Skip to main content
Redhat Developers  Logo
  • Products

    Platforms

    • Red Hat Enterprise Linux
      Red Hat Enterprise Linux Icon
    • Red Hat AI
      Red Hat AI
    • Red Hat OpenShift
      Openshift icon
    • Red Hat Ansible Automation Platform
      Ansible icon
    • View All Red Hat Products

    Featured

    • Red Hat build of OpenJDK
    • Red Hat Developer Hub
    • Red Hat JBoss Enterprise Application Platform
    • Red Hat OpenShift Dev Spaces
    • Red Hat OpenShift Local
    • Red Hat Developer Sandbox

      Try Red Hat products and technologies without setup or configuration fees for 30 days with this shared Openshift and Kubernetes cluster.
    • Try at no cost
  • Technologies

    Featured

    • AI/ML
      AI/ML Icon
    • Linux
      Linux Icon
    • Kubernetes
      Cloud icon
    • Automation
      Automation Icon showing arrows moving in a circle around a gear
    • View All Technologies
    • Programming Languages & Frameworks

      • Java
      • Python
      • JavaScript
    • System Design & Architecture

      • Red Hat architecture and design patterns
      • Microservices
      • Event-Driven Architecture
      • Databases
    • Developer Productivity

      • Developer productivity
      • Developer Tools
      • GitOps
    • Automated Data Processing

      • AI/ML
      • Data Science
      • Apache Kafka on Kubernetes
    • Platform Engineering

      • DevOps
      • DevSecOps
      • Ansible automation for applications and services
    • Secure Development & Architectures

      • Security
      • Secure coding
  • Learn

    Featured

    • Kubernetes & Cloud Native
      Openshift icon
    • Linux
      Rhel icon
    • Automation
      Ansible cloud icon
    • AI/ML
      AI/ML Icon
    • View All Learning Resources

    E-Books

    • GitOps Cookbook
    • Podman in Action
    • Kubernetes Operators
    • The Path to GitOps
    • View All E-books

    Cheat Sheets

    • Linux Commands
    • Bash Commands
    • Git
    • systemd Commands
    • View All Cheat Sheets

    Documentation

    • Product Documentation
    • API Catalog
    • Legacy Documentation
  • Developer Sandbox

    Developer Sandbox

    • Access Red Hat’s products and technologies without setup or configuration, and start developing quicker than ever before with our new, no-cost sandbox environments.
    • Explore Developer Sandbox

    Featured Developer Sandbox activities

    • Get started with your Developer Sandbox
    • OpenShift virtualization and application modernization using the Developer Sandbox
    • Explore all Developer Sandbox activities

    Ready to start developing apps?

    • Try at no cost
  • Blog
  • Events
  • Videos

Testing GCC in the wild

March 10, 2016
Marek Polacek

Share:

    Currently, the GCC testsuite contains more than fifty thousand tests which make up to two million lines of code. Since we, the GCC developers, try hard to avoid regressions in the compiler, almost every change to the compiler or to a related library requires a new test or a set of tests to be added. Hence the internal testsuite keeps growing at a rapid pace. It goes without saying that a new change should not break any existing test.

    Despite all this effort, new bugs still creep in. Because it is best to discover as many bugs as possible before a particular version of the compiler is released, many distribution maintainers try to rebuild all the packages in the distro with a still unreleased compiler. They file bug reports if they find any bugs to the upstream Bugzilla so that the compiler is up to snuff when it is released. This rebuild typically happens once a year in regression fixes-only stage of the development, i.e. a stage when no new features are being introduced and the development revolves around fixing bugs.

    While a new version of GCC is usually released in April, the Fedora project performs a mass rebuild of all the Fedora packages in January or February. Due to limited resources, this mass rebuild is only done on x86_64 architecture.
    Since the point is to test GCC, this is a mass rebuild of so-called first order.

    We are looking for several classes of bugs here. These include:

    • ICEs, internal compiler errors, are when the compiler just crashes or segfaults. These are always bugs even if the compiler crashes on a code that is not valid or contains undefined behavior. This sort of bug is usually rather easy to deal with. The compiler is clever enough to produce a pre-processed source file in /tmp.
    • false positive warnings occur when the compiler warns on a code even though it should not. This may cause a package build to fail in combination with the -Werror command-line option.
    • Another class of bugs is when the compiler fails to compile valid code. It is sometimes unclear whether the code in question is valid or not in a given language and requires significant expertise to assess; it might even involve raising a DR (Defect Report) with the relevant standard committee.
    • wrong-code issues are the most formidable ones. They very often manifest themselves by causing a package's testsuite to fail. However, the failures can be caused by a host of things; too often these are merely undefined behavior in applications that seemingly worked in the past. Some packages misuse the preprocessor in twisted ways, others might be relying on internal details of libstdc++ headers, and so on. All that makes it hard and quite time-consuming to gauge whether the bug is in the compiler or in the package. Fortunately sanitizers such as UBSan can be extremely useful when examining such failures. Another option is to build the package with e.g. -fno-strict-aliasing or -fno-aggressive-loop-optimizations to see if the build still fails. If any of these options helps, it is most likely a bug in the package.

    When compiler bug is found, it is important to reduce the problem to a stand-alone test case. This helps insure that future versions of the compiler will continue to appropriately detect invalid code or unspecified behaviors. It is understood that some problems such as those which involve LTO are nearly impossible to reduce into a stand alone test case. Given that not every package uses standard GNU Makefiles, it is sometimes needed to dabble with the internals of a package.

    The following is a general overview of how we perform a mass rebuild

    First, we rebuild every package in the distro with the new GCC. The number of packages is ever-increasing; while in 2008 there were 5118 packages, in 2011 it was 10404 packages, and this year there were 17741 packages. Naturally, this number of packages requires the rebuild to run in parallel. We start by creating a repository of all the source RPMs and list of their names. Then we create a second repository, this time with the new GCC and corresponding libtool packages. Afterwards we set up mock and chroot configs on every build machine and prepare a script which downloads an SRPM, rebuilds the SRPM in mock, and saves the logs of the rebuild. Now we run this script fed by the list created before (in parallel) on every build box.

    Secondly, we rebuild failed builds with the old GCC to quickly evaluate packages that failed due to non-GCC related reason.

    Thirdly, we need to investigate FTBFS (Fails To Build From Source) that only happen with the new GCC.

    The result of a mass rebuild is so-called "porting to" document, whose purpose is to provide help with porting a project to the new GCC. This year's "porting to" is still somewhat in flux, but interested readers might want to take a look
    at https://gcc.gnu.org/gcc-6/porting_to.html. Furthermore, we post a summary mail on the Fedora devel mailing list which concludes on the particular mass rebuild. For the latest summary, see https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/DH7M2ADHM6XCRFTRRSKZD6MWFUJKHBZK/.

    Ideally, all the bugs that were discovered during the mass rebuild are fixed, either in the compiler or in the packages built by it.  This annual rebuild should insure that the compiler is of a good quality and is prepared to be deployed as a new system compiler.

    Recent Posts

    • Why some agentic AI developers are moving code from Python to Rust

    • Confidential VMs: The core of confidential containers

    • Benchmarking with GuideLLM in air-gapped OpenShift clusters

    • Run Qwen3-Next on vLLM with Red Hat AI: A step-by-step guide

    • How to implement observability with Python and Llama Stack

    Red Hat Developers logo LinkedIn YouTube Twitter Facebook

    Products

    • Red Hat Enterprise Linux
    • Red Hat OpenShift
    • Red Hat Ansible Automation Platform

    Build

    • Developer Sandbox
    • Developer Tools
    • Interactive Tutorials
    • API Catalog

    Quicklinks

    • Learning Resources
    • E-books
    • Cheat Sheets
    • Blog
    • Events
    • Newsletter

    Communicate

    • About us
    • Contact sales
    • Find a partner
    • Report a website issue
    • Site Status Dashboard
    • Report a security problem

    RED HAT DEVELOPER

    Build here. Go anywhere.

    We serve the builders. The problem solvers who create careers with code.

    Join us if you’re a developer, software engineer, web designer, front-end designer, UX designer, computer scientist, architect, tester, product manager, project manager or team lead.

    Sign me up

    Red Hat legal and privacy links

    • About Red Hat
    • Jobs
    • Events
    • Locations
    • Contact Red Hat
    • Red Hat Blog
    • Inclusion at Red Hat
    • Cool Stuff Store
    • Red Hat Summit
    © 2025 Red Hat

    Red Hat legal and privacy links

    • Privacy statement
    • Terms of use
    • All policies and guidelines
    • Digital accessibility

    Report a website issue