[PATCH] Fix array stride bug

[PATCH] Fix array stride bug

[Tom Tromey]

Investigation of using the Python API with an Ada program showed that
an array of dynamic types was not being handled properly.  I tracked
this down to an oddity of how array strides are handled.

In gdb, an array stride can be attached to the range type, via the
range_bounds object.  However, the stride can also be put into the
array's first field.  From create_range_type_with_stride:

  else if (bit_stride > 0)
    TYPE_FIELD_BITSIZE (result_type, 0) = bit_stride;

It's hard to be sure why this is done, but I would guess a combination
of historical reasons plus a desire (mentioned in a comment somewhere)
to avoid modifying the range type.

This patch fixes the problem by changing type::bit_stride to
understand this convention.  It also fixes one spot that reproduces
this logic.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-06-30  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (update_static_array_size): Update.
	* gdbtypes.h (type::bit_stride): Handle array bit stride
	convention.

gdb/testsuite/ChangeLog
2021-06-30  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/array_of_variant.exp: Add Python tests.
---
 gdb/ChangeLog                              |  6 ++++++
 gdb/gdbtypes.c                             |  8 +-------
 gdb/gdbtypes.h                             |  2 ++
 gdb/testsuite/ChangeLog                    |  4 ++++
 gdb/testsuite/gdb.ada/array_of_variant.exp | 17 +++++++++++++++++
 5 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index dbc82f0a4ac..1a261719422 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -1280,13 +1280,7 @@ update_static_array_size (struct type *type)
       int stride;
       struct type *element_type;
 
-      /* If the array itself doesn't provide a stride value then take
-	 whatever stride the range provides.  Don't update BIT_STRIDE as
-	 we don't want to place the stride value from the range into this
-	 arrays bit size field.  */
-      stride = TYPE_FIELD_BITSIZE (type, 0);
-      if (stride == 0)
-	stride = range_type->bit_stride ();
+      stride = type->bit_stride ();
 
       if (!get_discrete_bounds (range_type, &low_bound, &high_bound))
 	low_bound = high_bound = 0;
diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index 0cc00e74a20..d754f2fcd36 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -1029,6 +1029,8 @@ struct type
 
   ULONGEST bit_stride () const
   {
+    if (this->code () == TYPE_CODE_ARRAY && this->field (0).bitsize != 0)
+      return this->field (0).bitsize;
     return this->bounds ()->bit_stride ();
   }
 
diff --git a/gdb/testsuite/gdb.ada/array_of_variant.exp b/gdb/testsuite/gdb.ada/array_of_variant.exp
index dfcbe751b57..d93ad4704a0 100644
--- a/gdb/testsuite/gdb.ada/array_of_variant.exp
+++ b/gdb/testsuite/gdb.ada/array_of_variant.exp
@@ -14,6 +14,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 load_lib "ada.exp"
+load_lib "gdb-python.exp"
 
 if { [skip_ada_tests] } { return -1 }
 
@@ -49,4 +50,20 @@ foreach_with_prefix scenario {all minimal} {
     gdb_test "print objects(2 .. 2)" \
 	[string_to_regexp " = (2 => $v2)"] \
 	"print second array slice"
+
+    # This is only supported for the DWARF encoding.
+    if {$scenario == "minimal" && ![skip_python_tests]} {
+	gdb_test_no_output \
+	    "python o = gdb.parse_and_eval('objects')" \
+	    "fetch value for python"
+	gdb_test "python print(o)" \
+	    [string_to_regexp "($v1, $v2)"] \
+	    "python print array"
+	gdb_test "python print(o\[1\])" \
+	    [string_to_regexp "$v1"] \
+	    "python print first array element"
+	gdb_test "python print(o\[2\])" \
+	    [string_to_regexp "$v2"] \
+	    "python print second array element"
+    }
 }
-- 
2.26.3

Re: [PATCH] Fix array stride bug

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tromey@adacore.com> writes:

Tom> Investigation of using the Python API with an Ada program showed that
Tom> an array of dynamic types was not being handled properly.  I tracked
Tom> this down to an oddity of how array strides are handled.

...
Tom> This patch fixes the problem by changing type::bit_stride to
Tom> understand this convention.  It also fixes one spot that reproduces
Tom> this logic.

I'm checking this in now.

Tom