Implement a simple register allocator that tries to keep instruction results in registers.

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index 612f84423f2f561b70801639693d01728e372aac..00d2f28e6471efe770949cdfa4e6fb65ed463ae3 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -634,14 +634,13 @@ void FlowGraphCompiler::EmitInstructionPrologue(Instruction* instr) {
   LocationSummary* locs = instr->locs();
   ASSERT(locs != NULL);
 
-  locs->AllocateRegisters();
+  frame_register_allocator()->AllocateRegisters(instr);
 
-  // Load instruction inputs into allocated registers.
-  for (intptr_t i = locs->input_count() - 1; i >= 0; i--) {
-    Location loc = locs->in(i);
-    ASSERT(loc.kind() == Location::kRegister);
-    __ popq(loc.reg());
-  }
+  // TODO(vegorov): adjust assertion when we start removing comparison from the
+  // graph when it is merged with a branch.
+  ASSERT(locs->is_call() ||
+         (instr->IsBranch() && instr->AsBranch()->is_fused_with_comparison()) ||
+         (locs->input_count() == instr->InputCount()));
 }
 
 
@@ -989,6 +988,7 @@ void FlowGraphCompiler::GenerateCall(intptr_t token_index,
                                      intptr_t try_index,
                                      const ExternalLabel* label,
                                      PcDescriptors::Kind kind) {
+  ASSERT(frame_register_allocator()->IsSpilled());
   __ call(label);
   AddCurrentDescriptor(kind, AstNode::kNoId, token_index, try_index);
 }
@@ -998,6 +998,7 @@ void FlowGraphCompiler::GenerateCallRuntime(intptr_t cid,
                                             intptr_t token_index,
                                             intptr_t try_index,
                                             const RuntimeEntry& entry) {
+  ASSERT(frame_register_allocator()->IsSpilled());
   __ CallRuntime(entry);
   AddCurrentDescriptor(PcDescriptors::kOther, cid, token_index, try_index);
 }