getCodeModel();
   // Only support the small and large code model.
   if (CM != CodeModel::Small && CM != CodeModel::Large)

test/CodeGen/AArch64/arm64-abi_align.ll

@@ -511,7 +511,9 @@ entry:
 ; CHECK: str {{w[0-9]+}}, [sp]
 ; FAST-LABEL: i64_split
 ; FAST: ldr x7, [{{x[0-9]+}}]
-; FAST: str {{w[0-9]+}}, [sp]
+; FAST: mov x[[R0:[0-9]+]], sp
+; FAST: orr w[[R1:[0-9]+]], wzr, #0x8
+; FAST: str w[[R1]], {{\[}}x[[R0]]{{\]}}
   %0 = load i64* bitcast (%struct.s41* @g41 to i64*), align 16
   %call = tail call i32 @callee_i64(i32 1, i32 2, i32 3, i32 4, i32 5,
                                     i32 6, i32 7, i64 %0, i32 8) #5

test/CodeGen/AArch64/tailcall-fastisel.ll

@@ -0,0 +1,11 @@
+; RUN: llc < %s -mtriple=arm64-apple-darwin -O0 | FileCheck %s
+
+; CHECK: b _foo0
+
+define i32 @foo1() {
+entry:
+  %call = tail call i32 @foo0()
+  ret i32 %call
+}
+
+declare i32 @foo0()