Interface hashes include time stamp of dependent files (UsageFile mtime)

See https://github.com/nh2/ghc-bug-time-dependent-interface-hashes for a fast test case.

Having Test.hs:

{-# LANGUAGE CPP #-}

module Test () where

and an empty cabal_macros.h,

touch cabal_macros.h
ghc -c Test.hs -optP-include -optPcabal_macros.h

will generate you a different interface hash most of the time in ghc --show-iface Test.hi.

It looks like the modification time of the .h file makes it into the interface hash, but only at second resolution, and you get compilation IS NOT required if you manage to run it twice per second.

I think the interface hash should be independent from the time stamp of the header file.

This bug can trigger unnecessary recompilation and makes compilations with identical inputs non-deterministic.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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