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

[Simon Marchi <simon.marchi@polymtl.ca>]

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"