--- /dev/null
+Notes on the GNU Implementation of DWARF Debugging Information
+--------------------------------------------------------------
+Last Updated: Sun Jul 17 08:17:42 PDT 1994 by rfg@segfault.us.com
+------------------------------------------------------------
+
+This file describes special and unique aspects of the GNU implementation
+of the DWARF debugging information language, as provided in the GNU version
+2.x compiler(s).
+
+For general information about the DWARF debugging information language,
+you should obtain the DWARF version 1 specification document (and perhaps
+also the DWARF version 2 draft specification document) developed by the
+UNIX International Programming Languages Special Interest Group. A copy
+of the the DWARF version 1 specification (in PostScript form) may be
+obtained either from me <rfg@netcom.com> or from the main Data General
+FTP server. (See below.) The file you are looking at now only describes
+known deviations from the DWARF version 1 specification, together with
+those things which are allowed by the DWARF version 1 specification but
+which are known to cause interoperability problems (e.g. with SVR4 SDB).
+
+To obtain a copy of the DWARF Version 1 and/or DWARF Version 2 specification
+from Data General's FTP server, use the following procedure:
+
+---------------------------------------------------------------------------
+ ftp to machine: "dg-rtp.dg.com" (128.222.1.2).
+
+ Log in as "ftp".
+ cd to "plsig"
+ get any of the following file you are interested in:
+
+ dwarf.1.0.3.ps
+ dwarf.2.0.0.index.ps
+ dwarf.2.0.0.ps
+---------------------------------------------------------------------------
+
+The generation of DWARF debugging information by the GNU version 2.x C
+compiler has now been tested rather extensively for m88k, i386, i860, and
+Sparc targets. The DWARF output of the GNU C compiler appears to inter-
+operate well with the standard SVR4 SDB debugger on these kinds of target
+systems (but of course, there are no guarantees).
+
+DWARF generation for the GNU g++ compiler is still not operable. This is
+due primarily to the many remaining cases where the g++ front end does not
+conform to the conventions used in the GNU C front end for representing
+various kinds of declarations in the TREE data structure. It is not clear
+at this time how these problems will be addressed.
+
+Future plans for the dwarfout.c module of the GNU compiler(s) includes the
+addition of full support for GNU FORTRAN. (This should, in theory, be a
+lot simpler to add than adding support for g++... but we'll see.)
+
+Many features of the DWARF version 2 specification have been adapted to
+(and used in) the GNU implementation of DWARF (version 1). In most of
+these cases, a DWARF version 2 approach is used in place of (or in addition
+to) DWARF version 1 stuff simply because it is apparent that DWARF version
+1 is not sufficiently expressive to provide the kinds of information which
+may be necessary to support really robust debugging. In all of these cases
+however, the use of DWARF version 2 features should not interfere in any
+way with the interoperability (of GNU compilers) with generally available
+"classic" (pre version 1) DWARF consumer tools (e.g. SVR4 SDB).
+
+The DWARF generation enhancement for the GNU compiler(s) was initially
+donated to the Free Software Foundation by Network Computing Devices.
+(Thanks NCD!) Additional development and maintenance of dwarfout.c has
+been largely supported (i.e. funded) by Intel Corporation. (Thanks Intel!)
+
+If you have questions or comments about the DWARF generation feature, please
+send mail to me <rfg@netcom.com>. I will be happy to investigate any bugs
+reported and I may even provide fixes (but of course, I can make no promises).
+
+The DWARF debugging information produced by GCC may deviate in a few minor
+(but perhaps significant) respects from the DWARF debugging information
+currently produced by other C compilers. A serious attempt has been made
+however to conform to the published specifications, to existing practice,
+and to generally accepted norms in the GNU implementation of DWARF.
+
+ ** IMPORTANT NOTE ** ** IMPORTANT NOTE ** ** IMPORTANT NOTE **
+
+Under normal circumstances, the DWARF information generated by the GNU
+compilers (in an assembly language file) is essentially impossible for
+a human being to read. This fact can make it very difficult to debug
+certain DWARF-related problems. In order to overcome this difficulty,
+a feature has been added to dwarfout.c (enabled by the -fverbose-asm
+option) which causes additional comments to be placed into the assembly
+language output file, out to the right-hand side of most bits of DWARF
+material. The comments indicate (far more clearly that the obscure
+DWARF hex codes do) what is actually being encoded in DWARF. Thus, the
+-fverbose-asm option can be highly useful for those who must study the
+DWARF output from the GNU compilers in detail.
+
+---------
+
+(Footnote: Within this file, the term `Debugging Information Entry' will
+be abbreviated as `DIE'.)
+
+
+Release Notes (aka known bugs)
+-------------------------------
+
+In one very obscure case involving dynamically sized arrays, the DWARF
+"location information" for such an array may make it appear that the
+array has been totally optimized out of existence, when in fact it
+*must* actually exist. (This only happens when you are using *both* -g
+*and* -O.) This is due to aggressive dead store elimination in the
+compiler, and to the fact that the DECL_RTL expressions associated with
+variables are not always updated to correctly reflect the effects of
+GCC's aggressive dead store elimination.
+
+-------------------------------
+
+When attempting to set a breakpoint at the "start" of a function compiled
+with -g1, the debugger currently has no way of knowing exactly where the
+end of the prologue code for the function is. Thus, for most targets,
+all the debugger can do is to set the breakpoint at the AT_low_pc address
+for the function. But if you stop there and then try to look at one or
+more of the formal parameter values, they may not have been "homed" yet,
+so you may get inaccurate answers (or perhaps even addressing errors).
+
+Some people may consider this simply a non-feature, but I consider it a
+bug, and I hope to provide some some GNU-specific attributes (on function
+DIEs) which will specify the address of the end of the prologue and the
+address of the beginning of the epilogue in a future release.
+
+-------------------------------
+
+It is believed at this time that old bugs relating to the AT_bit_offset
+values for bit-fields have been fixed.
+
+There may still be some very obscure bugs relating to the DWARF description
+of type `long long' bit-fields for target machines (e.g. 80x86 machines)
+where the alignment of type `long long' data objects is different from
+(and less than) the size of a type `long long' data object.
+
+Please report any problems with the DWARF description of bit-fields as you
+would any other GCC bug. (Procedures for bug reporting are given in the
+GNU C compiler manual.)
+
+--------------------------------
+
+At this time, GCC does not know how to handle the GNU C "nested functions"
+extension. (See the GCC manual for more info on this extension to ANSI C.)
+
+--------------------------------
+
+The GNU compilers now represent inline functions (and inlined instances
+thereof) in exactly the manner described by the current DWARF version 2
+(draft) specification. The version 1 specification for handling inline
+functions (and inlined instances) was known to be brain-damaged (by the
+PLSIG) when the version 1 spec was finalized, but it was simply too late
+in the cycle to get it removed before the version 1 spec was formally
+released to the public (by UI).
+
+--------------------------------
+
+At this time, GCC does not generate the kind of really precise information
+about the exact declared types of entities with signed integral types which
+is required by the current DWARF draft specification.
+
+Specifically, the current DWARF draft specification seems to require that
+the type of an non-unsigned integral bit-field member of a struct or union
+type be represented as either a "signed" type or as a "plain" type,
+depending upon the the exact set of keywords that were used in the
+type specification for the given bit-field member. It was felt (by the
+UI/PLSIG) that this distinction between "plain" and "signed" integral types
+could have some significance (in the case of bit-fields) because ANSI C
+does not constrain the signedness of a plain bit-field, whereas it does
+constrain the signedness of an explicitly "signed" bit-field. For this
+reason, the current DWARF specification calls for compilers to produce
+type information (for *all* integral typed entities... not just bit-fields)
+which explicitly indicates the signedness of the relevant type to be
+"signed" or "plain" or "unsigned".
+
+Unfortunately, the GNU DWARF implementation is currently incapable of making
+such distinctions.
+
+--------------------------------
+
+
+Known Interoperability Problems
+-------------------------------
+
+Although the GNU implementation of DWARF conforms (for the most part) with
+the current UI/PLSIG DWARF version 1 specification (with many compatible
+version 2 features added in as "vendor specific extensions" just for good
+measure) there are a few known cases where GCC's DWARF output can cause
+some confusion for "classic" (pre version 1) DWARF consumers such as the
+System V Release 4 SDB debugger. These cases are described in this section.
+
+--------------------------------
+
+The DWARF version 1 specification includes the fundamental type codes
+FT_ext_prec_float, FT_complex, FT_dbl_prec_complex, and FT_ext_prec_complex.
+Since GNU C is only a C compiler (and since C doesn't provide any "complex"
+data types) the only one of these fundamental type codes which GCC ever
+generates is FT_ext_prec_float. This fundamental type code is generated
+by GCC for the `long double' data type. Unfortunately, due to an apparent
+bug in the SVR4 SDB debugger, SDB can become very confused wherever any
+attempt is made to print a variable, parameter, or field whose type was
+given in terms of FT_ext_prec_float.
+
+(Actually, SVR4 SDB fails to understand *any* of the four fundamental type
+codes mentioned here. This will fact will cause additional problems when
+there is a GNU FORTRAN front-end.)
+
+--------------------------------
+
+In general, it appears that SVR4 SDB is not able to effectively ignore
+fundamental type codes in the "implementation defined" range. This can
+cause problems when a program being debugged uses the `long long' data
+type (or the signed or unsigned varieties thereof) because these types
+are not defined by ANSI C, and thus, GCC must use its own private fundamental
+type codes (from the implementation-defined range) to represent these types.
+
+--------------------------------
+
+
+General GNU DWARF extensions
+----------------------------
+
+In the current DWARF version 1 specification, no mechanism is specified by
+which accurate information about executable code from include files can be
+properly (and fully) described. (The DWARF version 2 specification *does*
+specify such a mechanism, but it is about 10 times more complicated than
+it needs to be so I'm not terribly anxious to try to implement it right
+away.)
+
+In the GNU implementation of DWARF version 1, a fully downward-compatible
+extension has been implemented which permits the GNU compilers to specify
+which executable lines come from which files. This extension places
+additional information (about source file names) in GNU-specific sections
+(which should be totally ignored by all non-GNU DWARF consumers) so that
+this extended information can be provided (to GNU DWARF consumers) in a way
+which is totally transparent (and invisible) to non-GNU DWARF consumers
+(e.g. the SVR4 SDB debugger). The additional information is placed *only*
+in specialized GNU-specific sections, where it should never even be seen
+by non-GNU DWARF consumers.
+
+To understand this GNU DWARF extension, imagine that the sequence of entries
+in the .lines section is broken up into several subsections. Each contiguous
+sequence of .line entries which relates to a sequence of lines (or statements)
+from one particular file (either a `base' file or an `include' file) could
+be called a `line entries chunk' (LEC).
+
+For each LEC there is one entry in the .debug_srcinfo section.
+
+Each normal entry in the .debug_srcinfo section consists of two 4-byte
+words of data as follows:
+
+ (1) The starting address (relative to the entire .line section)
+ of the first .line entry in the relevant LEC.
+
+ (2) The starting address (relative to the entire .debug_sfnames
+ section) of a NUL terminated string representing the
+ relevant filename. (This filename name be either a
+ relative or an absolute filename, depending upon how the
+ given source file was located during compilation.)
+
+Obviously, each .debug_srcinfo entry allows you to find the relevant filename,
+and it also points you to the first .line entry that was generated as a result
+of having compiled a given source line from the given source file.
+
+Each subsequent .line entry should also be assumed to have been produced
+as a result of compiling yet more lines from the same file. The end of
+any given LEC is easily found by looking at the first 4-byte pointer in
+the *next* .debug_srcinfo entry. That next .debug_srcinfo entry points
+to a new and different LEC, so the preceding LEC (implicitly) must have
+ended with the last .line section entry which occurs at the 2 1/2 words
+just before the address given in the first pointer of the new .debug_srcinfo
+entry.
+
+The following picture may help to clarify this feature. Let's assume that
+`LE' stands for `.line entry'. Also, assume that `* 'stands for a pointer.
+
+
+ .line section .debug_srcinfo section .debug_sfnames section
+ ----------------------------------------------------------------
+
+ LE <---------------------- *
+ LE * -----------------> "foobar.c" <---
+ LE |
+ LE |
+ LE <---------------------- * |
+ LE * -----------------> "foobar.h" <| |
+ LE | |
+ LE | |
+ LE <---------------------- * | |
+ LE * -----------------> "inner.h" | |
+ LE | |
+ LE <---------------------- * | |
+ LE * ------------------------------- |
+ LE |
+ LE |
+ LE |
+ LE |
+ LE <---------------------- * |
+ LE * -----------------------------------
+ LE
+ LE
+ LE
+
+In effect, each entry in the .debug_srcinfo section points to *both* a
+filename (in the .debug_sfnames section) and to the start of a block of
+consecutive LEs (in the .line section).
+
+Note that just like in the .line section, there are specialized first and
+last entries in the .debug_srcinfo section for each object file. These
+special first and last entries for the .debug_srcinfo section are very
+different from the normal .debug_srcinfo section entries. They provide
+additional information which may be helpful to a debugger when it is
+interpreting the data in the .debug_srcinfo, .debug_sfnames, and .line
+sections.
+
+The first entry in the .debug_srcinfo section for each compilation unit
+consists of five 4-byte words of data. The contents of these five words
+should be interpreted (by debuggers) as follows:
+
+ (1) The starting address (relative to the entire .line section)
+ of the .line section for this compilation unit.
+
+ (2) The starting address (relative to the entire .debug_sfnames
+ section) of the .debug_sfnames section for this compilation
+ unit.
+
+ (3) The starting address (in the execution virtual address space)
+ of the .text section for this compilation unit.
+
+ (4) The ending address plus one (in the execution virtual address
+ space) of the .text section for this compilation unit.
+
+ (5) The date/time (in seconds since midnight 1/1/70) at which the
+ compilation of this compilation unit occurred. This value
+ should be interpreted as an unsigned quantity because gcc
+ might be configured to generate a default value of 0xffffffff
+ in this field (in cases where it is desired to have object
+ files created at different times from identical source files
+ be byte-for-byte identical). By default, these timestamps
+ are *not* generated by dwarfout.c (so that object files
+ compiled at different times will be byte-for-byte identical).
+ If you wish to enable this "timestamp" feature however, you
+ can simply place a #define for the symbol `DWARF_TIMESTAMPS'
+ in your target configuration file and then rebuild the GNU
+ compiler(s).
+
+Note that the first string placed into the .debug_sfnames section for each
+compilation unit is the name of the directory in which compilation occurred.
+This string ends with a `/' (to help indicate that it is the pathname of a
+directory). Thus, the second word of each specialized initial .debug_srcinfo
+entry for each compilation unit may be used as a pointer to the (string)
+name of the compilation directory, and that string may in turn be used to
+"absolutize" any relative pathnames which may appear later on in the
+.debug_sfnames section entries for the same compilation unit.
+
+The fifth and last word of each specialized starting entry for a compilation
+unit in the .debug_srcinfo section may (depending upon your configuration)
+indicate the date/time of compilation, and this may be used (by a debugger)
+to determine if any of the source files which contributed code to this
+compilation unit are newer than the object code for the compilation unit
+itself. If so, the debugger may wish to print an "out-of-date" warning
+about the compilation unit.
+
+The .debug_srcinfo section associated with each compilation will also have
+a specialized terminating entry. This terminating .debug_srcinfo section
+entry will consist of the following two 4-byte words of data:
+
+ (1) The offset, measured from the start of the .line section to
+ the beginning of the terminating entry for the .line section.
+
+ (2) A word containing the value 0xffffffff.
+
+--------------------------------
+
+In the current DWARF version 1 specification, no mechanism is specified by
+which information about macro definitions and un-definitions may be provided
+to the DWARF consumer.
+
+The DWARF version 2 (draft) specification does specify such a mechanism.
+That specification was based on the GNU ("vendor specific extension")
+which provided some support for macro definitions and un-definitions,
+but the "official" DWARF version 2 (draft) specification mechanism for
+handling macros and the GNU implementation have diverged somewhat. I
+plan to update the GNU implementation to conform to the "official"
+DWARF version 2 (draft) specification as soon as I get time to do that.
+
+Note that in the GNU implementation, additional information about macro
+definitions and un-definitions is *only* provided when the -g3 level of
+debug-info production is selected. (The default level is -g2 and the
+plain old -g option is considered to be identical to -g2.)
+
+GCC records information about macro definitions and undefinitions primarily
+in a section called the .debug_macinfo section. Normal entries in the
+.debug_macinfo section consist of the following three parts:
+
+ (1) A special "type" byte.
+
+ (2) A 3-byte line-number/filename-offset field.
+
+ (3) A NUL terminated string.
+
+The interpretation of the second and third parts is dependent upon the
+value of the leading (type) byte.
+
+The type byte may have one of four values depending upon the type of the
+.debug_macinfo entry which follows. The 1-byte MACINFO type codes presently
+used, and their meanings are as follows:
+
+ MACINFO_start A base file or an include file starts here.
+ MACINFO_resume The current base or include file ends here.
+ MACINFO_define A #define directive occurs here.
+ MACINFO_undef A #undef directive occur here.
+
+(Note that the MACINFO_... codes mentioned here are simply symbolic names
+for constants which are defined in the GNU dwarf.h file.)
+
+For MACINFO_define and MACINFO_undef entries, the second (3-byte) field
+contains the number of the source line (relative to the start of the current
+base source file or the current include files) when the #define or #undef
+directive appears. For a MACINFO_define entry, the following string field
+contains the name of the macro which is defined, followed by its definition.
+Note that the definition is always separated from the name of the macro
+by at least one whitespace character. For a MACINFO_undef entry, the
+string which follows the 3-byte line number field contains just the name
+of the macro which is being undef'ed.
+
+For a MACINFO_start entry, the 3-byte field following the type byte contains
+the offset, relative to the start of the .debug_sfnames section for the
+current compilation unit, of a string which names the new source file which
+is beginning its inclusion at this point. Following that 3-byte field,
+each MACINFO_start entry always contains a zero length NUL terminated
+string.
+
+For a MACINFO_resume entry, the 3-byte field following the type byte contains
+the line number WITHIN THE INCLUDING FILE at which the inclusion of the
+current file (whose inclusion ends here) was initiated. Following that
+3-byte field, each MACINFO_resume entry always contains a zero length NUL
+terminated string.
+
+Each set of .debug_macinfo entries for each compilation unit is terminated
+by a special .debug_macinfo entry consisting of a 4-byte zero value followed
+by a single NUL byte.
+
+--------------------------------
+
+In the current DWARF draft specification, no provision is made for providing
+a separate level of (limited) debugging information necessary to support
+tracebacks (only) through fully-debugged code (e.g. code in system libraries).
+
+A proposal to define such a level was submitted (by me) to the UI/PLSIG.
+This proposal was rejected by the UI/PLSIG for inclusion into the DWARF
+version 1 specification for two reasons. First, it was felt (by the PLSIG)
+that the issues involved in supporting a "traceback only" subset of DWARF
+were not well understood. Second, and perhaps more importantly, the PLSIG
+is already having enough trouble agreeing on what it means to be "conforming"
+to the DWARF specification, and it was felt that trying to specify multiple
+different *levels* of conformance would only complicate our discussions of
+this already divisive issue. Nonetheless, the GNU implementation of DWARF
+provides an abbreviated "traceback only" level of debug-info production for
+use with fully-debugged "system library" code. This level should only be
+used for fully debugged system library code, and even then, it should only
+be used where there is a very strong need to conserve disk space. This
+abbreviated level of debug-info production can be used by specifying the
+-g1 option on the compilation command line.
+
+--------------------------------
+
+As mentioned above, the GNU implementation of DWARF currently uses the DWARF
+version 2 (draft) approach for inline functions (and inlined instances
+thereof). This is used in preference to the version 1 approach because
+(quite simply) the version 1 approach is highly brain-damaged and probably
+unworkable.
+
+--------------------------------
+
+
+GNU DWARF Representation of GNU C Extensions to ANSI C
+------------------------------------------------------
+
+The file dwarfout.c has been designed and implemented so as to provide
+some reasonable DWARF representation for each and every declarative
+construct which is accepted by the GNU C compiler. Since the GNU C
+compiler accepts a superset of ANSI C, this means that there are some
+cases in which the DWARF information produced by GCC must take some
+liberties in improvising DWARF representations for declarations which
+are only valid in (extended) GNU C.
+
+In particular, GNU C provides at least three significant extensions to
+ANSI C when it comes to declarations. These are (1) inline functions,
+and (2) dynamic arrays, and (3) incomplete enum types. (See the GCC
+manual for more information on these GNU extensions to ANSI C.) When
+used, these GNU C extensions are represented (in the generated DWARF
+output of GCC) in the most natural and intuitively obvious ways.
+
+In the case of inline functions, the DWARF representation is exactly as
+called for in the DWARF version 2 (draft) specification for an identical
+function written in C++; i.e. we "reuse" the representation of inline
+functions which has been defined for C++ to support this GNU C extension.
+
+In the case of dynamic arrays, we use the most obvious representational
+mechanism available; i.e. an array type in which the upper bound of
+some dimension (usually the first and only dimension) is a variable
+rather than a constant. (See the DWARF version 1 specification for more
+details.)
+
+In the case of incomplete enum types, such types are represented simply
+as TAG_enumeration_type DIEs which DO NOT contain either AT_byte_size
+attributes or AT_element_list attributes.
+
+--------------------------------
+
+
+Future Directions
+-----------------
+
+The codes, formats, and other paraphernalia necessary to provide proper
+support for symbolic debugging for the C++ language are still being worked
+on by the UI/PLSIG. The vast majority of the additions to DWARF which will
+be needed to completely support C++ have already been hashed out and agreed
+upon, but a few small issues (e.g. anonymous unions, access declarations)
+are still being discussed. Also, we in the PLSIG are still discussing
+whether or not we need to do anything special for C++ templates. (At this
+time it is not yet clear whether we even need to do anything special for
+these.)
+
+Unfortunately, as mentioned above, there are quite a few problems in the
+g++ front end itself, and these are currently responsible for severely
+restricting the progress which can be made on adding DWARF support
+specifically for the g++ front-end. Furthermore, Richard Stallman has
+expressed the view that C++ friendships might not be important enough to
+describe (in DWARF). This view directly conflicts with both the DWARF
+version 1 and version 2 (draft) specifications, so until this small
+misunderstanding is cleared up, DWARF support for g++ is unlikely.
+
+With regard to FORTRAN, the UI/PLSIG has defined what is believed to be a
+complete and sufficient set of codes and rules for adequately representing
+all of FORTRAN 77, and most of Fortran 90 in DWARF. While some support for
+this has been implemented in dwarfout.c, further implementation and testing
+will have to await the arrival of the GNU Fortran front-end (which is
+currently in early alpha test as of this writing).
+
+GNU DWARF support for other languages (i.e. Pascal and Modula) is a moot
+issue until there are GNU front-ends for these other languages.
+
+GNU DWARF support for DWARF version 2 will probably not be attempted until
+such time as the version 2 specification is finalized. (More work needs
+to be done on the version 2 specification to make the new "abbreviations"
+feature of version 2 more easily implementable. Until then, it will be
+a royal pain the ass to implement version 2 "abbreviations".) For the
+time being, version 2 features will be added (in a version 1 compatible
+manner) when and where these features seem necessary or extremely desirable.
+
+As currently defined, DWARF only describes a (binary) language which can
+be used to communicate symbolic debugging information from a compiler
+through an assembler and a linker, to a debugger. There is no clear
+specification of what processing should be (or must be) done by the
+assembler and/or the linker. Fortunately, the role of the assembler
+is easily inferred (by anyone knowledgeable about assemblers) just by
+looking at examples of assembly-level DWARF code. Sadly though, the
+allowable (or required) processing steps performed by a linker are
+harder to infer and (perhaps) even harder to agree upon. There are
+several forms of very useful `post-processing' steps which intelligent
+linkers *could* (in theory) perform on object files containing DWARF,
+but any and all such link-time transformations are currently both disallowed
+and unspecified.
+
+In particular, possible link-time transformations of DWARF code which could
+provide significant benefits include (but are not limited to):
+
+ Commonization of duplicate DIEs obtained from multiple input
+ (object) files.
+
+ Cross-compilation type checking based upon DWARF type information
+ for objects and functions.
+
+ Other possible `compacting' transformations designed to save disk
+ space and to reduce linker & debugger I/O activity.
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