PATCH to fix infinite loop in CSE

[Mark Mitchell]

If we had something like this:
  
     BB1 <-
      |    |
     BB2   |
      |    |
   ->BB3 --
  |   | 
   --BB4
    
We could loop forever in CSE.  When we fall off BB4, we return to
BB3.  We realize that we've already done BB3, so we want to skip it.
But, we call cse_end_of_basic_block, which cleverly thinks to follow
branches, and points us all the way back to the beginning of BB2.

That might not be exactly the right picture, but that's the idea.

I checked in the following fix.

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com

Wed Oct 20 20:41:46 1999  Mark Mitchell  <mark@codesourcery.com>

	* cse.c (cse_end_of_basic_block): Don't return the end of a basic
	block reached by a branch if we're not going to actually process
	this block.

Index: cse.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/cse.c,v
retrieving revision 1.98
diff -c -p -r1.98 cse.c
*** cse.c	1999/10/18 23:05:03	1.98
--- cse.c	1999/10/21 03:32:42
*************** cse_end_of_basic_block (insn, data, foll
*** 8442,8447 ****
--- 8442,8456 ----
  	path_size--;
      }
  
+   /* If the first instruction is marked with QImode, that means we've
+      already processed this block.  Our caller will look at DATA->LAST
+      to figure out where to go next.  We want to return the next block
+      in the instruction stream, not some branched-to block somewhere
+      else.  We accomplish this by pretending our called forbid us to
+      follow jumps, or skip blocks.  */
+   if (GET_MODE (insn) == QImode)
+     follow_jumps = skip_blocks = 0;
+ 
    /* Scan to end of this basic block.  */
    while (p && GET_CODE (p) != CODE_LABEL)
      {