[PATCH 5/9] Fix issues in write_pieced_value when targeting bit-fields

Andreas Arnez
2017-04-07T17:42:32Z

There are multiple issues in write_pieced_value when dealing with a
bit-field as the target value:

(1) The number of bits preceding the bit-field is calculated without
considering the relative offset of the value's parent.

(2) On big-endian targets the source value's *most* significant bits are
transferred to the target value, instead of its least significant
bits.

(3) The number of bytes containing a portion of the bit-field in a given
piece is calculated with the wrong starting offset; thus the result
may be off by one.

(4) When checking whether the data can be transferred byte-wise, the
transfer size is not verified to be byte-aligned.

(5) When transferring the data via a buffer, the bit offset within the
target value is not reduced to its sub-byte fraction before using it
as a bit offset into the buffer.

These issues are fixed. For consistency, the affected logic that exists
in read_pieced_value as well is changed there in the same way.

gdb/ChangeLog:

* dwarf2loc.c (write_pieced_value): Fix various issues with
bit-field handling.
(read_pieced_value): Sync with changes in write_pieced_value.

gdb/testsuite/ChangeLog:

* gdb.dwarf2/var-access.exp: Add test for accessing bit-fields
whose containing structure is located in several DWARF pieces.
---
gdb/dwarf2loc.c | 54 +++++++++++++++------------------
gdb/testsuite/gdb.dwarf2/var-access.exp | 50 +++++++++++++++++++++++++++++-
2 files changed, 74 insertions(+), 30 deletions(-)

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 76a58a3..1f89a08 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1775,7 +1775,8 @@ read_pieced_value (struct value *v)
bits_to_skip = 8 * value_offset (v);
if (value_bitsize (v))
{
- bits_to_skip += value_bitpos (v);
+ bits_to_skip += (8 * value_offset (value_parent (v))
+ + value_bitpos (v));
type_len = value_bitsize (v);
}
else
@@ -1796,18 +1797,11 @@ read_pieced_value (struct value *v)
bits_to_skip -= this_size_bits;
continue;
}
- if (bits_to_skip > 0)
- {
- dest_offset_bits = 0;
- source_offset_bits = bits_to_skip;
- this_size_bits -= bits_to_skip;
- bits_to_skip = 0;
- }
- else
- {
- dest_offset_bits = offset;
- source_offset_bits = 0;
- }
+ source_offset_bits = bits_to_skip;
+ this_size_bits -= bits_to_skip;
+ bits_to_skip = 0;
+ dest_offset_bits = offset;
+
if (this_size_bits > type_len - offset)
this_size_bits = type_len - offset;

@@ -1942,8 +1936,16 @@ write_pieced_value (struct value *to, struct value *from)
bits_to_skip = 8 * value_offset (to);
if (value_bitsize (to))
{
- bits_to_skip += value_bitpos (to);
+ bits_to_skip += (8 * value_offset (value_parent (to))
+ + value_bitpos (to));
type_len = value_bitsize (to);
+ /* Use the least significant bits of FROM. */
+ if (gdbarch_byte_order (get_type_arch (value_type (from)))
+ == BFD_ENDIAN_BIG)
+ {
+ offset = 8 * TYPE_LENGTH (value_type (from)) - type_len;
+ type_len += offset;
+ }
}
else
type_len = 8 * TYPE_LENGTH (value_type (to));
@@ -1962,25 +1964,19 @@ write_pieced_value (struct value *to, struct value *from)
bits_to_skip -= this_size_bits;
continue;
}
- if (bits_to_skip > 0)
- {
- dest_offset_bits = bits_to_skip;
- source_offset_bits = 0;
- this_size_bits -= bits_to_skip;
- bits_to_skip = 0;
- }
- else
- {
- dest_offset_bits = 0;
- source_offset_bits = offset;
- }
+ dest_offset_bits = bits_to_skip;
+ this_size_bits -= bits_to_skip;
+ bits_to_skip = 0;
+ source_offset_bits = offset;
+
if (this_size_bits > type_len - offset)
this_size_bits = type_len - offset;

- this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
+ this_size = (this_size_bits + dest_offset_bits % 8 + 7) / 8;
source_offset = source_offset_bits / 8;
dest_offset = dest_offset_bits / 8;
- if (dest_offset_bits % 8 == 0 && source_offset_bits % 8 == 0)
+ if (dest_offset_bits % 8 == 0 && source_offset_bits % 8 == 0
+ && this_size_bits % 8 == 0)
{
source_buffer = contents + source_offset;
need_bitwise = 0;
@@ -2049,7 +2045,7 @@ write_pieced_value (struct value *to, struct value *from)
read_memory (p->v.mem.addr + dest_offset, buffer.data (), 1);
read_memory (p->v.mem.addr + dest_offset + this_size - 1,
&buffer[this_size - 1], 1);
- copy_bitwise (buffer.data (), dest_offset_bits,
+ copy_bitwise (buffer.data (), dest_offset_bits % 8,
contents, source_offset_bits,
this_size_bits,
bits_big_endian);
diff --git a/gdb/testsuite/gdb.dwarf2/var-access.exp b/gdb/testsuite/gdb.dwarf2/var-access.exp
index 56a635a..c6abc87 100644
--- a/gdb/testsuite/gdb.dwarf2/var-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/var-access.exp
@@ -62,7 +62,7 @@ Dwarf::assemble $asm_file {
} {
declare_labels char_type_label
declare_labels int_type_label short_type_label
- declare_labels array_a8_label struct_s_label
+ declare_labels array_a8_label struct_s_label struct_t_label

char_type_label: base_type {
{name "char"}
@@ -111,6 +111,34 @@ Dwarf::assemble $asm_file {
}
}

+ struct_t_label: structure_type {
+ {name "t"}
+ {byte_size 8 DW_FORM_sdata}
+ } {
+ member {
+ {name u}
+ {type :$int_type_label}
+ }
+ member {
+ {name x}
+ {type :$int_type_label}
+ {data_member_location 4 DW_FORM_udata}
+ {bit_size 9 DW_FORM_udata}
+ }
+ member {
+ {name y}
+ {type :$int_type_label}
+ {data_bit_offset 41 DW_FORM_udata}
+ {bit_size 13 DW_FORM_udata}
+ }
+ member {
+ {name z}
+ {type :$int_type_label}
+ {data_bit_offset 54 DW_FORM_udata}
+ {bit_size 10 DW_FORM_udata}
+ }
+ }
+
DW_TAG_subprogram {
{name "main"}
{DW_AT_external 1 flag}
@@ -152,6 +180,18 @@ Dwarf::assemble $asm_file {
piece 1
} SPECIAL_expr}
}
+ # Memory pieces for bitfield access.
+ DW_TAG_variable {
+ {name "t1"}
+ {type :$struct_t_label}
+ {location {
+ piece 4
+ addr "$buf_var + 1"
+ piece 3
+ addr "$buf_var"
+ piece 1
+ } SPECIAL_expr}
+ }
}
}
}
@@ -196,3 +236,11 @@ gdb_test_no_output "set var s2 = {191, 73, 231, 123}" \
"re-initialize s2"
gdb_test "print/d s2" " = \\{a = 191, b = 73, c = 231, d = 123\\}" \
"verify re-initialized s2"
+
+# Unaligned bitfield access through byte-aligned pieces.
+gdb_test_no_output "set var t1.x = -7"
+gdb_test_no_output "set var t1.z = 340"
+gdb_test_no_output "set var t1.y = 1234"
+gdb_test "print t1" " = \\{u = <optimized out>, x = -7, y = 1234, z = 340\\}" \
+ "verify t1"
+
--
2.3.0