DEFAULT_VTABLE_THUNKS ?

[Jim Pick <jim@jimpick.com>]

Hi,

I rebuilt egcs, and it still had the same problem.

I looked into the code, and I can see in gcc/cp/cp-tree.h:

cp-tree.h:#define VTABLE_NAME_FORMAT (flag_vtable_thunks ? "__vt_%s" : "_vt$%s")

The flag_vtable_thunks variable gets set if the -fvtable-thunks option
is used when invoking egcs, or if the DEFAULT_VTABLE_THUNKS variable
is set when building egcs.

The egcs info file says this about DEFAULT_VTABLE_THUNKS:

     GNU CC supports two ways of implementing C++ vtables:  traditional
     or with so-called "thunks".  The flag `-fvtable-thunk' chooses
     between them.  Define this macro to be a C expression for the
     default value of that flag.  If `DEFAULT_VTABLE_THUNKS' is 0, GNU
     CC uses the traditional implementation by default.  The "thunk"
     implementation is more efficient (especially if you have provided
     an implementation of `ASM_OUTPUT_MI_THUNK', see *Note Function
     Entry::), but is not binary compatible with code compiled using
     the traditional implementation.  If you are writing a new ports,
     define `DEFAULT_VTABLE_THUNKS' to 1.

     If you do not define this macro, the default for `-fvtable-thunk'
     is 0.

config/mt-linux says:

  # When using glibc 2 on Linux we must always use vtable thunks.
  CXXFLAGS_FOR_TARGET = $(CXXFLAGS) -fvtable-thunks -D_GNU_SOURCE

So this causes parts of egcs to be compiled with -fvtable-thunks.
(But apparently not all the parts)

It seems that DEFAULT_VTABLE_THUNKS is normally set to 1 for the other
Linux ports.

./linux.h:#undef DEFAULT_VTABLE_THUNKS
./linux.h:#define DEFAULT_VTABLE_THUNKS 1
./alpha/linux-elf.h:#undef DEFAULT_VTABLE_THUNKS
./alpha/linux-elf.h:#define DEFAULT_VTABLE_THUNKS 1
./rs6000/linux.h:#undef DEFAULT_VTABLE_THUNKS
./rs6000/linux.h:#define DEFAULT_VTABLE_THUNKS 1
./sparc/linux.h:#undef DEFAULT_VTABLE_THUNKS
./sparc/linux.h:#define DEFAULT_VTABLE_THUNKS 1

However, for the ARM port, it doesn't seem to be enabled.

When I recompile the offending elements that were messing up in
libgcc.a with -fvtable-tunks, the symbols used the newer mangling
scheme.

I checked Corel's gcc stuff, and DEFAULT_VTABLE_THUNKS is defined
there.  So, I'm going to try this patch:

  
--- linux-elf.h~        Tue Jan  5 15:48:06 1999
+++ linux-elf.h Tue Jan  5 20:09:22 1999
@@ -35,6 +35,10 @@
        " %{mapcs-26:-mapcs-26} %(!mapcs-26:-mapcs-32}"
 #endif
 
+/* This was defined in linux.h.  Define it here also. */
+#undef DEFAULT_VTABLE_THUNKS
+#define DEFAULT_VTABLE_THUNKS   1
+
 /* Now we define the strings used to build the spec file */
 
 #define ASM_SPEC "%{mbig-endian:-EB} "


I'll recompile egcs, and tell you all if it worked.

Cheers,

 - Jim