[PATCH] Treat -g1 as -gline-tables-only

Hal Finkel hfinkel at anl.gov
Mon Oct 20 13:19:55 PDT 2014

----- 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++ code). 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.)

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

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