wiki:Commentary/Compiler/Backends/PprC

Version 3 (modified by simonmar, 6 years ago) (diff)

add information about prototypes

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.

Prototypes

When a label is referenced by an expression, the compiler needs to know whether to declare the label first, and if so, at what type.

  • all labels referenced as a result of an FFI declaration are declared as extern StgWord[], including funciton labels. If the label is called, it is first cast to the correct function type. This is because the same label might be referred to both as a function and an untyped data label in the same module (e.g. Foreign.Marsal.Alloc refers to "free" this way).
  • all RTS symbols already have declarations (mostly with the correct type) in includes/StgMiscClosures.h, so no declarations are generated.
  • certain labels are known to have been defined earlier in the same file, so a declaration can be omitted (e.g. SRT labels)
  • certain math functions (sin(), cos() etc.) are already declared because we #include math.h, so we don't emit declarations for these. We need to #include math.h because some of these fuctions have inline definitions, and we get terrible code otherwise.

When compiling the RTS cmm code, we have almost no information about labels referenced in the code. The only information we have is whether the label is defined in the RTS or in another package: a label that is declared with an import statement in the .cmm file is assumed to be defined in another package (this is for dynamic linking, where we need to emit special code to reference these labels).

For all other labels referenced by RTS .cmm code, we assume they are RTS labels, and hence already declared in includes/StgMiscClosures.h. This is the only choice here: since we don't know the type of the label (info, entry etc.), we can't generate a correct declaration.