[Dwarf-Discuss] DWARF on systems where memory is not byte addressable

Joeri van Ruth joeri at ace.nl
Wed Jul 25 22:45:48 PDT 2012


Hello all, I am wondering about how to deal with platforms with word
memories, by which I mean that the smallest addressable unit in memory
is (in our current case) 32 bits wide.  This means that at the C level, 

	sizeof(char) == sizeof(short) == sizeof(int) == 1,

so far so good.  However, we are having problems with gdb.  I am aware
that this may be entirely gdb specific but I do note that the standard
does not spend a lot of words on the issues that arise here, that's
why I bring this up here.

The standard does not seem to define anywhere how large a byte is
supposed to be.  Historically, older architectures used anything out
of 6, 7, 8 and 9 bits bytes which is why networking standards tend to
speak of octets instead.  DWARF seems to assume 8 bit bytes, hence the
LEB128 encoding, but it does not state so explicitly unless I
overlooked something.

A C oriented view might consider that sizeof(char) == sizeof(int), and
as C does not distinguish clearly between byte and char, take a byte
to be 32 bits wide.  But even that's not always the case as sometimes
we see word oriented platforms which still take the arithmetic size of
char to be 8 bits, requiring frequent sign- and zero-extension when
assigning to a char or short variable.

However, I assume that if the DWARF standard were explicit about the
size of a byte, it would define a byte to be 8 bits.

The problem we see with gdb hinges on the DW_AT_byte_size attribute of
a type descriptor.  Gdb uses it for at least two purposes:

	- to perform address arithmetic

	- to determine the bit size of values

If we set the DW_AT_byte_size of an integer to 1, gdb will do the
address arithmetic correctly, that is, look for int_array[1] at
address int_array + 1, not + 4, but if you ask for the value of an int
variable it will only display the lower 8 bits.

If we set the DW_AT_byte_size of int to 4, which indeed sounds
more consistent given the name _byte_size, gdb will extract the full
32 bits of the value but get the address arithmetic wrong as
int_array[1] now actually accesses int_array[4].

It seems to me that the proper way would be to fix gdb to take the
addressing size unit into account as general knowledge of the target
platform, but I can't believe we're the first to come across this.  I
wonder if anyone on this list has already faced similar issues and
what they did about it.

Best regards,

Joeri van Ruth

-- 
Joeri van Ruth, ACE Associated Compiler Experts
De Ruyterkade 113, 1011 AB Amsterdam, The Netherlands.
Tel: +31 20 6646416, Fax: +31 20 6750389,
mailto:joeri at ace.nl, http://www.ace.nl
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