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Subject:

Re: Difference between -trace and -g

From:

David LaFrance-Linden <[log in to unmask]>

Reply-To:

Fortran 90 List <[log in to unmask]>

Date:

Fri, 18 Mar 2005 11:54:13 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (75 lines)

   Date:         Fri, 18 Mar 2005 11:03:24 -0500
   From: Aleksandar Donev <[log in to unmask]>

   Most compilers offer a switch to enable tracing the location of an error
   (say -gline. -traceback, --trace, etc.) and also -g to put debugging
   info. What exactly is the difference? I presume -g puts in a lot more
   info, and -gline just puts line numbers on procedure calls? Is it
   expected that either one will significantly affect performance (without
   -O0, but rather high optimizations)?

Most compilers or most fortran compilers?  I'm unfamiliar with these
the tracing options to the compiler, but I can give you some general
information.

Getting tracebacks requires the ability to capture enough context
about a procedure and then be able to "unwind" to previous
procedures.  This context typically includes the program counter and
stack pointer, and depending on the architecture it could be many
other registers.  Modern systems often have libraries to assist in
some of this.  Debuggers sometimes use them, and sometimes write their
own.  The unwind mechanism often requires tables, typically indexed by
procedure, saying what it does with the registers so the unwinder can
undo it and "virtually return" to the caller.  I suspect -traceback
and --trace ensure these tables exist for the unwinding mechanism
being used.  Note that C++'s exception system demands sufficient
tables for all procedures so it can accomplish catch/throw.

From it's name, -gline is something different.  If you have a PC
(program counter), you probably want to know what procedure it is in.
That's a relatively simple PCRange->procedureDescriptor table, also
needed by the unwinder.  If you want line number information, you also
need <procDesc,offset> to <sourceFile,lineNum> tables.  I suspect
-gline makes sure these exist, perhaps just for an external debugger,
or perhaps so that the program can see it introspectively.

-g traditionally means "include debugging information" and by this it
is more than just line numbers, but also the names of variables, where
they are (memory, stack, register), what type they are (complex*8,
type(tree)), what their lifetime is, arguments to procedures, etc,
etc.  Things that make 'print <expr>' work; and other things, too.
You can unwind and get line numbers without all the argument and
variable information that -g provides, and -g often triples (or more)
the size of the executable due to the debug tables.

There has always been a tension between optimization and debugging
information.  Traditionally, -g by itself turns off optimization.  A
common effect of this is that each source line's instructions are
scheduled not to overlap with another line's instructions.  No CSE
collapsing, no loop hoisting, nothing, because that would cause the
line number to jump around as the PC advanced, which makes the 'step'
and 'next' operations do unhelpful things.

When you start to increase the optimization level, it is still
possible to retain some debugging information, but the quality starts
to drop, the utility starts to drop, and at very high levels the
compiler vendor may not try very hard to preserve debug info.  One of
the first things that happens is that the instructions for a source
line start to get shuffled with the instructions for another source
line.  'step' and 'next' start to look non-sensical, but with some
practice you get used to it.  (I know of a research idea to do
"semantic stepping" which would find the semantic points of the code
and step among them instead of by lines.  Alas, there's not much
funding for debugger research these days...)  loop unrolling starts to
replicate a variable, and the compiler may have given each instance a
different name (such as i_1, i_2, i_3, i_4) and may or may not have
told the debug tables about it (if indeed the debug tables have the
ability to represent it).  Some variables disappear completely because
their lifetime is over, or the compiler noticed it was conceptually a
temporary and threw it away as it folded its computation into
whereever it was used.  The list goes on.

But from what I've seen, when the user requests optimization and
debugability, compilers will honor the optimization request and will
do their best with the debugability request.  Your milage will vary.

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