gdb/fortran: Support negative array stride in one limited case

This commit adds support for negative Fortran array strides in one
limited case, that is the case of a single element array with a
negative array stride.

The changes in this commit will be required in order for more general
negative array stride support to work correctly, however, right now
other problems in GDB prevent negative array strides from working in
the general case.

The reason negative array strides don't currently work in the general
case is that when dealing with such arrays, the base address for the
objects data is actually the highest addressed element, subsequent
elements are then accessed with a negative offset from that address,
and GDB is not currently happy with this configuration.

The changes here can be summarised as, stop treating signed values as
unsigned, specifically, the array stride, and offsets calculated using
the array stride.

This issue was identified on the mailing list by Sergio:

  https://sourceware.org/ml/gdb-patches/2020-01/msg00360.html

The test for this issue is a new one written by me as the copyright
status of the original test is currently unknown.

gdb/ChangeLog:

	* gdbtypes.c (create_array_type_with_stride): Handle negative
	array strides.
	* valarith.c (value_subscripted_rvalue): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.fortran/derived-type-striding.exp: Add a new test.
	* gdb.fortran/derived-type-striding.f90: Add pointer variable for
	new test.

6 files changed, 31 insertions, 6 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 879a17a..cb3e185 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-25  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdbtypes.c (create_array_type_with_stride): Handle negative
+	array strides.
+	* valarith.c (value_subscripted_rvalue): Likewise.
+
 2020-02-25  Luis Machado  <luis.machado@linaro.org>
 
 	* aarch64-tdep.c (aarch64_vnv_type): Fix comment typo.
diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index 8575893..ef110b3 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -1223,7 +1223,7 @@ create_array_type_with_stride (struct type *result_type,
 	  && !type_not_allocated (result_type)))
     {
       LONGEST low_bound, high_bound;
-      unsigned int stride;
+      int stride;
 
       /* If the array itself doesn't provide a stride value then take
 	 whatever stride the range provides.  Don't update BIT_STRIDE as
@@ -1241,9 +1241,18 @@ create_array_type_with_stride (struct type *result_type,
 	 In such cases, the array length should be zero.  */
       if (high_bound < low_bound)
 	TYPE_LENGTH (result_type) = 0;
-      else if (stride > 0)
-	TYPE_LENGTH (result_type) =
-	  (stride * (high_bound - low_bound + 1) + 7) / 8;
+      else if (stride != 0)
+	{
+	  /* Ensure that the type length is always positive, even in the
+	     case where (for example in Fortran) we have a negative
+	     stride.  It is possible to have a single element array with a
+	     negative stride in Fortran (this doesn't mean anything
+	     special, it's still just a single element array) so do
+	     consider that case when touching this code.  */
+	  LONGEST element_count = abs (high_bound - low_bound + 1);
+	  TYPE_LENGTH (result_type)
+	    = ((abs (stride) * element_count) + 7) / 8;
+	}
       else
 	TYPE_LENGTH (result_type) =
 	  TYPE_LENGTH (element_type) * (high_bound - low_bound + 1);
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index ccbb5be..720febb 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2020-02-25  Andrew Burgess  <andrew.burgess@embecosm.com>
 
+	* gdb.fortran/derived-type-striding.exp: Add a new test.
+	* gdb.fortran/derived-type-striding.f90: Add pointer variable for
+	new test.
+
+2020-02-25  Andrew Burgess  <andrew.burgess@embecosm.com>
+
 	* gdb.base/cached-source-file.exp: Avoid source file paths in test
 	names.
 
diff --git a/gdb/testsuite/gdb.fortran/derived-type-striding.exp b/gdb/testsuite/gdb.fortran/derived-type-striding.exp
index 094843c..639dc4c 100644
--- a/gdb/testsuite/gdb.fortran/derived-type-striding.exp
+++ b/gdb/testsuite/gdb.fortran/derived-type-striding.exp
@@ -41,3 +41,5 @@ gdb_test "p point_dimension" "= \\\(2, 2, 2, 2, 2, 2, 2, 2, 2\\\)"
 # Test mixed type derived type.
 if { $gcc_with_broken_stride } { setup_kfail *-*-* gcc/92775 }
 gdb_test "p point_mixed_dimension" "= \\\(3, 3, 3, 3\\\)"
+
+gdb_test "p cloud_slice" " = \\\(\\\( x = 1, y = 2, z = 3 \\\)\\\)"
diff --git a/gdb/testsuite/gdb.fortran/derived-type-striding.f90 b/gdb/testsuite/gdb.fortran/derived-type-striding.f90
index 26829f5..fb53757 100644
--- a/gdb/testsuite/gdb.fortran/derived-type-striding.f90
+++ b/gdb/testsuite/gdb.fortran/derived-type-striding.f90
@@ -28,9 +28,11 @@ program derived_type_member_stride
     type(mixed_cartesian), dimension(10), target :: mixed_cloud
     integer(kind=8), dimension(:), pointer :: point_dimension => null()
     integer(kind=8), dimension(:), pointer :: point_mixed_dimension => null()
+    type(cartesian), dimension(:), pointer :: cloud_slice => null()
     cloud(:)%x = 1
     cloud(:)%y = 2
     cloud(:)%z = 3
+    cloud_slice => cloud(3:2:-2)
     point_dimension => cloud(1:9)%y
     mixed_cloud(:)%x = 1
     mixed_cloud(:)%y = 2
diff --git a/gdb/valarith.c b/gdb/valarith.c
index 79b1486..be0e073 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -187,7 +187,7 @@ value_subscripted_rvalue (struct value *array, LONGEST index, LONGEST lowerbound
 {
   struct type *array_type = check_typedef (value_type (array));
   struct type *elt_type = check_typedef (TYPE_TARGET_TYPE (array_type));
-  ULONGEST elt_size = type_length_units (elt_type);
+  LONGEST elt_size = type_length_units (elt_type);
 
   /* Fetch the bit stride and convert it to a byte stride, assuming 8 bits
      in a byte.  */
@@ -199,7 +199,7 @@ value_subscripted_rvalue (struct value *array, LONGEST index, LONGEST lowerbound
       elt_size = stride / (unit_size * 8);
     }
 
-  ULONGEST elt_offs = elt_size * (index - lowerbound);
+  LONGEST elt_offs = elt_size * (index - lowerbound);
 
   if (index < lowerbound
       || (!TYPE_ARRAY_UPPER_BOUND_IS_UNDEFINED (array_type)