The current ZL prototype supports most of C and an important subset of C++. For C, the only major feature not supported is bitfields, mainly because the need has not arisen. C++ is a rather complicated language, and fully implementing it correctly is beyond the scope of our research. We aim to implement enough of C++ to demonstrate our approach; in particular, we support single inheritance, but currently do not support multiple inheritance, exceptions, or templates.
As ZL is at present only a prototype compiler, the overall compile time (in version 0.3) when compared to compiling with GCC 4.4 is 2 to 3 times slower. However, ZL is designed to have little to no impact on the resulting code. ZL’s macro system imposes no run-time overhead.
The ZL compiler transforms higher level ZL into a low-level S-expression-like language that can best be described as C with Scheme syntax. Syntactically, the output is very similar to fully expanded ZL as shown in Figure 3.2. The transformed code is then passed to a modified version of GCC 4.4. When pure C is passed in we are very careful to avoid any transformations that might affect performance. The class macro currently implements a C++ ABI that is comparable to a traditional ABI, and hence should have no impact on performance.
To demonstrate that ZL can support C programs, two well-known programs were compiled with ZL: bzip2 and gzip. Bzip2 was compiled without modifications, but gzip required some minor modification because it was an older C program and used some C syntax that is not a subset of C++: K&R-style function declarations were transformed into the newer ANSI C style, and one instance of new as a variable was renamed to new_.
Overall, compile times were 2 to 3 times slower with ZL in comparison to compiling with GCC 4.4. However, both programs compiled correctly, produced correct results, and had similar run times to the GCC-compiled versions.
To evaluate ZL’s suitability to compile C++ programs, we chose to compile randprog , which is a small C++ program that generates random C programs. Randprog uses inheritance and other important C++ features, such as overloading and nondefault constructors. In addition, it uses a few C++ features that ZL does not yet support, so we changed randprog in small ways to compensate. These changes include reworking the command-line argument parsing, which used of a library that requires many modern C++ features; explicit instantiation of vector instances; changing uses of the for_each template function into normal for loops; and reworking some functions to avoid returning complex objects.
Randprog was verified to produce correct results by fixing the seed and comparing the generated program with a version of randprog compiled with GCC for several different seeds. It was also instrumented with Valgrind and found free of memory errors.
Overall compile time was around 2.5 times slower with ZL when compared to GCC 4.4. A direct run-time performance comparison is of limited usefulness, since ZL does not use the same C++ library as GCC, but the runtime performance of the ZL-compiled version of randprog was up to twice as fast as the GCC-compiled version.
In error messages, ZL provides a full backtrace of what was expanded from where in the case of a compile time error involving macros.
ZL provides very basic source level debugging support. ZL makes an effort to keep track of line numbers and passes this information onto the debugger to provide for meaningful backtrace. In addition, variable names are also available, but in most cases the name has been mangled. For most cases this means adding a $ and a number to the name.
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