Add an alias "var" for "frame var" and "vo" for "frame var -O"

[jingham]

Summary

"expression" is a hugely inefficient way to get the value of a local variable. There are a few cases where "frame variable" and "expression" will produce different results on the same expression (e.g. "foo->bar" when foo has an -> operator overload). But in general, if you are printing a local var, "frame var" is way more efficient. I've been trying to teach people to use 'frame var' more when it is appropriate, and it was suggested that adding a top-level alias for it might make the command easier to find and remember.

This patch adds two aliases:

var - frame var
vo - frame var -O

The latter is for our ObjC friends who often see the object description, rather than the object's ivars, as the primary source of information about the object.

I also added some caveats to the frame var help page.

[davide]

LG. If you can add a line (so that we can test this) to an existing inline test, I think it would be excellent.

[aprantl]

Currently v is an alias for version. I'd wager that — compared to printing a variable — LLDB user's almost never want to print LLDB's version. Would it make sense to re-purpose the single-letter v abbreviation for the much more frequently used frame variable action?

[xbolva00]

In D53010#1260956, @aprantl wrote:

Currently v is an alias for version. I'd wager that — compared to printing a variable — LLDB user's almost never want to print LLDB's version. Would it make sense to re-purpose the single-letter v abbreviation for the much more frequently used frame variable action?

+1 for v as an alias for frame variable.

source/Commands/CommandObjectFrame.cpp

 public:
   CommandObjectFrameVariable(CommandInterpreter &interpreter)
       : CommandObjectParsed(
             interpreter, "frame variable",
             "Show variables for the current stack frame. Defaults to all "
             "arguments and local variables in scope. Names of argument, "
             "local, file static and file global variables can be specified. "
             "Children of aggregate variables can be specified such as "
-            "'var->child.x'.",
+            "'var->child.x'.  The -> and [] operators in 'frame variable' do "
+            "not invoke operator overloads if they exist, but directly access "
+            "the specified element.  If you want to trigger operator overloads "
+            "use the expression command to print the variable instead."
+            "\nIt is worth noting that except for overloaded "
+            "operators, when printing local variables 'expr local_var' and "
+            "'frame var local_var' produce the same "
+            "results.  However, 'frame variable' is more efficient, since it "
+            "uses debug information and memory reads directly, rather than "
+            "parsing and evaluating an expression, which may even involve "
+            "JITing and running code in the target program.",
             nullptr, eCommandRequiresFrame | eCommandTryTargetAPILock |
                          eCommandProcessMustBeLaunched |
                          eCommandProcessMustBePaused | eCommandRequiresProcess),
         m_option_group(),
         m_option_variable(
             true), // Include the frame specific options by passing "true"
         m_option_format(eFormatDefault),
         m_varobj_options() {

source/Interpreter/CommandInterpreter.cpp

#endif
   if (cmd_obj_sp) {
     AddAlias("add-dsym", cmd_obj_sp);
   }
 
   cmd_obj_sp = GetCommandSPExact("breakpoint set", false);
   if (cmd_obj_sp) {
     AddAlias("rbreak", cmd_obj_sp, "--func-regex %1");
   }
+
+  cmd_obj_sp = GetCommandSPExact("frame variable", false);
+  if (cmd_obj_sp) {
+    AddAlias("var", cmd_obj_sp);
+    AddAlias("vo", cmd_obj_sp, "--object-description");
+  }
+  
 }
 
 void CommandInterpreter::Clear() {
   m_command_io_handler_sp.reset();
 
   if (m_script_interpreter_sp)
     m_script_interpreter_sp->Clear();
 }