Version 2 (modified by simonmar, 7 years ago) (diff)

add bit about header files

GHC Commentary: The C code generator

Source: compiler/cmm/PprC.hs

This phase takes Cmm and generates plain C code. The C code generator is very simple these days, in fact it can almost be considered pretty-printing.

There are some slight subtleties:

  • info tables, which are expressed in Cmm as being laid out before the entry code for a closure, are compiled into separate top-level structures in the generated C, because C has no support for laying out data next to functions. The desired layout is reconstructed in the assembly file by the Evil Mangler, or not if we're compiling unregisterised (see TABLES_NEXT_TO_CODE).

Header files

GHC was changed (from version 6.10) so that the C backend no longer uses header files specified by the user in any way. The c-includes field of a .cabal file is ignored, as is the -#include flag on the command-line. There were several reasons for making this change:

This has several advantages:

  • -fvia-C is consistent with -fasm with respect to FFI declarations:

both bind to the ABI, not the API.

  • foreign calls can now be inlined freely across module boundaries, since a header file is not required when compiling the call.

  • bootstrapping via C will be more reliable, because this difference in behavour between the two backends has been removed.

There is one disadvantage:

  • we get no checking by the C compiler that the FFI declaration is correct.