[PATCH] Treat -g1 as -gline-tables-only
hfinkel at anl.gov
Mon Oct 20 13:52:52 PDT 2014
----- Original Message -----
> From: "David Blaikie" <dblaikie at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Eric Christopher" <echristo at gmail.com>, "llvm cfe" <cfe-commits at cs.uiuc.edu>,
> reviews+D5862+public+e7b928154d4d08d9 at reviews.llvm.org
> Sent: Monday, October 20, 2014 3:38:39 PM
> Subject: Re: [PATCH] Treat -g1 as -gline-tables-only
> On Mon, Oct 20, 2014 at 1:19 PM, Hal Finkel < hfinkel at anl.gov >
> ----- Original Message -----
> > From: "David Blaikie" < dblaikie at gmail.com >
> > To: reviews+D5862+public+e7b928154d4d08d9 at reviews.llvm.org
> > Cc: "Hal Finkel" < hfinkel at anl.gov >, "Eric Christopher" <
> > echristo at gmail.com >, "llvm cfe" < cfe-commits at cs.uiuc.edu >
> > Sent: Sunday, October 19, 2014 8:35:13 PM
> > Subject: Re: [PATCH] Treat -g1 as -gline-tables-only
> > On Sun, Oct 19, 2014 at 4:06 PM, hfinkel at anl.gov < hfinkel at anl.gov
> > >
> > wrote:
> > Hi echristo, dblaikie,
> > To follow-up on this thread on cfe-dev:
> > http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-October/039649.html
> > -g1 on gcc (and also IBM's xlc) are documented to be very similar
> > to
> > -gline-tables-only. Our -gline-tables-only might still be more
> > verbose than -g1 on other compilers, but currently we treat -g1 as
> > -g, and so we're producing much more debug info at -g1 than
> > everybody else. Treating -g1 as -gline-tables-only brings us much
> > closer to what everyone else is doing.
> > A follow-up: which user(s) did you have who hit problems with -g1?
> > Do
> > you know what /they/ want when they specify -g1? It'd be useful to
> > at least have some actual user of -g1 with a documented use case so
> > we know if we're meeting their needs, etc.
> Okay, this is how the issue came up...
> At our supercomputing facility, we have three compilers: IBM's xlc,
> gcc and clang (all customized for the BG/Q). The facility is
> responsible for analyzing all job failures, both for our own
> purposes, and to help our users diagnose and fix problems with their
> code (when their code is at fault). When running tens of thousands
> of processes in parallel, it is not feasible to generate full core
> files for every crash from every process (that could be hundreds of
> TB of data per crash), and so our kernel generates "light weight"
> core files. For the most part, these core files contain only enough
> information to reconstruct the stack backtrace, but not enough for
> interactive debugging. Nevertheless, as you might imagine, the
> backtraces themselves are very useful (compared to nothing), and are
> much more useful with line-table information (especially for C++
> Do you happen to know if your backtracing/symbolication uses inlining
> information to improve the quality?
It uses whatever advanced technology is made available by addr2line -C -e <filename> <addr> ;)
> If so, it doesn't look like XLC will help you there, and neither will
> GCC, but Clang will.
> Beyond that, for basic, non-inlining backtraces, -g1 in xlc and gcc
> (and as you've patched here in Clang) will all provide the basic
> line table good for symbolication.
> Arguably we could have -g1 do something more like xlc to save on
> space more if non-inlined symbolication is what users of this
> feature need - so if you find the debug info is too big even with
> this new -g1 you're proposing, we can chat about what to do there.
> (note that the debug info with Clang's -gmlt (this new -g1) is
> substantially bigger in optimized binaries (with lots of inlining)
> so if you're assessing the build impact of this feature, be sure to
> test it on whatever build modes you care about, not just -O0)
Certainly will do.
> One proposal we've been evaluating is automatically adding -g1 to all
> compiles on the system, so that we can always get line-table
> information for use with these light-weight core files. In order to
> add this to every compile, there needs to be only a small effect on
> compile/link times (the smaller the better -- and the line table
> information is really all we need). The fact that clang treats -g1
> like -g (instead of generating much less debugging info like gcc/xlc
> do) was discovered in the process of evaluating this proposal.
> I could obviously just "fix" this locally, but it seemed like it
> might be generally desirable behavior to better match gcc/xlc in
> this case (thus the thread on cfe-dev, etc.)
> Nah, seems fine as a notion.
> Please commit
> - David
> Thanks again,
> > http://reviews.llvm.org/D5862
> > Files:
> > lib/Driver/Tools.cpp
> > test/Driver/debug-options.c
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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