Building the whole Debian archive with GCC 4.1: a summary
Over the last 2.5 weeks I have built the complete Debian archive
on a quad-core MIPS machine donated by Broadcom using the recently
released version 4.1 of GCC. In parallel, I have done the same
on an EM64T box donated to Debian by Intel and hosted by Stephen
Frost.
The purpose of this exercise was three-fold:
- Find out about compiler problems in GCC 4.1 itself as well
as in packages that may fail with the new version *before*
GCC 4.1 is uploaded to unstable. GCC, in particular G++,
is becoming stricter regarding adherence to standards and
packages may fail to build with 4.1 due to invalid code
that was accepted previously.
- Find out about MIPS specific problems in GCC 4.1 and to answer
Matthias Klose's question [1] as to which platforms can move to
GCC 4.1 as the default compiler once it is uploaded to unstable.
- Find MIPS specific assembler warnings and create a list of all
users of xgot (a MIPS specific toolchain problem).
Executive summary
-----------------
GCC 4.1 itself appears to be very stable, both on MIPS and AMD64.
There are, however, a large number of packages using code (especially
C++) which GCC 4.1 treats as errors. Fortunately, most of them are
trivial to fix. By compiling about 6200 packages, over 500 new
bugs have been discovered and submitted, 280 of which are specific
to the increased strictness of GCC 4.1. Patches for 2/3 of those
GCC 4.1 specific bugs have been submitted.
Based on my findings, Ben Hutchings has prepared a summary of the most
common C++ syntax errors (that weren't treated as such before G++ 4.1):
http://womble.decadentplace.org.uk/c++/syntax-errors.html
Methodology
-----------
I generated a list of package that are "Architecture: any" or "mips",
sorted by upload (old packages first). I then started compiling these
packages, and after the mirror pulse I would update my packages list
again (excluding packages which had no new version). What I explicitly
did *not* do was to exclude packages which have known build failures
because I wanted to see if they might have GCC 4.1 issues too. Another
thing I did on MIPS which the official build machines would not do
is to compile as "mips64" rather than "mips" (using a 64-bit kerneel but
32-bit userland, uname -m shows "mips64"; this can be changed by using the
linux32 program). The aim of this was to identify mips64 specific
problems. I compiled every packages that failed with "mips64" using
"mips" too, though.
In total, 6192 individual packages were compiled on MIPS, with 6761
compilations (because of new versions uploaded to the archive during
those two weeks). A listing is available from [2] and all build logs
from MIPS from [3]. On AMD64, the number of individual packages
compiled was 5862. This number is lower than the one for MIPS because
I started with MIPS first and then ignored packages on AMD64 with known
build errors.
Detailed summary of bugs found
------------------------------
While I've tried to keep count of the different errors, some of the
numbers are slightly off, partly because you do make some errors when
keeping track of so many bugs and partly because the classification
below is quite arbitrary and I slightly changed it over time.
(Normally, you'd go back and classify each bug again, but I didn't do
that because this was not a scientific study.)
1. New bugs I have filed in the BTS
- gcc/g++ 4.1 strictness: 277 (see [4] for a list]
- failures due to the new version of make: 4
- old or missing build-dependencies: 50
- host type cannot be recognized:
- config.* out of date: 26
- other method (mips64): 7
- other method (amd64): 1
- architecture specific bugs:
- mips: 9
- amd64: 7
- GCC 4.1 compiler bugs: 6
- packages that could (but don't) support mips: 5
- non-i386 brokenness: 4
- 64-bit brokenness: 2
- a cast loses precision: 2
- not using PIC: 1
- .orig.tar.gz missing from archive: 1
- other/generic: 50
2. The build is "successful" but there is a bug if you look closely
I tried to look at successful build logs but with over 6000 packages
I could obviously not do so in great detail. Therefore, I'm sure there
are other bugs in "successful" builds I missed.
- "Architecture" is "any" but should be "all": 45
- package contains nothing useful (no binary, no headers, etc): 1
- package contains no binary on !i386: 1
- build doesn't show what commands are run (wishlist bug): 2
- test suite not run (wrong command): 1
3. Bugs which I saw but which have already been reported
- gcc/g++ 4.1 strictness: 2
- gcc/g++ 3.4/4.0 strictness: 17
- failures due to the new version of make: 6
- old or missing build-dependencies: 66
- host type cannot be recognized:
- config.* out of date: 42
- architecture specific bugs:
- mips: 2
- amd64: didn't count but there some
- non-i386 brokenness: 27
- 64-bit brokenness: 3
- a cast loses precision: 2
- not using PIC: 2
- the "Architecture" is "any" but should be "all": 2
- other/generic: 40
4. Most often seen warning
- dh_*: Compatibility levels before 4 are deprecated.
Most common programming errors
------------------------------
Basically, it all boils down to broken C++ code. There were a few
bugs in C code, but the majority was in C++. The most common
errors I found (and some *approximate* numbers) are:
- extra qualification: about 187 bugs
- reliance on friend injection: 26 bugs
- wrong escape characters (e.g. "\."; most commonly seen in regular
expressions): 6 bugs
- iterator problems (such as assigning 0 or NULL to an iterator): 3
- template specialisation in wrong namespace
- template reliance on a function declared later
- use of template's base class members, unqualified, where the base class
is dependent
- use of "assert" without #include <cassert>: 5 bugs
- dereferencing type-punned pointer will break strict-aliasing rules
together (a warning) combined with -Werror: 6 bugs
If you intend to touch C++ code, please take some time to read
the following pages:
- http://womble.decadentplace.org.uk/c++/syntax-errors.html
- http://womble.decadentplace.org.uk/c++/template-faq.html
- http://gcc.gnu.org/gcc-4.1/changes.html - a detailed list of changes
in GCC 4.1.
Compiler bugs in GCC 4.1
------------------------
- gcc ICE: #356231 (gnubik_2.2-5)
http://gcc.gnu.org/PR26672 - fixed
- g++ ICE: #356569 (3ddesktop_0.2.9-5.1)
http://gcc.gnu.org/PR26757 - test case needs further reduction
- gcc bug: #356574 (address of register requested)
http://gcc.gnu.org/PR26004 - fixed
- g++ bug: #356548 (cannot bind packed field)
http://gcc.gnu.org/PR26670 - not yet assigned but related to known bugs
- g++ bug: #356435 (no constructor) [package bug: #356245]
http://gcc.gnu.org/PR26755 - waiting
This is a binary incompatibility! 4.0 has a different behaviour to
3.3 and 4.1, which means that linking 4.1 code against 4.0 libraries
may fail. 4.0 and 4.1 were supposed to be binary compatible so I'm not
sure what will be done about this.
- gcc bug: #356896 (miscompiles code, leading to segfault in program)
http://gcc.gnu.org/PR26763 - confirmed
- gcc or autoconf bug: #358644 (change in behaviour on mips regarding
-fstack-protector)
It's not clear right now whether this behaviour change was correct and
autoconf should be changed or whether GCC should revert that change.
Fortunately, this change only breaks 2 packages out of ~6200.
Status of 4.1 readiness
-----------------------
(These numbers are slightly off but give a good overview)
* 248 Outstanding
* 12 Forwarded
* 10 Pending Upload
* 3 Fixed in NMU
* 12 Resolved
* 186 Patch Available
* 1 Confirmed
* 38 Unclassified
Further work and unresolved issues
----------------------------------
These are issues I haven't had time to investigate yet:
- look for packages that use a specific version of GCC (hard-coded) and
see if they work with GCC 4.1.
- a number of Java programs fail to build because jni.h cannot be found.
- many gnustep packages didn't get built
- many package that require kaffe didn't get built
MIPS specific comments
----------------------
1. Assembler warnings
20 packages had assembler warnings on MIPS. A list will be sent to
debian-mips so Thiemo Seufer (or others) can investigate.
2. Packages that couldn't be built because of missing packages (depwait)
- ghc-cvs/ghc6: 33 [see #274942]
- libopenafs-dev: 3 [not ported to mips yet]
- linux source/headers 2.6.15-1: 6 [2.6 is in the archive for mips now]
- libdb4.2-java: 1 (openoffice)
- gjdoc: 7
- zeroc-ice/zeroc-icee-translators: 3
- hmake: 1
- libopenvrml5-dev: 1
- libavifile-0.7-dev: 2
- fp-compiler: 2
- eclipse: 1
- thunderbird-dev: 1
- libgtkada2-dev: 1
- mono/cil: 16
3. Packages not relevant to mips (notforus)
- depends on syslinux: 1
- mig: 1 (hurd)
- low-level stuff: 1
- cmucl: 1
- mondo: 1
- pbbuttonsd-dev (gtkpbbuttons): 1 [powerpc specific]
- svga: 1
- firebird2: 2
4. Packages which I've now ported to mips
- numactl: add mips syscall definitions
- liblinux-inotify2-perl: add mips syscall definitions
- zeroc-icee-translators: add some simple #ifdefs for __mips__
- zeroc-ice: add some simple #ifdefs for __mips__
5. Unclarified issue
7 packages fail to build because MIPS doesn't have a generic
cpu-feature-overrides.h header. We have to check whether those packages
should be fixed (userspace should not depend on kernel headers anyway)
or whether the kernel can provide a generic cpu-feature-overrides.h.
Musings
-------
Obviously, compiling the archive from source leads to many important
insights. What I found out over the last two weeks is just how much
work it actually is. We should try to create better infrastructure
to make manual compilation of the whole archive easier. It seems
that Roland Stigge has done some work on this already with his DARTS
project [5] but I have yet to take a look at it.
Acknowledgements
----------------
- Broadcom for supporting our MIPS port through the donation of hardware
to the project and developers. Without Broadcom, our little and big
endian MIPS ports would have a hard time meeting the release criteria
regarding autobuilders.
- Google for supporting my PhD, thereby allowing me to spend two weeks
compiling the archive with GCC 4.1 and sorting out failures.
- Intel for supporting some PhD work which led to this experiment.
- Ben Hutchings for explaining many of the C++ bugs I found to me.
I've learned more about C++ in these two weeks than I ever wanted to
know. ;-) Ben also submitted a number of patches for tricky bugs I
couldn't fix.
- Intel for donating an EM64T machine to Debian.
- Thiemo Seufer for fixing all the tricky MIPS problems in Debian.
- The GCC project for creating a great compiler, and in particular
Andrew Pinski for doing lots of bug triaging work.
- Roger Leigh for quickly implementing all my sbuild feature requests.
- Everyone who compiles the archive from scratch from time to time.
Kudos to you. I'm now aware how much work it is!
- Everyone who has fixed the bugs I filed already. :-)
References
----------
[1] http://lists.debian.org/debian-gcc/2006/02/msg00173.html
[2] http://people.debian.org/~tbm/logs/gcc-4.1/PACKAGES
[3] http://people.debian.org/~tbm/logs/gcc-4.1/
[4] http://bugs.debian.org/cgi-bin/pkgreport.cgi?which=submitter&includesubj=4.1&data=tbm%40cyrius.com&archive=no&version=&dist=unstable
[5] http://darts.alioth.debian.org/
--
Martin Michlmayr
http://www.cyrius.com/
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