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Diffstat (limited to 'libraries/ghc-heap/GHC/Exts/Heap/FFIClosures.hs')
| -rw-r--r-- | libraries/ghc-heap/GHC/Exts/Heap/FFIClosures.hs | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/libraries/ghc-heap/GHC/Exts/Heap/FFIClosures.hs b/libraries/ghc-heap/GHC/Exts/Heap/FFIClosures.hs new file mode 100644 index 0000000000..14004b0b1f --- /dev/null +++ b/libraries/ghc-heap/GHC/Exts/Heap/FFIClosures.hs @@ -0,0 +1,47 @@ +{-# LANGUAGE CPP #-} + +module GHC.Exts.Heap.FFIClosures (module Reexport) where + +-- NOTE [hsc and CPP workaround] +-- +-- # Problem +-- +-- Often, .hsc files are used to get the correct offsets of C struct fields. +-- Those structs may be affected by CPP directives e.g. profiled vs not profiled +-- closure headers is affected by the PROFILED cpp define. Since we are building +-- multiple variants of the RTS, we must support all possible offsets e.g. by +-- running hsc2hs with cpp defines corresponding to each RTS flavour. The +-- problem is that GHC's build system runs hsc2hs *only once* per .hsc file +-- without properly setting cpp defines. This results in the same (probably +-- incorrect) offsets into our C structs. +-- +-- +-- # Workaround +-- +-- To work around this issue, we create multiple .hsc files each manually +-- defining thir cpp defines (see e.g. FFIClosures_ProfilingDisabled.hsc and +-- FFIClosures_ProfilingEnabled.hsc). Then we rely on cpp defines working +-- correctly in .hs files and use CPP to switch on which .hsc module to +-- re-export (see below). In each case we import the desired .hsc module as +-- `Reexport` and we import `()` (i.e. nothing) from all other .hsc variants +-- just so that the build system sees all .hsc file as dependencies. +-- +-- +-- # Future Work +-- +-- * Duplication of the code in the .hsc files could be reduced simply by +-- placing the code in a single .hsc.in file and `#include`ing it from each +-- .hsc file. The .hsc files would only be responsible for setting the correct +-- cpp defines. This currently doesn't work as hadrian doesn't know to copy +-- the .hsc.in file to the build directory. +-- * The correct solution would be for the build system to run `hsc2hs` with the +-- correct cpp defines once per RTS flavour. +-- + +#if defined(PROFILING) +import GHC.Exts.Heap.FFIClosures_ProfilingEnabled as Reexport +import GHC.Exts.Heap.FFIClosures_ProfilingDisabled () +#else +import GHC.Exts.Heap.FFIClosures_ProfilingDisabled as Reexport +import GHC.Exts.Heap.FFIClosures_ProfilingEnabled () +#endif |