We describe our experiences successfully applying lightweight formal methods to substantially improve and reformulate an important part of Standard Portable Intermediate Representation (SPIR-V), an industrystandard language for GPU computing. The formal model that we present has allowed us to (1) identify several ambiguities and needless complexities in the way that structured control flow was defined in the SPIR-V specification; (2) interact with the authors of the SPIR-V specification to rectify these problems; (3) validate the developer tools and conformance test suites that support the SPIR-V language by cross-checking them against our formal model, improving the tools, test suites, and our models in the process; and (4) develop a novel method for fuzzing SPIR-V compilers to detect miscompilation bugs that leverages our formal model. The latest release of the SPIR-V specification incorporates the revised set of control-flow definitions that have arisen from our work. Furthermore, our novel compiler-fuzzing technique has led to the discovery of twenty distinct, previously unknown bugs in SPIR-V compilers from Google, the Khronos Group, Intel, and Mozilla. Our work showcases the practical impact that formal modelling and analysis techniques can have on the design and implementation of industry-standard programming languages.