Abstract: Techniques used in testing software are described. A test generator produces a source program used to test two or more compilers using a differential testing technique. The test generator includes a grammar with added semantic constraints to minimize the generation of non-conforming source programs. The source program is a conforming source program conforming to constraints included in a programming language standard. By using properties of a conforming source code, a differential testing technique is described in which a test failure indicates that one or more of the compilers is not processing the source program correctly in accordance with the programming language standard. If a test failure is detected, the source program causing the test failure is reduced using various reduction and simplification techniques.