http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39838
bin.cheng <amker.cheng at gmail dot com> changed:
What |Removed |Added
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CC| |amker.cheng at gmail dot com
--- Comment #15 from bin.cheng <amker.cheng at gmail dot com> ---
The situation gets a little bit better on 4_9 trunk. The Os assembly code on
cortex-m0 (thumb1 as reported) is like:
test:
push {r0, r1, r2, r4, r5, r6, r7, lr}
mov r6, r0
mov r4, #0
str r2, [sp, #4]
.L2:
ldr r2, [r6]
cmp r4, r2
bge .L7
mov r5, #0
lsl r7, r4, #2
add r2, r7, #4 <----move to before XXX
str r2, [sp] <----spill
.L3:
ldr r3, [sp, #4]
cmp r5, r3
bge .L8
ldr r3, [r6, #4]
ldr r2, [sp] <----spill
ldr r0, [r3, r7]
ldr r1, [r3, r2] <----XXX
bl func
add r5, r5, #1
b .L3
.L8:
add r4, r4, #1
b .L2
.L7:
@ sp needed
pop {r0, r1, r2, r4, r5, r6, r7, pc}
.size test, .-test
IVOPT chooses the original biv for all uses in outer loop, regression comes
from long live range of "r2" and the corresponding spill.
Then I realized that GCC IVOPT computes iv (for non-linear uses) at original
place, we may be able to teach IVOPT to compute the iv just before it's used in
order to shrink live range of iv. The patch I had at
http://gcc.gnu.org/ml/gcc-patches/2013-11/msg00535.html is similar to this,
only it computes iv uses at appropriate place for outside loop iv uses.
But this idea won't help this specific case because LIM will hoist all the
computation to basic block .L2 after IVOPT.
