Re: [PATCH 2/6] gdb: use TARGET_CHAR_BIT more in dwarf2/expr.c

[Tom Tromey <tom@tromey.com>]

>>>>> "Andrew" == Andrew Burgess <aburgess@redhat.com> writes:

Andrew> if (!check_optimized)
Andrew> copy_bitwise (v_contents, offset,
Andrew> -				buffer.data (), bits_to_skip % 8,
Andrew> +				buffer.data (), bits_to_skip % TARGET_CHAR_BIT,
Andrew> this_size_bits, bits_big_endian);

>> In this situation it seems to me that gdb is manipulating host-side data.

Andrew> Aren't the value contents pulled from the inferior though?  And
Andrew> bits_to_skip and this_size_bits are all from DWARF properties that
Andrew> describe the inferior's data, so I don't understand why TARGET_CHAR_BIT
Andrew> wouldn't be correct?  I got really confused trying to reason this all
Andrew> through.

Yeah, I think it's confusing as well, and I'm not really sure if I am
correct.

But, my reasoning is that here we are dealing with host-side data.  It
is stored as 8-bit bytes in 'buffer'.

And, in this spot we are dealing with bit offsets.  To find the correct
data, we have to convert this to a byte offset in 'buffer' -- but buffer
is in host-side units and so, I think, we need a host-side conversion.

If the DWARF originally specified the offset as a byte offset, then it
was already multiplied by TARGET_CHAR_BIT when converting to a bit
offset.

Alternatively if the DWARF originally specified the offset as a bit
offset, bit offsets aren't dependent on the byte size.  So using the
host size is still correct since that will find the correct data in
'buffer'.

Andrew> Maybe the 8-bit byte assumption is just baked into GDB too much?

My feeling is yes.  If gdb ever had to target a machine with 9-bit bytes
I would quit and start a farm or learn deep-sea welding.  Something
easy.

I think there's been some attempt at targeting a machine with 16-bit
"bytes" (see the addressable_memory_unit_size gdbarch hook).  Some
multiple of 8 is maybe doable.

Andrew> I eventually just decided to drop this patch from the series.

I'm sorry about that.  I do think it's fine to do, as a marker for what
might need to change.

Tom