[rfc] Only compare against dummy frame's top when explicitly set

Hello,

To follow up an earlier thread, the attached revised patch modifies 
generic_find_dummy_frame() so that it only tests TOP when it was set.
However, this time it uses the value of TOP instead of 
find_dummy_frame_p().  I think this is more robust since, as with the 
ia64, it allows an architecture to explicitly set TOS (rather than rely 
on hand_function_call() to set it).

I'm intending to commit this in a few days (~monday).  I should note 
that there is evidence that the change, while arguably correct, will 
break something.  cf, the fixes I recently made to the rs6000 because 
the change flushes out bugs in that target that the incorrect test was 
hiding.

Thoughts before the commit?

Andrew
2002-09-25  Andrew Cagney  <ac131313@xxxxxxxxxx>

        * blockframe.c (generic_find_dummy_frame): Rewrite.  Only test
        against TOP when TOP was explictly set.
        (generic_push_dummy_frame): Set TOP to zero.

Index: blockframe.c
===================================================================
RCS file: /cvs/src/src/gdb/blockframe.c,v
retrieving revision 1.40
diff -u -r1.40 blockframe.c
--- blockframe.c        17 Sep 2002 20:42:01 -0000      1.40
+++ blockframe.c        26 Sep 2002 16:19:30 -0000
@@ -1151,8 +1151,8 @@
 
 /* Function: find_dummy_frame(pc, fp, sp)
 
-   Search the stack of dummy frames for one matching the given PC, FP
-   and SP.  Unlike PC_IN_CALL_DUMMY, this function doesn't need to
+   Search the stack of dummy frames for one matching the given PC and
+   FP/SP.  Unlike PC_IN_CALL_DUMMY, this function doesn't need to
    adjust for DECR_PC_AFTER_BREAK.  This is because it is only legal
    to call this function after the PC has been adjusted.  */
 
@@ -1163,12 +1163,37 @@
 
   for (dummyframe = dummy_frame_stack; dummyframe != NULL;
        dummyframe = dummyframe->next)
-    if ((pc >= dummyframe->call_lo && pc < dummyframe->call_hi)
-       && (fp == dummyframe->fp
-           || fp == dummyframe->sp
-           || fp == dummyframe->top))
-      /* The frame in question lies between the saved fp and sp, inclusive */
+    {
+      /* Does the PC fall within the dummy frame's breakpoint
+         instruction.  If not, discard this one.  */
+      if (!(pc >= dummyframe->call_lo && pc < dummyframe->call_hi))
+       continue;
+      /* Does the FP match?  */
+      if (dummyframe->top != 0)
+       {
+         /* If the target architecture explicitly saved the
+            top-of-stack before the inferior function call, assume
+            that that same architecture will always pass in an FP
+            (frame base) value that eactly matches that saved TOS.
+            Don't check the saved SP and SP as they can lead to false
+            hits.  */
+         if (fp != dummyframe->top)
+           continue;
+       }
+      else
+       {
+         /* An older target that hasn't explicitly or implicitly
+             saved the dummy frame's top-of-stack.  Try matching the
+             FP against the saved SP and FP.  NOTE: If you're trying
+             to fix a problem with GDB not correctly finding a dummy
+             frame, check the comments that go with FRAME_ALIGN() and
+             SAVE_DUMMY_FRAME_TOS().  */
+         if (fp != dummyframe->fp && fp != dummyframe->sp)
+           continue;
+       }
+      /* The FP matches this dummy frame.  */
       return dummyframe->regcache;
+    }
 
   return 0;
 }
@@ -1265,7 +1290,7 @@
 
   dummy_frame->pc = read_pc ();
   dummy_frame->sp = read_sp ();
-  dummy_frame->top = dummy_frame->sp;
+  dummy_frame->top = 0;
   dummy_frame->fp = fp;
   regcache_cpy (dummy_frame->regcache, current_regcache);
   dummy_frame->next = dummy_frame_stack;