Abstract: A compiler can instrument a program during the front end of compilation to collect data about runtime behavior when the program is executed. Instrumentation can occur during source language processing rather than later in the compilation chain. The instrumentation can occur in between semantic analysis and code generation. The instrumentation can occur in an instrumenting rewriting phase. The instrumentation can occur in a first lowering phase in which high level constructs in a programming language are rewritten as lower level constructs in intermediate representation (IR). The compiler can inject the instrumentation code at particular points in the source code based on specified language constructs in the source program. The types of instrumentation injected can be extended by providing additional information to the compiler at compilation time, without the need to reprogram the compiler.