[Dwarf-Discuss] DW_FORM_data1 and DW_AT_const_value

David Anderson davea42 at earthlink.net
Mon Jul 26 16:38:33 PDT 2010


Michael Eager wrote:
>> There is a distinction between "ambiguous" and "unspecified".
>> The interpretation of the value of DW_FORM_data is unspecified.
>> I don't believe that there is any ambiguity.
>>      
>    

If gcc simply avoided using a DW_FORM_data<n> that was unspecified-meaning
the problem would go away.  For the value 204, DW_FORM_data2 (and
some other FORMs)  would do fine
without treading unspecified waters.

When the data<n> is an offset (as in DWARF3 data4, data8) then it 
clearly is 'unsigned'.

If a data type is 'char', and char is 8 bits,
and a data object's  DW_AT_const_value has form DW_FORM_data1
there is no ambiguity at all.

DW_TAG_subrange gives reason to think that if the basis type and the
DW_FORM_data<n> are the same size that the DW_AT_upper (and lower)
bounds would use the signedness of the basis type to 'extend' the bits
if one is expanding the value to a larger integer.

So not all uses of DW_FORM_data<n> have any value ambiguity.

But some  uses of data<n> stray into unspecified waters.

DavidA.










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