Disable this-return argument forwarding on ARM/AArch64

r275042 reverted function-attribute inference for the 'returned' attribute
because the feature triggered self-hosting failures on ARM and AArch64. James
Molloy determined that the this-return argument forwarding feature, which
directly ties the returned input argument to the returned value, was the cause.
It seems likely that this forwarding code contains, or triggers, a subtle bug.
Disabling for now until we can track that down.

llvm-svn: 275677

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index a435923c15b..ef019f98726 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -65,6 +65,13 @@ ARMInterworking("arm-interworking", cl::Hidden,
   cl::desc("Enable / disable ARM interworking (for debugging only)"),
   cl::init(true));
 
+// Disabled for causing self-hosting failures once returned-attribute inference
+// was enabled.
+static cl::opt<bool>
+EnableThisRetForwarding("arm-this-return-forwarding", cl::Hidden,
+                        cl::desc("Directly forward this return"),
+                        cl::init(false));
+
 namespace {
   class ARMCCState : public CCState {
   public:
@@ -1466,7 +1473,7 @@ SDValue ARMTargetLowering::LowerCallResult(
 
     // Pass 'this' value directly from the argument to return value, to avoid
     // reg unit interference
-    if (i == 0 && isThisReturn) {
+    if (i == 0 && isThisReturn && EnableThisRetForwarding) {
       assert(!VA.needsCustom() && VA.getLocVT() == MVT::i32 &&
              "unexpected return calling convention register assignment");
       InVals.push_back(ThisVal);