iface/FlagChecker.hs

    {-# LANGUAGE RecordWildCards #-}

    -- | This module manages storing the various GHC option flags in a modules
    -- interface file as part of the recompilation checking infrastructure.
    module FlagChecker (
            fingerprintDynFlags
          , fingerprintOptFlags
          , fingerprintHpcFlags
        ) where

    import GhcPrelude

    import Binary
    import DynFlags
    import HscTypes
    import Module
    import Name
    import Fingerprint
    import BinFingerprint
    -- import Outputable

    import qualified EnumSet
    import System.FilePath (normalise)

    -- | Produce a fingerprint of a @DynFlags@ value. We only base
    -- the finger print on important fields in @DynFlags@ so that
    -- the recompilation checker can use this fingerprint.
    --
    -- NB: The 'Module' parameter is the 'Module' recorded by the
    -- *interface* file, not the actual 'Module' according to our
    -- 'DynFlags'.
    fingerprintDynFlags :: DynFlags -> Module
                        -> (BinHandle -> Name -> IO ())
                        -> IO Fingerprint

    fingerprintDynFlags dflags@DynFlags{..} this_mod nameio =
        let mainis   = if mainModIs == this_mod then Just mainFunIs else Nothing
                          -- see #5878
            -- pkgopts  = (thisPackage dflags, sort $ packageFlags dflags)
            safeHs   = setSafeMode safeHaskell
            -- oflags   = sort $ filter filterOFlags $ flags dflags

            -- *all* the extension flags and the language
            lang = (fmap fromEnum language,
                    map fromEnum $ EnumSet.toList extensionFlags)

            -- -I, -D and -U flags affect CPP
            cpp = ( map normalise $ flattenIncludes includePaths
                -- normalise: eliminate spurious differences due to "./foo" vs "foo"
                  , picPOpts dflags
                  , opt_P_signature dflags)
                -- See Note [Repeated -optP hashing]

            -- Note [path flags and recompilation]
            paths = [ hcSuf ]

            -- -fprof-auto etc.
            prof = if gopt Opt_SccProfilingOn dflags then fromEnum profAuto else 0

            -- Ticky
            ticky =
              map (`gopt` dflags) [Opt_Ticky, Opt_Ticky_Allocd, Opt_Ticky_LNE, Opt_Ticky_Dyn_Thunk]

            flags = ((mainis, safeHs, lang, cpp), (paths, prof, ticky, debugLevel))

        in -- pprTrace "flags" (ppr flags) $
           computeFingerprint nameio flags

    -- Fingerprint the optimisation info. We keep this separate from the rest of
    -- the flags because GHCi users (especially) may wish to ignore changes in
    -- optimisation level or optimisation flags so as to use as many pre-existing
    -- object files as they can.
    -- See Note [Ignoring some flag changes]
    fingerprintOptFlags :: DynFlags
                          -> (BinHandle -> Name -> IO ())
                          -> IO Fingerprint
    fingerprintOptFlags DynFlags{..} nameio =
          let
            -- See https://gitlab.haskell.org/ghc/ghc/issues/10923
            -- We used to fingerprint the optimisation level, but as Joachim
            -- Breitner pointed out in comment 9 on that ticket, it's better
            -- to ignore that and just look at the individual optimisation flags.
            opt_flags = map fromEnum $ filter (`EnumSet.member` optimisationFlags)
                                              (EnumSet.toList generalFlags)

          in computeFingerprint nameio opt_flags

    -- Fingerprint the HPC info. We keep this separate from the rest of
    -- the flags because GHCi users (especially) may wish to use an object
    -- file compiled for HPC when not actually using HPC.
    -- See Note [Ignoring some flag changes]
    fingerprintHpcFlags :: DynFlags
                          -> (BinHandle -> Name -> IO ())
                          -> IO Fingerprint
    fingerprintHpcFlags dflags@DynFlags{..} nameio =
          let
            -- -fhpc, see https://gitlab.haskell.org/ghc/ghc/issues/11798
            -- hpcDir is output-only, so we should recompile if it changes
            hpc = if gopt Opt_Hpc dflags then Just hpcDir else Nothing

          in computeFingerprint nameio hpc

    {- Note [path flags and recompilation]

    There are several flags that we deliberately omit from the
    recompilation check; here we explain why.

    -osuf, -odir, -hisuf, -hidir
      If GHC decides that it does not need to recompile, then
      it must have found an up-to-date .hi file and .o file.
      There is no point recording these flags - the user must
      have passed the correct ones.  Indeed, the user may
      have compiled the source file in one-shot mode using
      -o to specify the .o file, and then loaded it in GHCi
      using -odir.

    -stubdir
      We omit this one because it is automatically set by -outputdir, and
      we don't want changes in -outputdir to automatically trigger
      recompilation.  This could be wrong, but only in very rare cases.

    -i (importPaths)
      For the same reason as -osuf etc. above: if GHC decides not to
      recompile, then it must have already checked all the .hi files on
      which the current module depends, so it must have found them
      successfully.  It is occasionally useful to be able to cd to a
      different directory and use -i flags to enable GHC to find the .hi
      files; we don't want this to force recompilation.

    The only path-related flag left is -hcsuf.
    -}

    {- Note [Ignoring some flag changes]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Normally, --make tries to reuse only compilation products that are
    the same as those that would have been produced compiling from
    scratch. Sometimes, however, users would like to be more aggressive
    about recompilation avoidance. This is particularly likely when
    developing using GHCi (see #13604). Currently, we allow users to
    ignore optimisation changes using -fignore-optim-changes, and to
    ignore HPC option changes using -fignore-hpc-changes. If there's a
    demand for it, we could also allow changes to -fprof-auto-* flags
    (although we can't allow -prof flags to differ). The key thing about
    these options is that we can still successfully link a library or
    executable when some of its components differ in these ways.

    The way we accomplish this is to leave the optimization and HPC
    options out of the flag hash, hashing them separately.
    -}

    {- Note [Repeated -optP hashing]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    We invoke fingerprintDynFlags for each compiled module to include
    the hash of relevant DynFlags in the resulting interface file.
    -optP (preprocessor) flags are part of that hash.
    -optP flags can come from multiple places:

      1. -optP flags directly passed on command line.
      2. -optP flags implied by other flags. Eg. -DPROFILING implied by -prof.
      3. -optP flags added with {-# OPTIONS -optP-D__F__ #-} in a file.

    When compiling many modules at once with many -optP command line arguments
    the work of hashing -optP flags would be repeated. This can get expensive
    and as noted on #14697 it can take 7% of time and 14% of allocations on
    a real codebase.

    The obvious solution is to cache the hash of -optP flags per GHC invocation.
    However, one has to be careful there, as the flags that were added in 3. way
    have to be accounted for.

    The current strategy is as follows:

      1. Lazily compute the hash of sOpt_p in sOpt_P_fingerprint whenever sOpt_p
         is modified. This serves dual purpose. It ensures correctness for when
         we add per file -optP flags and lets us save work for when we don't.
      2. When computing the fingerprint in fingerprintDynFlags use the cached
         value *and* fingerprint the additional implied (see 2. above) -optP flags.
         This is relatively cheap and saves the headache of fingerprinting all
         the -optP flags and tracking all the places that could invalidate the
         cache.
    -}