The compilation manager --- as implemented by `compiler/cm' and used by tools such as `raco make', `raco setup', and DrRacket --- previously based its decision to recompile a file using only timestamps. It now uses SHA-1 hashes to reduce recompilation when timestamps change but source-file content doesn't. (This is useful with various git operations that change the timestamps on source files when the content doesn't change.)
If a source file's timestamp changes but its SHA-1 hash does not, then the bytecode version of the file is updated by simply setting its timestamp. Similarly, if the bytecode file for a dependency changes, but the bytecode's SHA-1 doesn't change and neither do any of its further dependencies (recursively), then the dependent bytecode file's timestamp can be adjusted instead of recompiling. When you run a program, the load handler still chooses between source and bytecode based just on the immediate timestamps. That's why the compilation manager updates the bytecode file's timestamp when it doesn't need to recompile from source. If you change a file by just editing a comment, whitespace, or something else that doesn't affect a module's meaning, then it's possible that the generated bytecode will be the same, in which case no file that depends on the changed file will need to be recompiled. Compilation uses various `gensym'-like operations, however, so I'm not sure how often the bytecode file will turn out to be the same. The format of a ".dep" file has changed to record relevant hashes. Specifically, a ".dep" file now records a SHA-1 hash for the source and a hash that combines information for the compiled form of each dependency. For each dependency, the combined hash includes the hash of its bytecode plus the combined hash from all of the dependency's dependencies as recorded in the dependency's ".dep" file. A SHA-1 hash is computed through the OpenSSL library, which we already use/include in our distribution. _________________________________________________ For list-related administrative tasks: http://list.cs.brown.edu/mailman/listinfo/plt-dev
