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12.4 Limitations of the cross-referencing facility

No cross-reference information is available for interpreted functions. The cross-referencing database is not persistent: unless you save an image using save-lisp, the database will be empty each time CMUCL is restarted. There is no mechanism that saves cross-reference information in FASL files, so loading a system from compiled code will not populate the cross-reference database. The XREF database currently accumulates “stale” information: when compiling a file, it does not delete any cross-references that may have previously been generated for that file. This latter limitation will be removed in a future release.

The cross-referencing facility is only able to analyze the static dependencies in a program; it does not provide any information about runtime (dynamic) dependencies. For instance, XREF is able to identify the list of program contexts where a given function may be called, but is not able to determine which contexts will be activated when the program is executed with a specific set of input parameters. However, the static analysis that is performed by the CMUCL compiler does allow XREF to provide more information than would be available from a mere syntactic analysis of a program. References that occur from within unreachable code will not be displayed by XREF, because the CMUCL compiler deletes dead code before cross-references are analyzed. Certain “trivial” function calls (where the result of the function call can be evaluated at compile-time) may be eliminated by optimizations carried out by the compiler; see the example below.

If you examine the entire database of cross-reference information (by accessing undocumented internals of the XREF package), you will note that XREF notes “bogus” cross-references to function calls that are inserted by the compiler. For example, in safe code, the CMUCL compiler inserts a call to an internal function called c::%verify-argument-count, so that the number of arguments passed to the function is checked each time it is called. The XREF facility does not distinguish between user code and these forms that are introduced during compilation. This limitation should not be visible if you use the documented functions in the XREF package.

As of the 18e release of CMUCL, the cross-referencing facility is experimental; expect details of its implementation to change in future releases. In particular, the names given to CLOS methods and to inner functions will change in future releases.