# Compilation caching (CompilationCaching)

Errors and warnings related to the compiler's CAS compilation caching.


## Overview

Compilation caching allows Swift compiler to reuse previously compiled outputs if the identical
compilation has been done before.

When compilation caching is enabled (via `-cache-compile-job` flag), the build commands are
expected to be generated by Swift driver, and the command-line of compilation tasks contains
CASIDs that are objects constructed during planning phase. These CAS objects need to be backed
by CAS storage and in a specific scheme accepted by the compiler. Any malformed command-line
will cause error in this category. If the inputs changed, always fallback to re-run `swiftc`
command to plan the new build commands.

The other category of error is unexpected failures when performing reads/writes to Content
Addressable Storage. Please refer to the error message (contains errors provided by CAS storage) for clues.
If the error persists, consider clearing the cache. To clear the cache, delete the directory
pointed to by the `-cas-path` option. Note that the CAS directory should only be deleted as a whole when not used.