PowerPC reusing CTR register for nested loops

[David Edelsohn <dje at watson dot ibm dot com>]

	The PowerPC target has not been using the CTR register for loops.
This is caused by a special optimization that Haifa added which never is
enabled for any optimization level.  Enabling it causes a bootstrap
failure which I have been trying to track down.

	What appears to be happening is the CTR register is used for two
nested loops making the count for the outer loop invalid.

	loop.c:scan_loop is called for each loop from the inside loop
outward.  When the CTR register optimization finds a loop that can utilize
the CTR regsiter, it walks up loop_outer_loop[] vector marking the outer
loops with a CTR busy flag.

	Looking at the loop phase output, the optimization is finding loop
10 can use the register, then loop 9 can use the register.  It clearly is
searching the loops from highest number to lowest -- inner to outer.  But
the assembly shows two nested loops using the CTR register:

        mtctr 20	<--- load CTR
L..531:
        cmpw 0,5,27
        bc 4,1,L..475
        add 10,3,22
        add 11,4,22
        mtctr 6		<--- load CTR
L..530:
        lwz 8,0(10)
        lwz 7,0(11)
        lwz 9,12(8)
        lwz 0,12(7)
        cmpw 0,9,0
        bc 4,0,L..479
        stw 8,0(11)
        stw 7,0(10)
L..479:
        addi 10,10,-4
        addi 11,11,-4
        bdnz L..530	<--- decrement and branch
L..475:
        addi 6,6,-1
        addi 27,27,1
        bdnz L..531	<--- decrement and branch

Does anyone have any idea how this could be getting confused?  I slowly am
digging into what the compiler thinks it is doing as time permits, but I
am curious if anybody knows of changes to loop.c that could be affecting
this. 

Thanks, David