RFC: Adding VxWorks PIC support to various backends

[Richard Sandiford <richard at codesourcery dot com>]

CodeSourcery is gearing up to submit support for the VxWorks RTP PIC
model.  Six targets are affected: arm, i386, mips, rs6000, sh and sparc.
All this code is conditional on TARGET_VXWORKS_RTP being true and refers
to two other macros: VXWORKS_GOTT_BASE and VXWORKS_GOTT_INDEX.

Before submitting patches, I just wanted to ask how this code should
be integrated into the surrounding non-VxWorks code.  Some obvious
alternatives are:

  1) Wrap it in #ifdef TARGET_VXWORKS_RTP.

  2) Provide an always-false definition of TARGET_VXWORKS_RTP in
     arm.h, i386.h, etc.  Also provide dummy definitions of the
     VXWORKS_GOTT_* macros.

  3) Provide an always-false definition of TARGET_VXWORKS_RTP in
     defaults.h.  Also provide dummy definitions of the VXWORKS_GOTT_*
     macros.

  4) Like (3), but provide the real VxWorks default values of the
     VXWORKS_GOTT_* macros (rather than dummy ones).

but I'm open to others.

FWIW, I've attached a proto-patch for (4) below.  One good thing about
(4) is that it seems to be the least redundant option.  One bad thing
is that it leaks OS-specific stuff into the main part of the compiler.
I suppose we do have some precedent for that with the ELF-related code,
but I can see that it's not something we want to encourage.

Richard


gcc/
	* defaults.h (TARGET_VXWORKS_RTP, VXWORKS_GOTT_BASE)
	(VXWORKS_GOTT_INDEX): Define.
	* config/vxworks.h (VXWORKS_GOTT_BASE, VXWORKS_GOTT_INDEX): Delete.
	* doc/tm.texi (TARGET_VXWORKS_RTP, VXWORKS_GOTT_BASE
	(VXWORKS_GOTT_INDEX): Document.

Index: gcc/defaults.h
===================================================================
--- gcc/defaults.h	(revision 122476)
+++ gcc/defaults.h	(working copy)
@@ -906,4 +906,14 @@ #define HARD_REGNO_NREGS_HAS_PADDING(REG
 #define HARD_REGNO_NREGS_WITH_PADDING(REGNO, MODE) -1
 #endif
 
+#ifndef TARGET_VXWORKS_RTP
+#define TARGET_VXWORKS_RTP false
+#endif
+#ifndef VXWORKS_GOTT_BASE
+#define VXWORKS_GOTT_BASE "__GOTT_BASE__"
+#endif
+#ifndef VXWORKS_GOTT_INDEX
+#define VXWORKS_GOTT_INDEX "__GOTT_INDEX__"
+#endif
+
 #endif  /* ! GCC_DEFAULTS_H */
Index: gcc/config/vxworks.h
===================================================================
--- gcc/config/vxworks.h	(revision 122473)
+++ gcc/config/vxworks.h	(working copy)
@@ -97,11 +97,4 @@ #define TARGET_ASM_DESTRUCTOR vxworks_as
 extern void vxworks_asm_out_constructor (rtx symbol, int priority);
 extern void vxworks_asm_out_destructor (rtx symbol, int priority);
 
-/* The name of the symbol for the table of GOTs in a particular
-   RTP.  */
-#define VXWORKS_GOTT_BASE "__GOTT_BASE__"
-/* The name of the symbol for the index into the table of GOTs for the
-   GOT associated with the current shared library.  */
-#define VXWORKS_GOTT_INDEX "__GOTT_INDEX__"
-
 #define VXWORKS_KIND VXWORKS_KIND_NORMAL
Index: gcc/doc/tm.texi
===================================================================
--- gcc/doc/tm.texi	(revision 122473)
+++ gcc/doc/tm.texi	(working copy)
@@ -53,6 +53,7 @@ through the macros defined in the @file{
 * MIPS Coprocessors::   MIPS coprocessor support and how to customize it.
 * PCH Target::          Validity checking for precompiled headers.
 * C++ ABI::             Controlling C++ ABI changes.
+* VxWorks::             VxWorks-specific macros.
 * Misc::                Everything else.
 @end menu
 
@@ -9176,6 +9177,29 @@ defined.  Use this hook to make adjustme
 visibility or perform any other required target modifications).
 @end deftypefn
 
+@node VxWorks
+@section VxWorks-specific Parameters
+@cindex VxWorks
+
+Several backends have code to support the VxWorks PIC model.
+Rather than hide this code in preprocessor conditions, it is
+convenient to ensure that certain cross-target macros are always
+defined.
+
+@defmac TARGET_VXWORKS_RTP
+True if generating code for a VxWorks RTP.
+@end defmac
+
+@defmac VXWORKS_GOTT_BASE
+A symbol that points to the table of GOTs used in RTP PIC.
+The default is @code{__GOTT_BASE__}.
+@end defmac
+
+@defmac VXWORKS_GOTT_INDEX
+A symbol that gives the index of the current module's GOT in
+@code{VXWORKS_GOTT_BASE}.  The default is @code{__GOTT_INDEX__}.
+@end defmac
+
 @node Misc
 @section Miscellaneous Parameters
 @cindex parameters, miscellaneous