[Dwarf-Discuss] DW_FORM_data1 and DW_AT_const_value
eager at eagercon.com
Mon Jul 26 15:20:16 PDT 2010
Roland McGrath wrote:
> Here's the way I've always thought about this.
> The clearest way to get an exact target byte pattern in general is just
> to use DW_FORM_block*, which is unambiguous about both the size and the
> contents of the constant block.
> For a quantity that is 64 bits or smaller, you can use constant forms
> instead. I take any constant form to represent a bit pattern that is up
> to 64 bits wide. I take DW_FORM_data to be a bit pattern whose
> implicit high bits are all zero. I take DW_FORM_sdata to be
> sign-extended by SLEB128 rules to 64 bits. The resulting 64 bits is a
> target bit pattern to be interpreted according to the context of the
> attribute, such as the type indicated by a related DW_AT_type attribute.
> If the quantity of interest is smaller than 64 bits, then the extra high
> bits are ignored. That bit pattern of the indicated width is then
> interpreted according to the associated DW_ATE_* and the target format
> rules it indicates (i.e. an FP format, an unsigned integer, a signed integer).
> The italicized (and thus non-normative) paragraph that Tom quoted from
> the spec might suggest in a different direction, but it is quite
> ambiguous. On its face it seems to suggest that the issue of any
> implied high bits (or lack thereof) in DW_FORM_data is known to be
> fuzzy and not really specified. It's clear to everyone what the meaning
> is for DW_FORM_[us]data, and it "strongly encourages" using only them.
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.
> I think it's wise (and is manifest existing practice) to use
> DW_FORM_data freely and be clear that they imply zero-extension.
That is not the interpretation described in the DWARF standard.
> The only reason we have a variety of forms to encode each class is to
> offer compact options for values with few significant bits. Using
> data can save a byte of encoding space for various ranges of values
> vs always using LEB128.
Michael Eager eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077
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