gdb: PR mi/20395: Fix -var-update for registers in frames 1 and up

This patch fixes a problem with using the MI -var-update command
to access the values of registers in frames other than the current
frame.  The patch includes a test that demonstrates the problem:

* run so there are several frames on the stack
* create a fixed varobj for $pc in each frame, #'s 1 and above
* step one instruction, to modify the value of $pc
* call -var-update for each of the previously created varobjs
  to verify that they are not reported as having changed.

Without the patch, the -var-update command reported that $pc for all
frames 1 and above had changed to the value of $pc in frame 0.

A varobj is created as either fixed, the expression is evaluated within
the context of a specific frame, or floating, the expression is
evaluated within the current frame, whatever that may be.

When a varobj is created by -var-create we set two fields of the varobj
to track the context in which the varobj was created, these two fields
are varobj->root->frame and var->root->valid_block.

If a varobj is of type fixed, then, when we subsequently try to
reevaluate the expression associated with the varobj we must determine
if the original frame (and block) is still available, if it is not then
the varobj can no longer be evaluated.

The problem is that for register expressions varobj->root->valid_block
is not set correctly.  This block tracking is done using the global
'innermost_block' which is set in the various parser files (for example
c-exp.y).  However, this is not set for register expressions.

The fix then seems like it should be to just update the innermost block
when parsing register expressions, however, that solution causes several
test regressions.

The problem is that in some cases we rely on the expression parsing
code not updating the innermost block for registers, one example is
when we parse the expression for a 'display' command.  The display
commands treats registers like floating varobjs, but symbols are
treated like fixed varobjs.  So 'display $reg_name' will always show
the value of '$reg_name' even as the user moves from frame to frame,
while 'display my_variable' will only show 'my_variable' while it is
in the current frame and/or block, when the user moves to a new frame
and/or block (even one with a different 'my_variable' in) then the
display of 'my_variable' stops.  For the case of 'display', without
the option to force fixed or floating expressions, the current
behaviour is probably the best choice.  For the varobj system though,
we can choose between floating and fixed, and we should try to make
this work for registers.

There's only one existing test case that needs to be updated, in that
test a fixed varobj is created using a register, the MI output now
include the thread-id in which the varobj should be evaluated, which I
believe is correct behaviour.  I also added a new floating test case
into the same test script, however, right now this also includes the
thread-id in the expected output, which I believe is an existing gdb
bug, which I plan to fix next.

Tested on x86_64 Linux native and native-gdbserver, no regressions.

gdb/ChangeLog:

	PR mi/20395
	* ada-exp.y (write_var_from_sym): Pass extra parameter when
	updating innermost block.
	* parse.c (innermost_block_tracker::update): Take extra type
	parameter, and check types match before updating innermost block.
	(write_dollar_variable): Update innermost block for registers.
	* parser-defs.h (enum innermost_block_tracker_type): New enum.
	(innermost_block_tracker::innermost_block_tracker): Initialise
	m_types member.
	(innermost_block_tracker::reset): Take type parameter.
	(innermost_block_tracker::update): Take type parameter, and pass
	type through as needed.
	(innermost_block_tracker::m_types): New member.
	* varobj.c (varobj_create): Pass type when reseting innermost
	block.

gdb/testsuite/ChangeLog:

	* gdb.mi/basics.c: Add new global.
	* gdb.mi/mi-frame-regs.exp: New file.
	* gdb.mi/mi-var-create-rtti.exp: Update expected results, add new
	case.

1 files changed, 38 insertions, 9 deletions

diff --git a/gdb/parser-defs.h b/gdb/parser-defs.h
index 01ac0cd..c67b8d56 100644
--- a/gdb/parser-defs.h
+++ b/gdb/parser-defs.h
@@ -75,6 +75,24 @@ extern const struct block *expression_context_block;
    then look up the macro definitions active at that point.  */
 extern CORE_ADDR expression_context_pc;
 
+/* While parsing expressions we need to track the innermost lexical block
+   that we encounter.  In some situations we need to track the innermost
+   block just for symbols, and in other situations we want to track the
+   innermost block for symbols and registers.  These flags are used by the
+   innermost block tracker to control which blocks we consider for the
+   innermost block.  These flags can be combined together as needed.  */
+
+enum innermost_block_tracker_type
+{
+  /* Track the innermost block for symbols within an expression.  */
+  INNERMOST_BLOCK_FOR_SYMBOLS = (1 << 0),
+
+  /* Track the innermost block for registers within an expression.  */
+  INNERMOST_BLOCK_FOR_REGISTERS = (1 << 1)
+};
+DEF_ENUM_FLAGS_TYPE (enum innermost_block_tracker_type,
+		     innermost_block_tracker_types);
+
 /* When parsing expressions we track the innermost block that was
    referenced.  */
 
@@ -82,24 +100,32 @@ class innermost_block_tracker
 {
 public:
   innermost_block_tracker ()
-    : m_innermost_block (NULL)
+    : m_types (INNERMOST_BLOCK_FOR_SYMBOLS),
+      m_innermost_block (NULL)
   { /* Nothing.  */ }
 
   /* Reset the currently stored innermost block.  Usually called before
-     parsing a new expression.  */
-  void reset ()
+     parsing a new expression.  As the most common case is that we only
+     want to gather the innermost block for symbols in an expression, this
+     becomes the default block tracker type.  */
+  void reset (innermost_block_tracker_types t = INNERMOST_BLOCK_FOR_SYMBOLS)
   {
-    m_innermost_block = nullptr;
+    m_types = t;
+    m_innermost_block = NULL;
   }
 
   /* Update the stored innermost block if the new block B is more inner
-     than the currently stored block, or if no block is stored yet.  */
-  void update (const struct block *b);
+     than the currently stored block, or if no block is stored yet.  The
+     type T tells us whether the block B was for a symbol or for a
+     register.  The stored innermost block is only updated if the type T is
+     a type we are interested in, the types we are interested in are held
+     in M_TYPES and set during RESET.  */
+  void update (const struct block *b, innermost_block_tracker_types t);
 
   /* Overload of main UPDATE method which extracts the block from BS.  */
   void update (const struct block_symbol &bs)
   {
-    update (bs.block);
+    update (bs.block, INNERMOST_BLOCK_FOR_SYMBOLS);
   }
 
   /* Return the stored innermost block.  Can be nullptr if no symbols or
@@ -111,13 +137,16 @@ public:
   }
 
 private:
+  /* The type of innermost block being looked for.  */
+  innermost_block_tracker_types m_types;
+
   /* The currently stored innermost block found while parsing an
      expression.  */
   const struct block *m_innermost_block;
 };
 
-/* The innermost context required by the stack and register variables we've
-   encountered so far.  This should be cleared before parsing an
+/* The innermost context required by the stack and register variables
+   we've encountered so far.  This should be cleared before parsing an
    expression, and queried once the parse is complete.  */
 extern innermost_block_tracker innermost_block;