sccache/docs/Caching.md

How caching works

To know if the storage contains the artifact we need, we are computing some hashes to make sure the input is the same.

Because the configuration and environment matter, the hash computation takes a few parameters into account.

How hash keys are computed.

Rust

We generate a blake3 digest for each file compiled. In parallel, we also take into account in the hash:

• Path to the rustc executable
• Host triple for this rustc
• Path to the rustc sysroot
• digests of all the shared libraries in rustc's $sysroot/lib
• A shared, caching reader for rlib dependencies (for dist-client)
• Parsed arguments from the rustc invocation See 8567bbe2ba/src/compiler/rust.rs (L122) for the full list

C/C++ compiler

For C/C++, the hash is generated with a blake3 digest of the preprocessed file (-E with gcc/clang). For compilations that specify multiple -arch flags, these flags are rewritten to their corresponding preprocessor defines to allow pre-processing the file (e.g -arch x86_64 is rewritten to -D__X86_64__=1).

We also take into account in the hash:

• Hash of the compiler binary
• Programming language
• Flag required to compile for the given language
• File in which to generate dependencies.
• Commandline arguments for dependency generation
• Commandline arguments for the preprocessor
• Commandline arguments specifying the architecture to compile for
• Extra files that need to have their contents hashed
• Whether the compilation is generating profiling or coverage data
• Color mode
• Environment variables See 8567bbe2ba/src/compiler/c.rs (L84)