Split external symbol prototypes (EF_) (Trac #11395)

Before the patch both Cmm and C symbols were declared
with 'EF_' macro:

    #define EF_(f)    extern StgFunPtr f()

but for Cmm symbols we know exact prototypes.

The patch splits there prototypes in to:

    #define EFF_(f)   void f() /* See Note [External function prototypes] */
    #define EF_(f)    StgFunPtr f(void)

Cmm functions are 'EF_' (External Functions),
C functions are 'EFF_' (External Foreign Functions).

While at it changed external C function prototype
to return 'void' to workaround ghc bug on m68k.
Described in detail in Trac #11395.

This makes simple tests work on m68k-linux target!

Thanks to Michael Karcher for awesome analysis
happening in Trac #11395.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Test Plan: ran "hello world" on m68k successfully

Reviewers: simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1975

GHC Trac Issues: #11395

1 files changed, 17 insertions, 3 deletions

diff --git a/includes/Stg.h b/includes/Stg.h
index 899e685635..a8ab5ca8e8 100644
--- a/includes/Stg.h
+++ b/includes/Stg.h
@@ -222,11 +222,21 @@ typedef StgFunPtr       F_;
 #define II_(X)          static StgWordArray (X) GNU_ATTRIBUTE(aligned (8))
 #define IF_(f)    static StgFunPtr GNUC3_ATTRIBUTE(used) f(void)
 #define FN_(f)    StgFunPtr f(void)
-#define EF_(f)    extern StgFunPtr f()   /* See Note [External function prototypes] */
+#define EF_(f)    StgFunPtr f(void) /* External Cmm functions */
+#define EFF_(f)   void f() /* See Note [External function prototypes] */
 
-/* Note [External function prototypes]  See Trac #8965
+/* Note [External function prototypes]  See Trac #8965, #11395
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The external-function macro EF_(F) used to be defined as
+In generated C code we need to distinct between two types
+of external symbols:
+1.  Cmm functions declared by 'EF_' macro (External Functions)
+2.    C functions declared by 'EFF_' macro (External Foreign Functions)
+
+Cmm functions are simple as they are internal to GHC.
+
+C functions are trickier:
+
+The external-function macro EFF_(F) used to be defined as
     extern StgFunPtr f(void)
 i.e a function of zero arguments.  On most platforms this doesn't
 matter very much: calls to these functions put the parameters in the
@@ -249,6 +259,10 @@ unspecified argument list rather than a void argument list.  This is no
 worse for platforms that don't care either way, and allows a successful
 bootstrap of GHC 7.8 on little-endian Linux ppc64 (which uses the ELFv2
 ABI).
+
+Another case is m68k ABI where 'void*' return type is returned by 'a0'
+register while 'long' return type is returned by 'd0'. Thus we trick
+external prototype return neither of these types to workaround #11395.
 */