Teach the flow graph compiler to play nice with the debugger

runtime/vm/flow_graph_compiler_x64.cc

 // Copyright (c) 2012, 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/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/ast_printer.h"
@@ skipping 130 matching lines @@
     __ LoadObject(RBX, function);
     __ pushq(RBX);
     GenerateCallRuntime(AstNode::kNoId,
                         0,
                         kTraceFunctionExitRuntimeEntry);
     __ popq(RAX);  // Remove argument.
     __ popq(RAX);  // Restore result.
   }
   __ LeaveFrame();
   __ ret();
+
+  // Generate 8 bytes of NOPs so that the debugger can patch the
+  // return pattern with a call to the debug stub.

[srdjan, 2012/02/28 21:37:40]

__ nop(8) instead ?

[hausner, 2012/02/28 21:42:34]

With 8 0x90 bytes it's easier to find the code pattern in the debugger to patch
it. this code is never executed, so speed is no issue.

[srdjan, 2012/02/28 21:51:48]

You may want to add a comment that the pattern of 8 1-nops is important,
otherwise it is very tempting to replace it with nop(8).

+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  __ nop(1);
+  AddCurrentDescriptor(PcDescriptors::kReturn,
+                       AstNode::kNoId,
+                       instr->token_index());
 }
 
 
 void FlowGraphCompiler::VisitBranch(BranchInstr* instr) {
   Bailout("BranchInstr");
 }
 
 
 void FlowGraphCompiler::CompileGraph() {
   const Function& function = parsed_function_.function();
@@ skipping 122 matching lines @@
 void FlowGraphCompiler::FinalizeExceptionHandlers(const Code& code) {
   // We don't compile exception handlers yet.
   code.set_exception_handlers(
       ExceptionHandlers::Handle(ExceptionHandlers::New(0)));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64