Compilers are part of the foundation upon which software systems are built; they need to be as correct as possible. This paper is about stress-testing loop optimizers; it presents a major reimplementation of Yet Another Random Program Generator (YARPGen), an open-source generative compiler fuzzer. This new version has found 122 bugs, both in compilers for data-parallel languages, such as the Intel® Implicit SPMD Program Compiler and the Intel® oneAPI DPC++ compiler, and in C++ compilers such as GCC and Clang/LLVM. The first main contribution of our work is a novel method for statically avoiding undefined behavior when generating loops; the resulting programs conform to the relevant language standard, enabling automated testing. The second main contribution is a collection of mechanisms for increasing the diversity of generated loop code; in our evaluation, we demonstrate that these make it possible to trigger loop optimizations significantly more often, providing opportunities to discover bugs in the optimizers.

Compilers should not crash and they should not miscompile applications. Random testing is an effective method for finding compiler bugs that have escaped other kinds of testing. This paper presents Yet Another Random Program Generator (YARPGen), a random test-case generator for C and C++ that we used to find and report more than 220 bugs in GCC, LLVM, and the Intel® C++ Compiler. Our research contributions include a method for generating expressive programs that avoid undefined behavior without using dynamic checks, and generation policies, a mechanism for increasing diversity of generated code and for triggering more optimizations. Generation policies decrease the testing time to find hard-to-trigger compiler bugs and, for the kinds of scalar optimizations YARPGen was designed to stress-test, increase the number of times these optimizations are applied by the compiler by an average of 20% for LLVM and 40% for GCC. We also created tools for automating most of the common tasks related to compiler fuzzing; these tools are also useful for fuzzers other than ours.