Re: [PATCH 7/9] Improve logic for buffer allocation in read/write_pieced_value

Simon Marchi
2017-04-14T14:51:32Z

On 2017-04-07 13:38, Andreas Arnez wrote:
> So far the main loop in read_pieced_value and write_pieced_value is
> structured like this:
>
> (1) Prepare a buffer and some variables we may need;
>
> (2) depending on the DWARF piece type to be handled, use the buffer and
> the prepared variables, ignore them, or even recalculate them.
>
> This approach reduces readability and may also lead to unnecessary
> copying
> of data. This patch moves the preparations to the places where
> sufficient
> information is available and removes some of the variables involved.
>
> gdb/ChangeLog:
>
> * dwarf2loc.c (read_pieced_value): Move the buffer allocation and
> some other preparations to the places where sufficient information
> is available.
> (write_pieced_value): Likewise.
> ---
> gdb/dwarf2loc.c | 111
> ++++++++++++++++++++++++++++----------------------------
> 1 file changed, 56 insertions(+), 55 deletions(-)
>
> diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
> index d13c0c5..67df598 100644
> --- a/gdb/dwarf2loc.c
> +++ b/gdb/dwarf2loc.c
> @@ -1794,8 +1794,7 @@ read_pieced_value (struct value *v)
> {
> struct dwarf_expr_piece *p = &c->pieces[i];
> size_t this_size, this_size_bits;
> - long dest_offset_bits, source_offset_bits, source_offset;
> - const gdb_byte *intermediate_buffer;
> + long dest_offset_bits, source_offset_bits;
>
> /* Compute size, source, and destination offsets for copying, in
> bits. */
> @@ -1813,11 +1812,6 @@ read_pieced_value (struct value *v)
> if (this_size_bits > type_len - offset)
> this_size_bits = type_len - offset;
>
> - this_size = bits_to_bytes (source_offset_bits, this_size_bits);
> - buffer.reserve (this_size);
> - source_offset = source_offset_bits / 8;
> - intermediate_buffer = buffer.data ();
> -
> /* Copy from the source to DEST_BUFFER. */
> switch (p->location)
> {
> @@ -1843,13 +1837,11 @@ read_pieced_value (struct value *v)
> this_size, buffer.data (),
> &optim, &unavail))
> {
> - /* Just so garbage doesn't ever shine through. */
> - memset (buffer.data (), 0, this_size);
> -
> if (optim)
> mark_value_bits_optimized_out (v, offset, this_size_bits);
> if (unavail)
> mark_value_bits_unavailable (v, offset, this_size_bits);
> + break;
> }
> copy_bitwise (contents, dest_offset_bits,
> buffer.data (), source_offset_bits % 8,
> @@ -1858,12 +1850,15 @@ read_pieced_value (struct value *v)
> break;
>
> case DWARF_VALUE_MEMORY:
> + this_size = bits_to_bytes (source_offset_bits, this_size_bits);
> + buffer.reserve (this_size);
> +
> read_value_memory (v, offset,
> p->v.mem.in_stack_memory,
> - p->v.mem.addr + source_offset,
> + p->v.mem.addr + source_offset_bits / 8,
> buffer.data (), this_size);
> copy_bitwise (contents, dest_offset_bits,
> - intermediate_buffer, source_offset_bits % 8,
> + buffer.data (), source_offset_bits % 8,
> this_size_bits, bits_big_endian);
> break;
>
> @@ -1889,17 +1884,18 @@ read_pieced_value (struct value *v)
>
> case DWARF_VALUE_LITERAL:
> {
> - size_t n = this_size;
> -
> - if (n > p->v.literal.length - source_offset)
> - n = (p->v.literal.length >= source_offset
> - ? p->v.literal.length - source_offset
> - : 0);
> - if (n != 0)
> - intermediate_buffer = p->v.literal.data + source_offset;
> + ULONGEST obj_size = 8 * p->v.literal.length;

I think this variable should be named "literal_size_bits". And now that
I see it, the obj_size variable in the DWARF_VALUE_STACK case (in a
previous patch) should be named "stack_element_size_bits".

> + size_t n = this_size_bits;
> +
> + /* Cut off at the end of the implicit value. */
> + if (source_offset_bits >= obj_size)
> + break;
> + if (n > obj_size - source_offset_bits)
> + n = obj_size - source_offset_bits;
> +
> copy_bitwise (contents, dest_offset_bits,
> - intermediate_buffer, source_offset_bits % 8,
> - this_size_bits, bits_big_endian);
> + p->v.literal.data, source_offset_bits,
> + n, bits_big_endian);
> }
> break;
>
> @@ -1956,9 +1952,7 @@ write_pieced_value (struct value *to, struct
> value *from)
> {
> struct dwarf_expr_piece *p = &c->pieces[i];
> size_t this_size_bits, this_size;
> - long dest_offset_bits, source_offset_bits, dest_offset,
> source_offset;
> - int need_bitwise;
> - const gdb_byte *source_buffer;
> + long dest_offset_bits, source_offset_bits;
>
> this_size_bits = p->size;
> if (bits_to_skip > 0 && bits_to_skip >= this_size_bits)
> @@ -1974,22 +1968,6 @@ write_pieced_value (struct value *to, struct
> value *from)
> if (this_size_bits > type_len - offset)
> this_size_bits = type_len - offset;
>
> - this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
> - source_offset = source_offset_bits / 8;
> - dest_offset = dest_offset_bits / 8;
> - if (dest_offset_bits % 8 == 0 && source_offset_bits % 8 == 0
> - && this_size_bits % 8 == 0)
> - {
> - source_buffer = contents + source_offset;
> - need_bitwise = 0;
> - }
> - else
> - {
> - buffer.reserve (this_size);
> - source_buffer = buffer.data ();
> - need_bitwise = 1;
> - }
> -
> switch (p->location)
> {
> case DWARF_VALUE_REGISTER:
> @@ -2040,21 +2018,44 @@ write_pieced_value (struct value *to, struct
> value *from)
> }
> break;
> case DWARF_VALUE_MEMORY:
> - if (need_bitwise)
> - {
> - /* Only the first and last bytes can possibly have any
> - bits reused. */
> - 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 % 8,
> - contents, source_offset_bits,
> - this_size_bits,
> - bits_big_endian);
> - }
> + {
> + CORE_ADDR start_addr = p->v.mem.addr + dest_offset_bits / 8;
>
> - write_memory (p->v.mem.addr + dest_offset,
> - source_buffer, this_size);
> + if (dest_offset_bits % 8 == 0 && this_size_bits % 8 == 0
> + && source_offset_bits % 8 == 0)
> + {
> + /* Everything is byte-aligned; no buffer needed. */
> + write_memory (start_addr,
> + contents + source_offset_bits / 8,
> + this_size_bits / 8);
> + break;
> + }
> +
> + this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
> + buffer.reserve (this_size);
> +
> + if (dest_offset_bits % 8 != 0 || this_size_bits % 8 != 0)
> + {
> + if (this_size <= 8)
> + {
> + /* Perform a single read for small sizes. */
> + read_memory (start_addr, buffer.data (), this_size);
> + }
> + else
> + {
> + /* Only the first and last bytes can possibly have any
> + bits reused. */
> + read_memory (start_addr, buffer.data (), 1);
> + read_memory (start_addr + this_size - 1,
> + &buffer[this_size - 1], 1);
> + }
> + }
> +
> + copy_bitwise (buffer.data (), dest_offset_bits % 8,
> + contents, source_offset_bits,
> + this_size_bits, bits_big_endian);
> + write_memory (start_addr, buffer.data (), this_size);
> + }

I believe that the complexity of read_pieced_value/write_pieced_value
could be greatly reduced but introducing functions such as:

- read_value_memory_bits
- write_memory_bits
- read_value_register_bits
- write_register_bits

It would hide a lot of nastiness behind clear interfaces. Those
functions could in turn use smaller functions that could be unit-tested.

I still like the current patch the way it is, as it takes relatively
small steps. Breaking it up in smaller functions could be done as an
additional patch to this series or a follow-up.

> break;
> default:
> mark_value_bytes_optimized_out (to, 0, TYPE_LENGTH (value_type
> (to)));

Thanks,

Simon