GHC Commentary: The C code generator
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. It is only used for unregisterised compilers.
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:
- Via C compilation is consistent with the other backend with respect to FFI declarations: all 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 behavior between the two backends has been removed.
There are some disadvantages:
- we get no checking by the C compiler that the FFI declaration is correct.
- we can't benefit from inline definitions in header files.
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.
C only lets us declare an external label at one type in any given source file, even if the scopes of the declarations don't overlap. So we either have to scan the whole code to figure out what the type of each label should be, or we opt for declaring all labels at the same type and then casting later. Currently we do the latter.
- all labels referenced as a result of an FFI declaration are declared as extern StgWord, including function 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).
- An exception is made to the above for functions declared with the stdcall calling convention on Windows. These functions must be declared with the stdcall attribute and a function type, otherwise the C compiler won't add the @n suffix to the symbol. We can't add the @n suffix ourselves, because it is illegal syntax in C. However, we always declare these labels with the type void (*)(void), to avoid conflicts if the same function is called at different types in one module (see Graphics.Win32.GDI.HDC.SelectObject).
- Another exception is made for functions that are marked never returns in C--. We have to put an __attribute__((noreturn)) on the declaration for these functions, and it only works if the function is declared with a proper function type and called without casting it to/from a pointer. So only the correct prototype will do here.
- 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 functions 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.