Compiler Requirements

Compiler Requirements

Correctness.

Correctness is absolutely paramount. A buggy compiler is next to useless in practice. Since we cannot formally prove the correctness of your compilers, we use extensive testing. This testing is end-to-end, verifying the correctness of the generated code on sample inputs. We also verify that your compiler rejects programs as expected when the input is not well-formed (lexically, syntactically, or with respect to the static semantics), and that the generated code raises an exception as expected if the language specification prescribes this. We go so far as to test that your generated code fails to terminate (with a time-out) when the source program should diverge.

Emphasis on correctness means that we very carefully define the semantics of the source language. The semantics of the target language is given by the GNU assembler on the lab machines together with the semantics of the actually machine. Unlike C, we try to make sure that as little as possible about the source language remains undefined. This is not just for testability, but also good language design practice since an unambiguously defined language is portable. The only part we do not fully define are precise resource constraints regarding the generated code (for example, the amount of memory available).

Efficiency.

 In a production compiler, efficiency of the generated code and also efficiency of the compiler itself are important considerations. In this course, we set very lax targets for both, emphasizing correctness instead. In one of the later labs in the course, you will have the opportunity to optimize the generated code.

The early emphasis on correctness has consequences for your approach to the design of the implementation. Modularity and simplicity of the code are important for two reasons: first, your code is much more likely to be correct, and, second, you will be able to respond to changes in the source language specification from lab to lab much more easily.

Interoperability.

Programs do not run in isolation, but are linked with library code before they are executed, or will be called as a library from other code. This puts some additional requirements on the compiler, which must respect certain interface specifications.

Your generated code will be required to execute correctly in the environment on the lab machines. This means that you will have to respect calling conventions early on (for example, properly save callee-save registers) and data layout conventions later, when your code will be calling library functions. You will have to carefully study the ABI specification as it applies to C and our target architecture.

Usability.

A compiler interacts with the programmer primarily when there are errors in the program. As such, it should give helpful error messages. Also, compilers may be instructed to generate debug information together with executable code in order help users debug runtime errors in their pro- gram.

In this course, we will not formally evaluate the quality or detail of your error messages, although you should strive to achieve at least a minimum standard so that you can use your own compiler effectively.

Retargetability.

At the outset, we think of a compiler of going from one source language to one target language. In practice, compilers may be required to generate more than one target from a given source (for example,x86-64 and ARM code), sometimes at very different levels of abstraction (for example, x86-64 assembly or LLVM intermediate code).

In this course we will deemphasize retargetability, although if you structure your compiler following the general outline presented in the next section, it should not be too difficult to retrofit another code generator. One of the options for the last lab in this course is to retarget your compiler to pro- duce code in a low-level virtual machine (LLVM). Using LLVM tools this means you will be able to produce efficient binaries for a variety of concrete machine architectures.