------- Comment #7 from ramana at gcc dot gnu dot org 2009-06-16 10:01 -------
(In reply to comment #5)
> See the attached pqp.c file.
>
> With gcc 4.3.3, on such simplistic examples, peephole ldm and stm works:
>
> sum:
> ldr r2, .L3
> ldmia r2, {r1, r3} @ phole ldm
> add r3, r0, r3
> add r0, r0, r1
> stmia r2, {r0, r3} @ phole stm
> bx lr
>
>
> With gcc 4.4.0 branch, built on 20090413, it fails:
>
> sum:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> ldr r3, .L3
> ldr r2, [r3, #0]
> ldr r1, [r3, #4]
> add r2, r0, r2
> add r1, r0, r1
> str r1, [r3, #4]
> str r2, [r3, #0]
> bx lr
>
We can't use stm or ldm on the second case because ldm's and stm's depend on
the lowest numbered register going to the lowest memory address. It's a relic
of the register allocator choosing a different order for the registers for such
cases.
ldm's and stm's are not easily describable in the RTL backend and are
semi-bolted on on top of the existing infrastructure using peepholes.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9831
