cc-with-tweaks: show dwz stderr and check exit code

From: Simon Marchi <simon.marchi@efficios.com>

  On 2019-05-10 5:03 a.m., Tom de Vries wrote:
> That's PR24301 - "dwz fails to return EXIT_FAILURE when processing two
> files" ( https://sourceware.org/bugzilla/show_bug.cgi?id=24301 ). I've
> just committed a fix.

Thanks!

>> Good point.  Why not check both actually (exit code and presence of the -m file)?
>> I did this change in the updated patch lower, tell me if you think it makes sense.
> 
> The rationale for checking for file changed in want_dwz mode and
> multifile presence in want_multi mode is because these are measures that
> are more reliable than the exit code, and will get better results for
> people using older dwz.
> 
> Checking for both goes against that rationale.

Ok, I understand your point for older dwzs.

>> What do you mean by "we could copy and compare for the want_dwz case"?  No new
>> file is created in that case, so what should we test for?
>>
> 
> I meant:
> ...
> if [ "$want_dwz" = true ]; then
>     cp $output_file ${output_file}.copy
>     $DWZ "$output_file" > /dev/null
>     if cmp "$output_file" "$output_file.copy"; rc=$?; then
> 	true
>     fi
>     rm -f ${output_file}.copy
>     if [ $rc == 0 ]; then exit 1; fi

Ah, interesting.  As you said earlier, this doesn't guarantee that the DWARF has been
modified, it could be something else in the executable that was changed.  But still, it
should catch a number of cases where dwz doesn't touch the executable at all.

I have included your suggestion in the patch below and made a few adjustments.

By the way, do you have a way to reproduce the case where dwz doesn't optimize anything
in single file mode?  Even with a file with just an empty main, dwz manages to do some
optimization.

Running dwz twice on the same executable is a way to get "DWARF compression not beneficial",
but this shouldn't happen in cc-with-tweaks.

Another question: in multifile mode, we check if the output dwz file exists.  Should we also
check that the original file now contains a .gnu_debugaltlink section?  If dwz succeeded to
generated the external dwz file, but failed to modify the original executable, we could also
end up running a test with a non-dwzified executable.

From ae61facce0dd0a480e265be1728fb1741b886180 Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@efficios.com>
Date: Tue, 7 May 2019 14:14:54 -0400
Subject: [PATCH] cc-with-tweaks: show dwz stderr and verify result

When running the gdb.base/index-cache.exp test case with the
cc-with-dwz-m board, I noticed that the final executable didn't actually
contain a .gnu_debugaltlink section with the name of the external dwz
file:

    $ readelf --debug-dump=links testsuite/outputs/gdb.base/index-cache/index-cache
    * empty *

Running dwz by hand, I realized it's because dwz complains that the
output .debug_info section is empty and fails:

    $ gcc ~/src/binutils-gdb/gdb/testsuite/gdb.base/index-cache.c -g3 -O0 -o a && cp a b
    $ dwz -m foo a b
    dwz: foo: .debug_info section not present
    $ echo $?
    1

This is because index-cache.c is trivial (just an empty main) and dwz
doesn't find anything to factor out to the dwz file. [1]

I think that cc-with-tweaks should fail in this scenario: if the user
asks for an external dwz file to be generated (the -m flag), then it
should be an error if cc-with-tweaks doesn't manage to produce an
executable with the proper link to this external dwz file.  Otherwise,
the test runs with a regular non-dwzified executable, which gives a
false sense of security about whether the feature under test works with
dwzified executables.

So this patch adds checks for that after invoking dwz.  It also removes
the 2>&1 to allow the error message to be printed like so:

    Running /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/index-cache.exp ...
    gdb compile failed, dwz: /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/index-cache/index-cache.dwz: .debug_info section not present

- In the -m case (multi-file compression), we check if the expected output file
  exists.
- In the -z case (single-file compression), we check if the file
  contents has changed.  This should catch cases where dwz doesn't modify the
  file because it's not worth it.

It was chosen not to check for dwz's exit code, as it is not very
reliable up to dwz 0.12.

With this patch, fewer tests will pass than before with the
cc-with-dwz and cc-with-dwz-m boards, but those were false positives
anyway, as the test ran with regular executables.

[1] Note that dwz has been patched by Tom de Vries to work correctly in
this case, so we can use dwz master to run the test:

https://sourceware.org/git/?p=dwz.git;a=commit;h=08becc8b33453b6d013a65e7eeae57fc1881e801

gdb/ChangeLog:

	* contrib/cc-with-tweaks.sh: Validate dwz's work.
---
 gdb/contrib/cc-with-tweaks.sh | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

cc-with-tweaks: show dwz stderr and check exit code

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