[PATCH] gdb: user variables with components of dynamic type

[Andrew Burgess <andrew.burgess@embecosm.com>]

Consider this Fortran type:

  type :: some_type
     integer, allocatable :: array_one (:,:)
     integer :: a_field
     integer, allocatable :: array_two (:,:)
  end type some_type

And a variable declared:

  type(some_type) :: some_var

Now within GDB we try this:

  (gdb) set $a = some_var
  (gdb) p $a
  $1 = ( array_one =
  ../../src/gdb/value.c:3968: internal-error: Unexpected lazy value type.

Normally, when an internalvar ($a in this case) is created, it is
non-lazy, the value is immediately copied out of the inferior into
GDB's memory.

When printing the internalvar ($a) GDB will extract each field in
turn, so in this case `array_one`.  As the original internalvar is
non-lazy then the extracted field will also be non-lazy, with its
contents immediately copied from the parent internalvar.

However, when the field has a dynamic type this is not the case,
value_primitive_field we see that any field with dynamic type is
always created lazy.  Further, the content of this field will usually
not have been captured in the contents buffer of the original value, a
field with dynamic location is effectively a pointer value contained
within the parent value, with rules in the DWARF for how to
dereference the pointer.

So, we end up with a lazy lval_internalvar_component representing a
field within an lval_internalvar.  This eventually ends up in
value_fetch_lazy, which currently does not support
lval_internalvar_component, and we see the error above.

My original plan for how to handle this involved extending
value_fetch_lazy to handle lval_internalvar_component.  However, when
I did this I ran into another error:

  (gdb) set $a = some_var
  (gdb) p $a
  $1 = ( array_one = ((1, 1) (1, 1) (1, 1)), a_field = 5, array_two = ((0, 0, 0) (0, 0, 0)) )
  (gdb) p $a%array_one
  $2 = ((1, 1) (1, 1) (1, 1))
  (gdb) p $a%array_one(1,1)
  ../../src/gdb/value.c:1547: internal-error: void set_value_address(value*, CORE_ADDR): Assertion `value->lval == lval_memory' failed.

The problem now is some patch up code inside
set_value_component_location, where we attempt to set the address for
a component, if the original parent value has a dynamic location.  GDB
does not expect to ever set the address on anything other than an
lval_memory value (which seems reasonable).

In order to resolve this issue I initially thought about how an
internalvar should "capture" the value of a program variable at the
moment the var is created.  In an ideal world (I think) GDB would be
able to do this even for values with dynamic type.  So in our above
example doing `set $a = some_var` would capture the content of
'some_var', but also the content of 'array_one', and also 'array_two',
even these content regions are not contained within the region of
'some_var'.

Supporting this would require GDB values to be able to carry around
multiple non-contiguous regions of memory at content in some way,
which sounds like a pretty huge change to a core part of GDB.

So, I wondered if there was some other solution that wouldn't require
such a huge change.

What if values with a dynamic location were though of like points with
automatic dereferencing?  Given this C structure:

  struct foo_t {
    int *val;
  }

  struct foo_t my_foo;

Then in GDB:

  (gdb) $a = my_foo

We would expect GDB to capture the pointer value in '$a', but not the
value pointed at by the pointer.  So maybe it's not that unreasonable
to think that given a dynamically typed field GDB will capture the
address of the content, but not the actual content itself.

That's what this patch does.

The approach is to catch this case in set_value_component_location,
when we create a component location (of an lval_internalvar) that has
a dynamic data location, the lval_internalvar_component is changed
into an lval_memory.  After this both of the above issues are
resolved.  In the first case, the lval_memory is still lazy, but
value_fetch_lazy knows how to handle that.  In the second case, when
we access the element of the array we are now accessing an element of
an lval_memory, not an lval_internalvar_component, and calling
set_value_address on an lval_memory is fine.

gdb/ChangeLog:

	* value.c (set_value_component_location): Adjust the VALUE_LVAL
	for internalvar components that have a dynamic location.

gdb/testsuite/ChangeLog:

	* gdb.fortran/intvar-dynamic-types.exp: New file.
	* gdb.fortran/intvar-dynamic-types.f90: New file.
---
 gdb/ChangeLog                                 |  5 +
 gdb/testsuite/ChangeLog                       |  5 +
 .../gdb.fortran/intvar-dynamic-types.exp      | 97 +++++++++++++++++++
 .../gdb.fortran/intvar-dynamic-types.f90      | 32 ++++++
 gdb/value.c                                   | 29 +++++-
 5 files changed, 166 insertions(+), 2 deletions(-)
 create mode 100644 gdb/testsuite/gdb.fortran/intvar-dynamic-types.exp
 create mode 100644 gdb/testsuite/gdb.fortran/intvar-dynamic-types.f90

diff --git a/gdb/testsuite/gdb.fortran/intvar-dynamic-types.exp b/gdb/testsuite/gdb.fortran/intvar-dynamic-types.exp
new file mode 100644
index 00000000000..9cf5cee02ff
--- /dev/null
+++ b/gdb/testsuite/gdb.fortran/intvar-dynamic-types.exp
@@ -0,0 +1,97 @@
+# Copyright 2020 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Places a value with components that have dynamic type into a GDB
+# user variable, and then prints the user variable.
+
+standard_testfile ".f90"
+load_lib "fortran.exp"
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} \
+    {debug f90 quiet}] } {
+    return -1
+}
+
+if ![fortran_runto_main] {
+    untested "could not run to main"
+    return -1
+}
+
+gdb_breakpoint [gdb_get_line_number "Break here"]
+gdb_continue_to_breakpoint "Break here"
+
+gdb_test_no_output "set \$a=some_var" "set \$a internal variable"
+
+foreach var { "\$a" "some_var" } {
+    with_test_prefix "print $var" {
+	gdb_test "print $var" \
+	    " = \\( array_one = \\(\\(1, 1\\) \\(1, 1\\) \\(1, 1\\)\\), a_field = 5, array_two = \\(\\(2, 2, 2\\) \\(2, 2, 2\\)\\) \\)" \
+	    "print full contents"
+
+	gdb_test "print $var%array_one" \
+	    " = \\(\\(1, 1\\) \\(1, 1\\) \\(1, 1\\)\\)" \
+	    "print array_one field"
+
+	gdb_test "print $var%a_field" \
+	    " = 5" \
+	    "print a_field field"
+
+	gdb_test "print $var%array_two" \
+	    " = \\(\\(2, 2, 2\\) \\(2, 2, 2\\)\\)" \
+	    "print array_two field"
+    }
+}
+
+# Create new user variables for the fields of some_var, and show that
+# modifying these variables does not change the original value from
+# the program.
+gdb_test_no_output "set \$b = some_var%array_one"
+gdb_test_no_output "set \$c = some_var%array_two"
+gdb_test "print \$b" \
+    " = \\(\\(1, 1\\) \\(1, 1\\) \\(1, 1\\)\\)"
+gdb_test "print \$c" \
+    " = \\(\\(2, 2, 2\\) \\(2, 2, 2\\)\\)"
+gdb_test_no_output "set \$b(2,2) = 3"
+gdb_test_no_output "set \$c(3,1) = 4"
+gdb_test "print \$b" \
+    " = \\(\\(1, 1\\) \\(1, 3\\) \\(1, 1\\)\\)" \
+    "print \$b after a change"
+gdb_test "print \$c" \
+    " = \\(\\(2, 2, 4\\) \\(2, 2, 2\\)\\)" \
+    "print \$c after a change"
+gdb_test "print some_var%array_one" \
+    " = \\(\\(1, 1\\) \\(1, 1\\) \\(1, 1\\)\\)"
+gdb_test "print some_var%array_two" \
+    " = \\(\\(2, 2, 2\\) \\(2, 2, 2\\)\\)"
+
+# Now modify the user variable '$a', which is a copy of 'some_var',
+# and then check how this change is reflected in the original
+# 'some_var', and the user variable $a.
+#
+# When we change 'a_field' which is a non-dynamic field within the
+# user variable, the change is only visible within the user variable.
+#
+# In contrast, when we change 'array_one' and 'array_two', which are
+# both fields of dynanic type, then the change is visible in both the
+# user variable and the original program variable 'some_var'.  This
+# makes sense if you consider the dynamic type as if it was a C
+# pointer with automatic indirection.
+gdb_test_no_output "set \$a%a_field = 3"
+gdb_test_no_output "set \$a%array_one(2,2) = 3"
+gdb_test_no_output "set \$a%array_two(3,1) = 4"
+gdb_test "print \$a" \
+    " = \\( array_one = \\(\\(1, 1\\) \\(1, 3\\) \\(1, 1\\)\\), a_field = 3, array_two = \\(\\(2, 2, 4\\) \\(2, 2, 2\\)\\) \\)"
+gdb_test "print some_var" \
+    " = \\( array_one = \\(\\(1, 1\\) \\(1, 3\\) \\(1, 1\\)\\), a_field = 5, array_two = \\(\\(2, 2, 4\\) \\(2, 2, 2\\)\\) \\)"
diff --git a/gdb/testsuite/gdb.fortran/intvar-dynamic-types.f90 b/gdb/testsuite/gdb.fortran/intvar-dynamic-types.f90
new file mode 100644
index 00000000000..ace864812de
--- /dev/null
+++ b/gdb/testsuite/gdb.fortran/intvar-dynamic-types.f90
@@ -0,0 +1,32 @@
+! Copyright 2020 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+program internal_var_test
+  type :: some_type
+     integer, allocatable :: array_one (:,:)
+     integer :: a_field
+     integer, allocatable :: array_two (:,:)
+  end type some_type
+
+  type(some_type) :: some_var
+
+  allocate (some_var%array_one (2,3))
+  allocate (some_var%array_two (3,2))
+  some_var%array_one (:,:) = 1
+  some_var%a_field = 5
+  some_var%array_two (:,:) = 2
+  deallocate (some_var%array_one) ! Break here.
+  deallocate (some_var%array_two)
+end program internal_var_test
diff --git a/gdb/value.c b/gdb/value.c
index 7c36c31dccd..12b0fd7b48c 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -1784,12 +1784,37 @@ set_value_component_location (struct value *component,
         component->location.computed.closure = funcs->copy_closure (whole);
     }
 
-  /* If type has a dynamic resolved location property
-     update it's value address.  */
+  /* If either the WHOLE value, or the COMPONENT value has a dynamic
+     resolved location property then update the address of the COMPONENT.
+
+     If the COMPONENT itself has a dynamic location, and was an
+     lval_internalvar_component, then we change this to lval_memory.
+     Usually a component of an internalvar is created non-lazy, and has its
+     content immediately copied from the parent internalvar.  However,
+     for components with a dynamic location, the content of the component
+     is not contained within the parent, but is instead accessed
+     indirectly.  Further, the component will be created as a lazy value.
+
+     By changing the type of the component to lval_memory we ensure that
+     value_fetch_lazy can successfully load the component.  */
   type = value_type (whole);
   if (NULL != TYPE_DATA_LOCATION (type)
       && TYPE_DATA_LOCATION_KIND (type) == PROP_CONST)
     set_value_address (component, TYPE_DATA_LOCATION_ADDR (type));
+
+  type = value_type (component);
+  if (NULL != TYPE_DATA_LOCATION (type)
+      && TYPE_DATA_LOCATION_KIND (type) == PROP_CONST)
+    {
+      if (VALUE_LVAL (component) == lval_internalvar_component)
+	{
+	  gdb_assert (value_lazy (component));
+	  VALUE_LVAL (component) = lval_memory;
+	}
+      else
+	gdb_assert (VALUE_LVAL (component) == lval_memory);
+      set_value_address (component, TYPE_DATA_LOCATION_ADDR (type));
+    }
 }
 
 /* Access to the value history.  */
-- 
2.25.4