Re: [rfa] mips heuristic_proc_start fix

[Daniel Jacobowitz <dmj+@andrew.cmu.edu>]

On Thu, Jul 12, 2001 at 03:20:38AM -0400, Andrew Cagney wrote:
> Hmm, with this patch UINT_MAX would give a fence of VM_MIN_ADDRESS yet 
> UINT_MAX-1 would give the above warning.
> 
> Could I suggest instead something like:
> 
>    pc = ADDR_BITS_REMOVE (pc);
>    start_pc = pc;
>    if (start_pc >= heuristic_fence_post)
>      fence = start_pc - heuristic_fence_post;
>    else
>      fence = 0;
>    if (start_pc == 0)
>      return 0;
> 
>    if (heuristic_fence_post == UINT_MAX
>        || fence < VM_MIN_ADDRESS)
>      fence = VM_MIN_ADDRESS;
> 
> The test being moved to before the assignment to make what is happening 
> more transparent.

I agree it would be clearer to check for overflow, but just that
won't solve the problem.  If start_pc is 2 and instlen is 4, it doesn't
matter what fence gets set to.  First time through the for loop we
decrement start_pc by instlen, and that's where the overflow is.

How's this instead?  Instead of checking for pc == 0, check for pc <
instlen.  If fence overflows, that's fine, because start_pc will be
less than fence; or I could explicitly check for that too.

diff -u -r1.57 mips-tdep.c
--- mips-tdep.c 2001/07/12 17:34:33     1.57
+++ mips-tdep.c 2001/07/12 19:12:20
@@ -1497,19 +1497,19 @@
   int seen_adjsp = 0;
 
   pc = ADDR_BITS_REMOVE (pc);
-  start_pc = pc;
-  fence = start_pc - heuristic_fence_post;
-  if (start_pc == 0)
+  instlen = pc_is_mips16 (pc) ? MIPS16_INSTLEN : MIPS_INSTLEN;
+
+  if (pc < instlen)
     return 0;
+  start_pc = pc - instlen;
+  fence = start_pc - heuristic_fence_post;
 
   if (heuristic_fence_post == UINT_MAX
       || fence < VM_MIN_ADDRESS)
     fence = VM_MIN_ADDRESS;
 
-  instlen = pc_is_mips16 (pc) ? MIPS16_INSTLEN : MIPS_INSTLEN;
-
   /* search back for previous return */
-  for (start_pc -= instlen;; start_pc -= instlen)
+  for (;; start_pc -= instlen)
     if (start_pc < fence)
       {
 	/* It's not clear to me why we reach this point when


-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer