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Deterministic Inputs.md

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Deterministic Inputs

We are aiming to make the compilers ultimately deterministic (#372). What that means is that the "same inputs" will cause the compilers to produce the "same outputs".

The following are considered inputs to the compiler for the purpose of determinism:

  • The sequence of command-line parameters
  • The contents of the compiler's .rsp response file.
  • The precise version of the compiler used, and its referenced assemblies
  • Current full directory path (you can reduce this to a relative path; see #949)
  • (Binary) contents of all files explicitly passed to the compiler, directly or indirectly, including
    • source files
    • referenced assemblies
    • referenced modules
    • resources
    • the strong name key file
    • @ response files
    • Analyzers
    • Rulesets
    • "additional files" that may be used by analyzers
  • The current culture (for the language in which diagnostics and exception messages are produced).
  • The default encoding (or the current code page) if the encoding is not specified
  • The existence, non-existence, and contents of files on the compiler's search paths (specified, e.g. by /lib or /recurse)
  • The CLR platform on which the compiler is run (e.g. the result of double arithmetic performed for constant-folding may use excess precision on some platforms).
  • The value of %LIBPATH%, as it can affect analyzer dependency loading.

At the moment the compiler also depends on the time of day and random numbers for GUIDs, so it is not deterministic unless you specify /deterministic.