[Bug tree-optimization/39468] Constant propagation in a number of tree passes does not take into account machine costs.

[ramana dot r at gmail dot com]

------- Comment #1 from ramana dot r at gmail dot com  2009-03-15 22:20 -------

I took a look at this for some time on Friday and I found that the
conditional constant propagation pass has pushed down the value
(tree-ssa-ccp.c). This is done by the CCP pass up in the optimization
pipeline because in general constant propagation is a good idea . In
any case there are a bunch of tree optimizers that identify these and
generally bring in constants into expressions as generally a good
idea. One might argue that constant propagation in general is a good
thing but the problem appears to be that the moment one has an
architecture where costs of loading immediate's is higher than the
cost of simple arithmetic operations the final code generated might
not be the most efficient.


With some more experimentation in the last hour or so I found that for
this particular case, I can get the following code

divby1e9:
       @ Function supports interworking.
       @ args = 0, pretend = 0, frame = 0
       @ frame_needed = 0, uses_anonymous_args = 0
       @ link register save eliminated.
       ldr     r3, .L7
       cmp     r0, r3
       mov     r2, #0
       bcc     .L2
       mov     r3, r3, asl #2
       cmp     r0, r3
       rsbcs   r0, r3, r0
       addcs   r2, r2, #4
       bcs     .L2
       mov     r3, r3, lsr #1
       cmp     r0, r3
       rsbcs   r0, r3, r0
       mov     r3, r3, lsr #1
       movcs   r2, #2
       cmp     r0, r3
       rsbcs   r0, r3, r0
       addcs   r2, r2, #1
.L2:
       str     r2, [r1, #0]
       bx      lr
.L8:
       .align  2
.L7:
       .word   1000000000
       .size   divby1e9, .-divby1e9
       .ident  "GCC: (GNU) 4.4.0 20090313 (experimental) [trunk revision
143499]"


but with the following command line options.

./xgcc -B`pwd` -S -Os newpr.c -fno-tree-ccp -fno-tree-fre
-fno-tree-vrp -fno-tree-dominator-opts -fno-gcse


I'm not sure about the best way to fix this but I've filed this for
the moment as

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39468



cheers
Ramana


-- 

ramana dot r at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Constant propagation in a   |Constant propagation in a
                   |number of tree passes does  |number of tree passes does
                   |not take into account       |not take into account
                   |machine costs.              |machine costs.


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39468