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Notes on the GNU Implementation of DWARF Debugging Information -------------------------------------------------------------- Last Updated: Sun Oct 4 10:04:13 PDT 1992 by rfg@netcom.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 UNIX International. (See below.) The file you are looking at now only describes known deviations from the UI/PLSIG 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 specification from UNIX International, use the following procedure: --------------------------------------------------------------------------- Send mail to archive@ui.org containing the following: path yourname@your.site send PUBLIC/dwarf.v1.mm for the troff source, or send PUBLIC/dwarf.v1.ps for the postscript. If you system supports uncompress and uudecode, you can request that the data be compressed by placing the command 'compress' in the message. If you have any questions about the archive service, please contact Shane P. McCarron, UI Project Manager, <s.mccarron@ui.org>. --------------------------------------------------------------------------- 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 from the evolving DWARF version 2 (draft) specification have been adapted to, and used in the GNU implementation of DWARF (version 1). In most of these cases, a DWARF version 2 (draft) 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 (draft) 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). Full support for DWARF version 2 should be available sometime after the DWARF version 2 specification has been finalized. 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. If you are interested in obtaining more information about DWARF or in participating in the continuing evolution of DWARF within the UI/PLSIG group, please contact either myself or the UI/PLSIG chairman, Dan Oldman <oldman@dg-rtp.dg.com>. The UI/PLSIG welcomes and encourages the participation of new members who might be interested in discussing debugging issues in general, and DWARF in particular. There are no dues and you DO NOT have to be a UI member in order to join the UI/PLSIG. The UI/PLSIG operates an E-mail mailing list and holds regular meeting in various cities. If you don't have time to participate actively, but would like to be kept abreast of recent developments, you con join the UI/PLSIG mailing list and just listen in on our lively discussions. ** 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 "conformant" 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 severly 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.