Formally verified compilation

CompCert is a formally verified optimizing C compiler. Its intended use is compiling safety-critical and mission-critical software written in C and meeting high levels of assurance. It ac­cepts most of the ISO C 99 language, with some exceptions and a few extensions. It produces machine code for ARM, PowerPC, and x86 architectures.

What sets CompCert apart?

CompCert is the only production compiler that is formally verified, using machine-assisted math­ematical proofs, to be exempt from miscompilation issues. The code it pro­duces is proved to behave exactly as specified by the semantics of the source C program.

This level of confidence in the correctness of the compilation process is unprecedented and con­tributes to meeting the highest levels of software assurance.

CompCert diagram

The formal proof covers all transformations from the abstract syntax tree to the generated assem­bly code. To preprocess and produce object and executable files, an external C pre­processor, assemblers, linkers, and C libraries have to be used. However, these unverified stages are well-understood and robust from an implementation perspective. This has been demon­strated on a devel­op­ment version of CompCert in a 2011 study by Regehr, Yang et al.:

The striking thing about our CompCert results is that the middle-end bugs we found in all other compilers are absent. As of early 2011, the under-development version of CompCert is the only compiler we have tested for which Csmith cannot find wrong-code errors. This is not for lack of trying: we have devoted about six CPU-years to the task. The apparent unbreak­ability of CompCert supports a strong argument that developing compiler optimizations within a proof framework, where safety checks are explicit and machine-checked, has tangible be­ne­fits for compiler users.”

Formally verified optimizations

CompCert implements the following optimizations, all of them formally verified:

Who uses CompCert?

The Institute of Flight System Dynamics at the Technical University of Munich uses CompCert in the development of flight control and navigation algorithms.

In November 2017 CompCert was successfully qualified by MTU Friedrichshafen according to IEC 60880, category A and IEC 61508-3:2010, SCL 3 for a certification project in the nuclear energy domain. The use of CompCert reduced development time and costs.

Airbus is employing CompCert at the Toulouse plant for a number of currently undisclosed projects.

Your benefits

Compilation with execution time in mind

On typical embedded processors, code generated by CompCert typi­cally runs twice as fast as code generated by GCC without optimizations, and only 20% slower than GCC code at optimization level 3.

Chart of CompCert vs. GCC execution times for 23 benchmarks
Execution times of 23 benchmark programs compiled with ■ gcc -O0, ■ CompCert, ■ gcc -O1, and ■ gcc -O2

With CompCert it is possible to decrease the execution time of our flight control algo­rithms by a sig­nif­icant amount. The reduction of the execution time can be used for additional functionality.”

TU Munich, Institute of Flight System Dynamics, 2016

The computed WCET bounds lead to a total processor load which is about 28% smaller with the CompCert-generated code than with the code generated by the conventional compiler. The main reason for this behaviour is the improved memory performance. The result is consistent with our expectations and with previously published CompCert research papers.”

MTU Friedrichshafen, 2018

Supported architectures

CompCert produces machine code for ARM, PowerPC (32-bit and 32/64-bit hybrid), IA32 (x86 32-bit), and AMD64 (x86 64-bit).

Supported tool chains

To preprocess and produce object and executable files, an external C preprocessor, assembler and linker have to be provided. CompCert is currently tested for compatibility with:

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