P

        * unroll.c (unroll_loop): Do not abort for an UNROLL_MODULO
        or UNROLL_COMPLETELY loop that starts with a jump to its
        exit code.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@21476 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/unroll.c b/gcc/unroll.c
index c6de53b..72f1f04 100644 (file)
--- a/gcc/unroll.c
+++ b/gcc/unroll.c
@@ -431,15 +431,34 @@ unroll_loop (loop_end, insn_count, loop_start, end_insert_before,
 
   if (unroll_type == UNROLL_COMPLETELY || unroll_type == UNROLL_MODULO)
     {
-      /* Loops of these types should never start with a jump down to
-        the exit condition test.  For now, check for this case just to
-        be sure.  UNROLL_NAIVE loops can be of this form, this case is
-        handled below.  */
+      /* Loops of these types can start with jump down to the exit condition
+        in rare circumstances.
+
+        Consider a pair of nested loops where the inner loop is part
+        of the exit code for the outer loop.
+
+        In this case jump.c will not duplicate the exit test for the outer
+        loop, so it will start with a jump to the exit code.
+
+        Then consider if the inner loop turns out to iterate once and
+        only once.  We will end up deleting the jumps associated with
+        the inner loop.  However, the loop notes are not removed from
+        the instruction stream.
+
+        And finally assume that we can compute the number of iterations
+        for the outer loop.
+
+        In this case unroll may want to unroll the outer loop even though
+        it starts with a jump to the outer loop's exit code.
+
+        We could try to optimize this case, but it hardly seems worth it.
+        Just return without unrolling the loop in such cases.  */
+
       insn = loop_start;
       while (GET_CODE (insn) != CODE_LABEL && GET_CODE (insn) != JUMP_INSN)
        insn = NEXT_INSN (insn);
       if (GET_CODE (insn) == JUMP_INSN)
-       abort ();
+       return;
     }
 
   if (unroll_type == UNROLL_COMPLETELY)