[Dwarf-Discuss] DW_FORM_data1 and DW_AT_const_value

Tom Tromey tromey at redhat.com
Mon Jul 26 14:05:08 PDT 2010


I got the following DWARF from gcc svn:

 <2><8e2>: Abbrev Number: 5 (DW_TAG_variable)
    <8e3>   DW_AT_name        : (indirect string, offset: 0x167): funclocal_ro	
    <8e7>   DW_AT_decl_file   : 1	
    <8e8>   DW_AT_decl_line   : 15	
    <8e9>   DW_AT_type        : <0x901>	
    <8ed>   DW_AT_const_value : 203	
[...]
 <1><8fa>: Abbrev Number: 8 (DW_TAG_base_type)
    <8fb>   DW_AT_byte_size   : 4	
    <8fc>   DW_AT_encoding    : 5	(signed)
    <8fd>   DW_AT_name        : int	
 <1><901>: Abbrev Number: 9 (DW_TAG_const_type)
    <902>   DW_AT_type        : <0x8fa>	


This dump doesn't say so, but the DW_AT_const_value is represented as
<DW_FORM_data1 203>.

This inspired some local discussion, and we thought it couldn't hurt to
ask a wider audience.  The question is whether a reader should
sign-extend that "203".

GDB currently does sign extension here, because the type of the variable
is signed.  However, at least Roland and Jakub are of the opinion that
DW_FORM_data1 should not be sign extended in this situation, I believe
on the basis that DW_FORM_data* should just supply bits.

The DWARF spec is reasonably unclear:

    The data in DW_FORM_data1, DW_FORM_data2, DW_FORM_data4 and
    DW_FORM_data8 can be anything. Depending on context, it may be a signed
    integer, an unsigned integer, a floating-point constant, or anything
    else. A consumer must use context to know how to interpret the bits,
    which if they are target machine data (such as an integer or floating
    point constant) will be in target machine byte-order.

    If one of the DW_FORM_data<n> forms is used to represent a signed or
    unsigned integer, it can be hard for a consumer to discover the context
    necessary to determine which interpretation is intended. Producers are
    therefore strongly encouraged to use DW_FORM_sdata or DW_FORM_udata for
    signed and unsigned integers respectively, rather than DW_FORM_data<n>.

Nothing here really indicates what contextual information should be
considered.

The DW_AT_const_value docs may support the no-extension interpretation,
based on the use of the word "value":

    The value is the actual constant value of the variable, represented as
    it would be on the target architecture.

It appears that GCC has worked this way for quite a while, so my
inclination is to change GDB to match.  However, at the very least it
would be good to see a clarification in the spec.

Tom



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