Support SameAsFirstInput policy in register allocator when inputs are paired machine registers

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index bb092dfc8c810bf52cb39c8c94129f9e5035f006..ddac6dae3ae65312486458cc28db78edab75d4e3 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -1150,9 +1150,18 @@ void FlowGraphAllocator::ProcessOneInstruction(BlockEntryInstr* block,
   // Normalize same-as-first-input output if input is specified as
   // fixed register.
   if (locs->out(0).IsUnallocated() &&
-      (locs->out(0).policy() == Location::kSameAsFirstInput) &&
-      (locs->in(0).IsMachineRegister())) {
-    locs->set_out(0, locs->in(0));
+      (locs->out(0).policy() == Location::kSameAsFirstInput)) {
+    if (locs->in(0).IsPairLocation()) {
+      // Pair input, pair output.
+      PairLocation* in_pair = locs->in(0).AsPairLocation();
+      if (in_pair->At(0).IsMachineRegister() ||
+          in_pair->At(1).IsMachineRegister()) {
+        locs->set_out(0, Location::Pair(in_pair->At(0), in_pair->At(1)));

[Vyacheslav Egorov (Google), 2014/07/16 16:26:42]

I wonder what happens if you have partial pair location (EAX, R) specified as
input and output having an SameAsFirstInput. I don't think this will build the
right thing.

I suggest prohibiting this to avoid dealing with it now.

[Cutch, 2014/07/16 16:39:29]

Done.

+      }
+    } else if (locs->in(0).IsMachineRegister()) {
+      // Single input, single output.
+      locs->set_out(0, locs->in(0));
+    }
   }
 
   const bool output_same_as_first_input =