--- a/src/cpu/sparc/vm/vtableStubs_sparc.cpp	Tue Apr 07 19:04:24 2009 -0700
+++ b/src/cpu/sparc/vm/vtableStubs_sparc.cpp	Wed Apr 08 00:12:59 2009 -0700
@@ -114,6 +114,9 @@
                   (int)(s->code_end() - __ pc()));
   }
   guarantee(__ pc() <= s->code_end(), "overflowed buffer");
+  // shut the door on sizing bugs
+  int slop = 2*BytesPerInstWord;  // 32-bit offset is this much larger than a 13-bit one
+  assert(vtable_index > 10 || __ pc() + slop <= s->code_end(), "room for sethi;add");
 
   s->set_exception_points(npe_addr, ame_addr);
   return s;
@@ -208,6 +211,9 @@
                   (int)(s->code_end() - __ pc()));
   }
   guarantee(__ pc() <= s->code_end(), "overflowed buffer");
+  // shut the door on sizing bugs
+  int slop = 2*BytesPerInstWord;  // 32-bit offset is this much larger than a 13-bit one
+  assert(itable_index > 10 || __ pc() + slop <= s->code_end(), "room for sethi;add");
 
   s->set_exception_points(npe_addr, ame_addr);
   return s;
@@ -233,6 +239,50 @@
       return (basic + slop);
     }
   }
+
+  // In order to tune these parameters, run the JVM with VM options
+  // +PrintMiscellaneous and +WizardMode to see information about
+  // actual itable stubs.  Look for lines like this:
+  //   itable #1 at 0x5551212[116] left over: 8
+  // Reduce the constants so that the "left over" number is 8
+  // Do not aim at a left-over number of zero, because a very
+  // large vtable or itable offset (> 4K) will require an extra
+  // sethi/or pair of instructions.
+  //
+  // The JVM98 app. _202_jess has a megamorphic interface call.
+  // The itable code looks like this:
+  // Decoding VtableStub itbl[1]@16
+  //   ld  [ %o0 + 4 ], %g3
+  //   save  %sp, -64, %sp
+  //   ld  [ %g3 + 0xe8 ], %l2
+  //   sll  %l2, 2, %l2
+  //   add  %l2, 0x134, %l2
+  //   and  %l2, -8, %l2        ! NOT_LP64 only
+  //   add  %g3, %l2, %l2
+  //   add  %g3, 4, %g3
+  //   ld  [ %l2 ], %l5
+  //   brz,pn   %l5, throw_icce
+  //   cmp  %l5, %g5
+  //   be  %icc, success
+  //   add  %l2, 8, %l2
+  // loop:
+  //   ld  [ %l2 ], %l5
+  //   brz,pn   %l5, throw_icce
+  //   cmp  %l5, %g5
+  //   bne,pn   %icc, loop
+  //   add  %l2, 8, %l2
+  // success:
+  //   ld  [ %l2 + -4 ], %l2
+  //   ld  [ %g3 + %l2 ], %l5
+  //   restore  %l5, 0, %g5
+  //   ld  [ %g5 + 0x44 ], %g3
+  //   jmp  %g3
+  //   nop
+  // throw_icce:
+  //   sethi  %hi(throw_ICCE_entry), %g3
+  //   ! 5 more instructions here, LP64_ONLY
+  //   jmp  %g3 + %lo(throw_ICCE_entry)
+  //   restore
 }