d4691e641b
non-i386, non-unix, and generatable files have been trimmed, but can easily be added in later if needed. gcc-2.7.2.1 will follow shortly, it's a very small delta to this and it's handy to have both available for reference for such little cost. The freebsd-specific changes will then be committed, and once the dust has settled, the bmakefiles will be committed to use this code.
575 lines
28 KiB
Plaintext
575 lines
28 KiB
Plaintext
Notes on the GNU Implementation of DWARF Debugging Information
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--------------------------------------------------------------
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Last Updated: Sun Jul 17 08:17:42 PDT 1994 by rfg@segfault.us.com
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------------------------------------------------------------
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This file describes special and unique aspects of the GNU implementation
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of the DWARF debugging information language, as provided in the GNU version
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2.x compiler(s).
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For general information about the DWARF debugging information language,
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you should obtain the DWARF version 1 specification document (and perhaps
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also the DWARF version 2 draft specification document) developed by the
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UNIX International Programming Languages Special Interest Group. A copy
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of the the DWARF version 1 specification (in PostScript form) may be
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obtained either from me <rfg@netcom.com> or from the main Data General
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FTP server. (See below.) The file you are looking at now only describes
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known deviations from the DWARF version 1 specification, together with
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those things which are allowed by the DWARF version 1 specification but
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which are known to cause interoperability problems (e.g. with SVR4 SDB).
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To obtain a copy of the DWARF Version 1 and/or DWARF Version 2 specification
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from Data General's FTP server, use the following procedure:
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---------------------------------------------------------------------------
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ftp to machine: "dg-rtp.dg.com" (128.222.1.2).
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Log in as "ftp".
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cd to "plsig"
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get any of the following file you are interested in:
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dwarf.1.0.3.ps
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dwarf.2.0.0.index.ps
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dwarf.2.0.0.ps
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---------------------------------------------------------------------------
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The generation of DWARF debugging information by the GNU version 2.x C
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compiler has now been tested rather extensively for m88k, i386, i860, and
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Sparc targets. The DWARF output of the GNU C compiler appears to inter-
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operate well with the standard SVR4 SDB debugger on these kinds of target
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systems (but of course, there are no guarantees).
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DWARF generation for the GNU g++ compiler is still not operable. This is
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due primarily to the many remaining cases where the g++ front end does not
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conform to the conventions used in the GNU C front end for representing
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various kinds of declarations in the TREE data structure. It is not clear
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at this time how these problems will be addressed.
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Future plans for the dwarfout.c module of the GNU compiler(s) includes the
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addition of full support for GNU FORTRAN. (This should, in theory, be a
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lot simpler to add than adding support for g++... but we'll see.)
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Many features of the DWARF version 2 specification have been adapted to
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(and used in) the GNU implementation of DWARF (version 1). In most of
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these cases, a DWARF version 2 approach is used in place of (or in addition
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to) DWARF version 1 stuff simply because it is apparent that DWARF version
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1 is not sufficiently expressive to provide the kinds of information which
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may be necessary to support really robust debugging. In all of these cases
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however, the use of DWARF version 2 features should not interfere in any
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way with the interoperability (of GNU compilers) with generally available
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"classic" (pre version 1) DWARF consumer tools (e.g. SVR4 SDB).
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The DWARF generation enhancement for the GNU compiler(s) was initially
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donated to the Free Software Foundation by Network Computing Devices.
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(Thanks NCD!) Additional development and maintenance of dwarfout.c has
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been largely supported (i.e. funded) by Intel Corporation. (Thanks Intel!)
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If you have questions or comments about the DWARF generation feature, please
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send mail to me <rfg@netcom.com>. I will be happy to investigate any bugs
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reported and I may even provide fixes (but of course, I can make no promises).
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The DWARF debugging information produced by GCC may deviate in a few minor
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(but perhaps significant) respects from the DWARF debugging information
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currently produced by other C compilers. A serious attempt has been made
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however to conform to the published specifications, to existing practice,
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and to generally accepted norms in the GNU implementation of DWARF.
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** IMPORTANT NOTE ** ** IMPORTANT NOTE ** ** IMPORTANT NOTE **
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Under normal circumstances, the DWARF information generated by the GNU
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compilers (in an assembly language file) is essentially impossible for
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a human being to read. This fact can make it very difficult to debug
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certain DWARF-related problems. In order to overcome this difficulty,
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a feature has been added to dwarfout.c (enabled by the -fverbose-asm
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option) which causes additional comments to be placed into the assembly
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language output file, out to the right-hand side of most bits of DWARF
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material. The comments indicate (far more clearly that the obscure
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DWARF hex codes do) what is actually being encoded in DWARF. Thus, the
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-fverbose-asm option can be highly useful for those who must study the
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DWARF output from the GNU compilers in detail.
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---------
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(Footnote: Within this file, the term `Debugging Information Entry' will
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be abbreviated as `DIE'.)
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Release Notes (aka known bugs)
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-------------------------------
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In one very obscure case involving dynamically sized arrays, the DWARF
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"location information" for such an array may make it appear that the
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array has been totally optimized out of existence, when in fact it
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*must* actually exist. (This only happens when you are using *both* -g
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*and* -O.) This is due to aggressive dead store elimination in the
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compiler, and to the fact that the DECL_RTL expressions associated with
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variables are not always updated to correctly reflect the effects of
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GCC's aggressive dead store elimination.
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-------------------------------
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When attempting to set a breakpoint at the "start" of a function compiled
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with -g1, the debugger currently has no way of knowing exactly where the
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end of the prologue code for the function is. Thus, for most targets,
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all the debugger can do is to set the breakpoint at the AT_low_pc address
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for the function. But if you stop there and then try to look at one or
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more of the formal parameter values, they may not have been "homed" yet,
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so you may get inaccurate answers (or perhaps even addressing errors).
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Some people may consider this simply a non-feature, but I consider it a
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bug, and I hope to provide some some GNU-specific attributes (on function
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DIEs) which will specify the address of the end of the prologue and the
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address of the beginning of the epilogue in a future release.
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-------------------------------
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It is believed at this time that old bugs relating to the AT_bit_offset
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values for bit-fields have been fixed.
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There may still be some very obscure bugs relating to the DWARF description
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of type `long long' bit-fields for target machines (e.g. 80x86 machines)
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where the alignment of type `long long' data objects is different from
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(and less than) the size of a type `long long' data object.
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Please report any problems with the DWARF description of bit-fields as you
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would any other GCC bug. (Procedures for bug reporting are given in the
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GNU C compiler manual.)
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--------------------------------
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At this time, GCC does not know how to handle the GNU C "nested functions"
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extension. (See the GCC manual for more info on this extension to ANSI C.)
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--------------------------------
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The GNU compilers now represent inline functions (and inlined instances
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thereof) in exactly the manner described by the current DWARF version 2
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(draft) specification. The version 1 specification for handling inline
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functions (and inlined instances) was known to be brain-damaged (by the
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PLSIG) when the version 1 spec was finalized, but it was simply too late
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in the cycle to get it removed before the version 1 spec was formally
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released to the public (by UI).
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--------------------------------
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At this time, GCC does not generate the kind of really precise information
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about the exact declared types of entities with signed integral types which
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is required by the current DWARF draft specification.
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Specifically, the current DWARF draft specification seems to require that
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the type of an non-unsigned integral bit-field member of a struct or union
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type be represented as either a "signed" type or as a "plain" type,
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depending upon the the exact set of keywords that were used in the
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type specification for the given bit-field member. It was felt (by the
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UI/PLSIG) that this distinction between "plain" and "signed" integral types
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could have some significance (in the case of bit-fields) because ANSI C
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does not constrain the signedness of a plain bit-field, whereas it does
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constrain the signedness of an explicitly "signed" bit-field. For this
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reason, the current DWARF specification calls for compilers to produce
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type information (for *all* integral typed entities... not just bit-fields)
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which explicitly indicates the signedness of the relevant type to be
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"signed" or "plain" or "unsigned".
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Unfortunately, the GNU DWARF implementation is currently incapable of making
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such distinctions.
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--------------------------------
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Known Interoperability Problems
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-------------------------------
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Although the GNU implementation of DWARF conforms (for the most part) with
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the current UI/PLSIG DWARF version 1 specification (with many compatible
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version 2 features added in as "vendor specific extensions" just for good
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measure) there are a few known cases where GCC's DWARF output can cause
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some confusion for "classic" (pre version 1) DWARF consumers such as the
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System V Release 4 SDB debugger. These cases are described in this section.
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--------------------------------
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The DWARF version 1 specification includes the fundamental type codes
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FT_ext_prec_float, FT_complex, FT_dbl_prec_complex, and FT_ext_prec_complex.
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Since GNU C is only a C compiler (and since C doesn't provide any "complex"
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data types) the only one of these fundamental type codes which GCC ever
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generates is FT_ext_prec_float. This fundamental type code is generated
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by GCC for the `long double' data type. Unfortunately, due to an apparent
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bug in the SVR4 SDB debugger, SDB can become very confused wherever any
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attempt is made to print a variable, parameter, or field whose type was
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given in terms of FT_ext_prec_float.
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(Actually, SVR4 SDB fails to understand *any* of the four fundamental type
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codes mentioned here. This will fact will cause additional problems when
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there is a GNU FORTRAN front-end.)
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--------------------------------
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In general, it appears that SVR4 SDB is not able to effectively ignore
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fundamental type codes in the "implementation defined" range. This can
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cause problems when a program being debugged uses the `long long' data
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type (or the signed or unsigned varieties thereof) because these types
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are not defined by ANSI C, and thus, GCC must use its own private fundamental
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type codes (from the implementation-defined range) to represent these types.
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--------------------------------
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General GNU DWARF extensions
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----------------------------
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In the current DWARF version 1 specification, no mechanism is specified by
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which accurate information about executable code from include files can be
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properly (and fully) described. (The DWARF version 2 specification *does*
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specify such a mechanism, but it is about 10 times more complicated than
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it needs to be so I'm not terribly anxious to try to implement it right
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away.)
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In the GNU implementation of DWARF version 1, a fully downward-compatible
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extension has been implemented which permits the GNU compilers to specify
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which executable lines come from which files. This extension places
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additional information (about source file names) in GNU-specific sections
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(which should be totally ignored by all non-GNU DWARF consumers) so that
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this extended information can be provided (to GNU DWARF consumers) in a way
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which is totally transparent (and invisible) to non-GNU DWARF consumers
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(e.g. the SVR4 SDB debugger). The additional information is placed *only*
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in specialized GNU-specific sections, where it should never even be seen
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by non-GNU DWARF consumers.
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To understand this GNU DWARF extension, imagine that the sequence of entries
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in the .lines section is broken up into several subsections. Each contiguous
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sequence of .line entries which relates to a sequence of lines (or statements)
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from one particular file (either a `base' file or an `include' file) could
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be called a `line entries chunk' (LEC).
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For each LEC there is one entry in the .debug_srcinfo section.
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Each normal entry in the .debug_srcinfo section consists of two 4-byte
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words of data as follows:
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(1) The starting address (relative to the entire .line section)
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of the first .line entry in the relevant LEC.
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(2) The starting address (relative to the entire .debug_sfnames
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section) of a NUL terminated string representing the
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relevant filename. (This filename name be either a
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relative or an absolute filename, depending upon how the
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given source file was located during compilation.)
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Obviously, each .debug_srcinfo entry allows you to find the relevant filename,
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and it also points you to the first .line entry that was generated as a result
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of having compiled a given source line from the given source file.
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Each subsequent .line entry should also be assumed to have been produced
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as a result of compiling yet more lines from the same file. The end of
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any given LEC is easily found by looking at the first 4-byte pointer in
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the *next* .debug_srcinfo entry. That next .debug_srcinfo entry points
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to a new and different LEC, so the preceding LEC (implicitly) must have
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ended with the last .line section entry which occurs at the 2 1/2 words
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just before the address given in the first pointer of the new .debug_srcinfo
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entry.
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The following picture may help to clarify this feature. Let's assume that
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`LE' stands for `.line entry'. Also, assume that `* 'stands for a pointer.
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.line section .debug_srcinfo section .debug_sfnames section
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----------------------------------------------------------------
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LE <---------------------- *
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LE * -----------------> "foobar.c" <---
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LE |
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LE |
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LE <---------------------- * |
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LE * -----------------> "foobar.h" <| |
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LE | |
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LE | |
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LE <---------------------- * | |
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LE * -----------------> "inner.h" | |
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LE | |
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LE <---------------------- * | |
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LE * ------------------------------- |
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LE |
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LE |
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LE |
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LE |
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LE <---------------------- * |
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LE * -----------------------------------
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LE
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LE
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LE
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In effect, each entry in the .debug_srcinfo section points to *both* a
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filename (in the .debug_sfnames section) and to the start of a block of
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consecutive LEs (in the .line section).
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Note that just like in the .line section, there are specialized first and
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last entries in the .debug_srcinfo section for each object file. These
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special first and last entries for the .debug_srcinfo section are very
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different from the normal .debug_srcinfo section entries. They provide
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additional information which may be helpful to a debugger when it is
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interpreting the data in the .debug_srcinfo, .debug_sfnames, and .line
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sections.
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The first entry in the .debug_srcinfo section for each compilation unit
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consists of five 4-byte words of data. The contents of these five words
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should be interpreted (by debuggers) as follows:
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(1) The starting address (relative to the entire .line section)
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of the .line section for this compilation unit.
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(2) The starting address (relative to the entire .debug_sfnames
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section) of the .debug_sfnames section for this compilation
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unit.
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(3) The starting address (in the execution virtual address space)
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of the .text section for this compilation unit.
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(4) The ending address plus one (in the execution virtual address
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space) of the .text section for this compilation unit.
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(5) The date/time (in seconds since midnight 1/1/70) at which the
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compilation of this compilation unit occurred. This value
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should be interpreted as an unsigned quantity because gcc
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might be configured to generate a default value of 0xffffffff
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in this field (in cases where it is desired to have object
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files created at different times from identical source files
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be byte-for-byte identical). By default, these timestamps
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are *not* generated by dwarfout.c (so that object files
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compiled at different times will be byte-for-byte identical).
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If you wish to enable this "timestamp" feature however, you
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can simply place a #define for the symbol `DWARF_TIMESTAMPS'
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in your target configuration file and then rebuild the GNU
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compiler(s).
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Note that the first string placed into the .debug_sfnames section for each
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compilation unit is the name of the directory in which compilation occurred.
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This string ends with a `/' (to help indicate that it is the pathname of a
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directory). Thus, the second word of each specialized initial .debug_srcinfo
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entry for each compilation unit may be used as a pointer to the (string)
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name of the compilation directory, and that string may in turn be used to
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"absolutize" any relative pathnames which may appear later on in the
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.debug_sfnames section entries for the same compilation unit.
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The fifth and last word of each specialized starting entry for a compilation
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unit in the .debug_srcinfo section may (depending upon your configuration)
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indicate the date/time of compilation, and this may be used (by a debugger)
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to determine if any of the source files which contributed code to this
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compilation unit are newer than the object code for the compilation unit
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itself. If so, the debugger may wish to print an "out-of-date" warning
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about the compilation unit.
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The .debug_srcinfo section associated with each compilation will also have
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a specialized terminating entry. This terminating .debug_srcinfo section
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entry will consist of the following two 4-byte words of data:
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(1) The offset, measured from the start of the .line section to
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the beginning of the terminating entry for the .line section.
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(2) A word containing the value 0xffffffff.
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--------------------------------
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In the current DWARF version 1 specification, no mechanism is specified by
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which information about macro definitions and un-definitions may be provided
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to the DWARF consumer.
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The DWARF version 2 (draft) specification does specify such a mechanism.
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That specification was based on the GNU ("vendor specific extension")
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which provided some support for macro definitions and un-definitions,
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but the "official" DWARF version 2 (draft) specification mechanism for
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handling macros and the GNU implementation have diverged somewhat. I
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plan to update the GNU implementation to conform to the "official"
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DWARF version 2 (draft) specification as soon as I get time to do that.
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Note that in the GNU implementation, additional information about macro
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definitions and un-definitions is *only* provided when the -g3 level of
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debug-info production is selected. (The default level is -g2 and the
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plain old -g option is considered to be identical to -g2.)
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GCC records information about macro definitions and undefinitions primarily
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in a section called the .debug_macinfo section. Normal entries in the
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.debug_macinfo section consist of the following three parts:
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(1) A special "type" byte.
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(2) A 3-byte line-number/filename-offset field.
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(3) A NUL terminated string.
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The interpretation of the second and third parts is dependent upon the
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value of the leading (type) byte.
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The type byte may have one of four values depending upon the type of the
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.debug_macinfo entry which follows. The 1-byte MACINFO type codes presently
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used, and their meanings are as follows:
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MACINFO_start A base file or an include file starts here.
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MACINFO_resume The current base or include file ends here.
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MACINFO_define A #define directive occurs here.
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MACINFO_undef A #undef directive occur here.
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(Note that the MACINFO_... codes mentioned here are simply symbolic names
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for constants which are defined in the GNU dwarf.h file.)
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For MACINFO_define and MACINFO_undef entries, the second (3-byte) field
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contains the number of the source line (relative to the start of the current
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base source file or the current include files) when the #define or #undef
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directive appears. For a MACINFO_define entry, the following string field
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contains the name of the macro which is defined, followed by its definition.
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Note that the definition is always separated from the name of the macro
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by at least one whitespace character. For a MACINFO_undef entry, the
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string which follows the 3-byte line number field contains just the name
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of the macro which is being undef'ed.
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For a MACINFO_start entry, the 3-byte field following the type byte contains
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the offset, relative to the start of the .debug_sfnames section for the
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current compilation unit, of a string which names the new source file which
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is beginning its inclusion at this point. Following that 3-byte field,
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each MACINFO_start entry always contains a zero length NUL terminated
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string.
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For a MACINFO_resume entry, the 3-byte field following the type byte contains
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the line number WITHIN THE INCLUDING FILE at which the inclusion of the
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current file (whose inclusion ends here) was initiated. Following that
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3-byte field, each MACINFO_resume entry always contains a zero length NUL
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terminated string.
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Each set of .debug_macinfo entries for each compilation unit is terminated
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by a special .debug_macinfo entry consisting of a 4-byte zero value followed
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by a single NUL byte.
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--------------------------------
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In the current DWARF draft specification, no provision is made for providing
|
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a separate level of (limited) debugging information necessary to support
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tracebacks (only) through fully-debugged code (e.g. code in system libraries).
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|
A proposal to define such a level was submitted (by me) to the UI/PLSIG.
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|
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.
|