Re: [PATCH 8/9] Respect piece offset for DW_OP_bit_piece

Simon Marchi
2017-04-14T15:07:33Z

On 2017-04-07 13:38, Andreas Arnez wrote:
> So far GDB ignores the piece offset of all kinds of DWARF bit
> pieces (DW_OP_bit_piece) and treats such pieces as if the offset was
> zero.
>
> This is fixed, and an appropriate test is added.
>
> gdb/ChangeLog:
>
> * dwarf2loc.c (read_pieced_value): Respect the piece offset, as
> given by DW_OP_bit_piece.
> (write_pieced_value): Likewise.
>
> Andreas Arnez <arnez@linux.vnet.ibm.com>
>
> * gdb.dwarf2/var-access.exp: Add test for composite location with
> nonzero piece offsets.
> ---
> gdb/dwarf2loc.c | 22 ++++++++++++++------
> gdb/testsuite/gdb.dwarf2/var-access.exp | 37
> +++++++++++++++++++++++++++++++++
> 2 files changed, 53 insertions(+), 6 deletions(-)
>
> diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
> index 67df598..045c2d3 100644
> --- a/gdb/dwarf2loc.c
> +++ b/gdb/dwarf2loc.c
> @@ -1824,11 +1824,13 @@ read_pieced_value (struct value *v)
> int optim, unavail;
>
> if (gdbarch_byte_order (arch) == BFD_ENDIAN_BIG
> - && p->size < reg_bits)
> + && p->offset + p->size < reg_bits)
> {
> /* Big-endian, and we want less than full size. */
> - source_offset_bits += reg_bits - p->size;
> + source_offset_bits += reg_bits - (p->offset + p->size);
> }
> + else
> + source_offset_bits += p->offset;
> this_size = bits_to_bytes (source_offset_bits, this_size_bits);
> buffer.reserve (this_size);
>
> @@ -1850,6 +1852,7 @@ read_pieced_value (struct value *v)
> break;
>
> case DWARF_VALUE_MEMORY:
> + source_offset_bits += p->offset;
> this_size = bits_to_bytes (source_offset_bits, this_size_bits);
> buffer.reserve (this_size);
>
> @@ -1869,12 +1872,14 @@ read_pieced_value (struct value *v)
> ULONGEST obj_size = 8 * TYPE_LENGTH (value_type (p->v.value));
>
> /* Use zeroes if piece reaches beyond stack value. */
> - if (p->size > obj_size)
> + if (p->offset + p->size > obj_size)
> break;
>
> /* Piece is anchored at least significant bit end. */
> if (gdbarch_byte_order (objfile_gdbarch) == BFD_ENDIAN_BIG)
> - source_offset_bits += obj_size - p->size;
> + source_offset_bits += obj_size - (p->offset + p->size);
> + else
> + source_offset_bits += p->offset;
> copy_bitwise (contents, dest_offset_bits,
> value_contents_all (p->v.value),
> source_offset_bits,
> @@ -1888,6 +1893,7 @@ read_pieced_value (struct value *v)
> size_t n = this_size_bits;
>
> /* Cut off at the end of the implicit value. */
> + source_offset_bits += p->offset;
> if (source_offset_bits >= obj_size)
> break;
> if (n > obj_size - source_offset_bits)
> @@ -1978,11 +1984,13 @@ write_pieced_value (struct value *to, struct
> value *from)
> ULONGEST reg_bits = 8 * register_size (arch, gdb_regnum);
>
> if (gdbarch_byte_order (arch) == BFD_ENDIAN_BIG
> - && p->size <= reg_bits)
> + && p->offset + p->size < reg_bits)
> {
> /* Big-endian, and we want less than full size. */
> - dest_offset_bits += reg_bits - p->size;
> + dest_offset_bits += reg_bits - (p->offset + p->size);
> }
> + else
> + dest_offset_bits += p->offset;
> this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
> buffer.reserve (this_size);
>
> @@ -2019,6 +2027,8 @@ write_pieced_value (struct value *to, struct
> value *from)
> break;
> case DWARF_VALUE_MEMORY:
> {
> + dest_offset_bits += p->offset;
> +
> CORE_ADDR start_addr = p->v.mem.addr + dest_offset_bits / 8;
>
> if (dest_offset_bits % 8 == 0 && this_size_bits % 8 == 0
> diff --git a/gdb/testsuite/gdb.dwarf2/var-access.exp
> b/gdb/testsuite/gdb.dwarf2/var-access.exp
> index 4787dfb..29d61f8 100644
> --- a/gdb/testsuite/gdb.dwarf2/var-access.exp
> +++ b/gdb/testsuite/gdb.dwarf2/var-access.exp
> @@ -204,6 +204,22 @@ Dwarf::assemble $asm_file {
> piece 1
> } SPECIAL_expr}
> }
> + # One piece per bitfield. Use piece offsets.
> + DW_TAG_variable {
> + {name "t3"}
> + {type :$struct_t_label}
> + {location {
> + implicit_value 0x12 0x34 0x56 0x78 0x9a
> + bit_piece 32 4
> + const2s -280
> + stack_value
> + bit_piece 9 2
> + regx [lindex $dwarf_regnum 0]
> + bit_piece 13 14
> + addr $buf_var
> + bit_piece 10 42
> + } SPECIAL_expr}
> + }
> }
> }
> }
> @@ -276,3 +292,24 @@ gdb_test_no_output "set var t2.y = 2641"
> gdb_test_no_output "set var t2.z = -400"
> gdb_test_no_output "set var t2.x = 200"
> gdb_test "print t2.x + t2.y + t2.z" " = 2441"
> +
> +# Bitfield access through pieces with nonzero piece offsets.
> +gdb_test_no_output "set var \$[lindex $regname 0] = 0xa8000" \
> + "init reg for t3.y"
> +gdb_test_no_output "set var *(char \[2\] *) (a + 5) = { 70, 82 }" \
> + "init mem for t3.z"
> +switch $endian {
> + little {set val "u = -1484430527, x = -70, y = 42, z = 145"}
> + big {set val "u = 591751049, x = -70, y = 42, z = 101"}
> +}

Just a nit, I think this (at least the part about u) would be slightly
easier to understand if the value was printed in hex, like so (for
little endian):

u = 0xa7856341, x = 0xffffffba, y = 0x2a, z = 0x91

since we can recognize the nibbles from the implicit_value up there.

> +gdb_test "print t3" " = \\{$val\\}" \
> + "initialized t3 from reg and mem"
> +gdb_test_no_output "set var t3.y = -1" \
> + "overwrite t3.y"
> +gdb_test "print/x \$[lindex $regname 0]" " = 0x7ffc000" \
> + "verify t3.y through reg"
> +gdb_test_no_output "set var t3.z = -614" \
> + "overwrite t3.z"
> +switch $endian {big {set val "0x59, 0xa2"} little {set val "0x6a,
> 0x56"}}
> +gdb_test "print/x *(char \[2\] *) (a + 5)" " = \\{$val\\}" \
> + "verify t3.z through mem"