------- 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
