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

runtime/vm/flow_graph_allocator.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_allocator.h"
 
 #include "vm/bit_vector.h"
 #include "vm/intermediate_language.h"
 #include "vm/il_printer.h"
 #include "vm/flow_graph.h"
@@ skipping 1132 matching lines @@
   }
 
   const intptr_t pos = current->lifetime_position();
   ASSERT(IsInstructionStartPosition(pos));
 
   ASSERT(locs->input_count() == current->InputCount());
 
   // 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 =
       locs->out(0).IsUnallocated() &&
       (locs->out(0).policy() == Location::kSameAsFirstInput);
 
   // Output is same as first input which is a pair.
   if (output_same_as_first_input && locs->in(0).IsPairLocation()) {
     // Make out into a PairLocation.
     locs->set_out(0, Location::Pair(Location::RequiresRegister(),
@@ skipping 1721 matching lines @@
     OS::Print("-- [after ssa allocator] ir [%s] -------------\n",
               function.ToFullyQualifiedCString());
     FlowGraphPrinter printer(flow_graph_, true);
     printer.PrintBlocks();
     OS::Print("----------------------------------------------\n");
   }
 }
 
 
 }  // namespace dart