[project @ 2000-03-08 10:58:38 by simonmar]

Attempt to fix the Sparc version of StgRun.  Since
RESERVED_STACK_SPACE increased beyond the size of a 13-bit immediate,
the Sparc code has been broken.

We try to fix this with some imaginative gcc inline assembly
constraints.  If this breaks again, I'll try recoding it directly in
assembler.

While I'm here, pull in some of the comments from the old pre-4.00 RTS
and add some new comments.

ghc/rts/StgCRun.c

 /* -----------------------------------------------------------------------------
- * $Id: StgCRun.c,v 1.13 2000/03/07 11:35:36 simonmar Exp $
+ * $Id: StgCRun.c,v 1.14 2000/03/08 10:58:38 simonmar Exp $
  *
  * (c) The GHC Team, 1998-2000
  *
@@ -264,7 +264,33 @@ StgRun(StgFunPtr f, StgRegTable *basereg) {
 #endif
 
 /* -----------------------------------------------------------------------------
-   sparc architecture
+   Sparc architecture
+
+   -- 
+   OLD COMMENT from GHC-3.02:
+
+   We want tailjumps to be calls, because `call xxx' is the only Sparc
+   branch that allows an arbitrary label as a target.  (Gcc's ``goto
+   *target'' construct ends up loading the label into a register and
+   then jumping, at the cost of two extra instructions for the 32-bit
+   load.)
+
+   When entering the threaded world, we stash our return address in a
+   known location so that \tr{%i7} is available as an extra
+   callee-saves register.  Of course, we have to restore this when
+   coming out of the threaded world.
+
+   I hate this god-forsaken architecture.  Since the top of the
+   reserved stack space is used for globals and the bottom is reserved
+   for outgoing arguments, we have to stick our return address
+   somewhere in the middle.  Currently, I'm allowing 100 extra
+   outgoing arguments beyond the first 6.  --JSM
+
+   Updated info (GHC 4.06): we don't appear to use %i7 any more, so
+   I'm not sure whether we still need to save it.  Incedentally, what
+   does the last paragraph above mean when it says "the top of the
+   stack is used for globals"?  What globals?  --SDM
+
    -------------------------------------------------------------------------- */
 	
 #ifdef sparc_TARGET_ARCH
@@ -276,10 +302,23 @@ StgRun(StgFunPtr f, StgRegTable *basereg) {
     register void *i7 __asm__("%i7");
     ((void **)(space))[100] = i7;
     f();
-    __asm__ volatile (".align 4\n"		
+    __asm__ volatile (
+	    ".align 4\n"		
             ".global " STG_RETURN "\n"
-       	    STG_RETURN ":\n"
-    	    "\tld %1,%0" : "=r" (i7) : "m" (((void **)(space))[100]));
+       	    STG_RETURN ":" 
+	    : : : "l0","l1","l2","l3","l4","l5","l6","l7");
+    /* we tell the C compiler that l0-l7 are clobbered on return to
+     * StgReturn, otherwise it tries to use these to save eg. the
+     * address of space[100] across the call.  The correct thing
+     * to do would be to save all the callee-saves regs, but we
+     * can't be bothered to do that.
+     *
+     * The code that gcc generates for this little fragment is now
+     * terrible.  We could do much better by coding it directly in
+     * assembler.
+     */
+    __asm__ volatile ("ld %1,%0" 
+		      : "=r" (i7) : "m" (((void **)(space))[100]));
     return (StgThreadReturnCode)R1.i;
 }