SLP reduction: why there is only loop-aware SLP and no straight line?

[Anton Youdkevitch <anton dot youdkevitch at bell-sw dot com>]

Looking at the examples for gcc SLP reduction I see
that the reduction variable is defined outside the
loop. It's OK to have this as an example, I thought.
However, looking at the actual implementation I found
out that this is the ONLY SLP reduction gcc does.

/* Detect SLP reduction of the form:

   #a1 = phi <a5, a0>
   a2 = operation (a1)
   a3 = operation (a2)
   a4 = operation (a3)
   a5 = operation (a4)

   #a = phi <a5>

   PHI is the reduction phi node (#a1 = phi <a5, a0> above)
   FIRST_STMT is the first reduction stmt in the chain
   (a2 = operation (a1)).

   Return TRUE if a reduction chain was detected.  */

Is there any reasoning behind this other than that
the loop-aware one is generally more profitable?

--
  Thanks,
  Anton