Compiling the whole Debian archive on MIPS with GCC 4.1

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. 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 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 errors before G++ 4.1).

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 are:

• extra qualification
• reliance on friend injection
• wrong escape characters (e.g. "\.", most commonly seen in regular expressions)
• iterator problems (such as assigning 0 or NULL to an iterator)
• 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>
• dereferencing type-punned pointer will break strict-aliasing rules (a warning) combined with -Werror

I've posted a detailed report to debian-devel.

Acknowledgements (abridged)

• 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. ;-)