Teach the flow graph compiler to play nice with the debugger

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 4720)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -148,6 +148,20 @@
   }
   __ 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());
 }