Hi,
Trying to figure out if it's GCC being broken or what else could be
causing this problem.
I am chasing an issue where the console hangs when I boot my self
compiled kernels under KVM. I have tracked down the problem to a
single instruction change in the vgacon driver, however if I
understand the rules correctly this specific change should not cause
any difference in code execution whatsoever?
Basically if I apply the below diff my code no longer works. However I
am under the impression that ld2.acq does sign extension? If thats the
case, does anybody have an explanation for why my bode breaks if I
don't perform the zxt2 instruction on the previously loaded register
before the comparison? Note the mov r19=r17 at the end, which should
mean there is no reason why this change should affect code flow
further down the lin...
Any input would be greatly appreciated, I have been trying to track
this down for a couple of weeks and I still don't get why this is
going wrong.
Thanks,
Jes
--- /tmp/vgacon-good.S 2008-05-16 06:09:08.000000000 -0500
+++ /tmp/vgacon-test2.S 2008-05-19 03:54:45.000000000 -0500
@@ -113,22 +113,22 @@
ld2.acq r21 = [r18], 2
;;
ld2.acq r16 = [r18]
nop 0
.mmi
st2.rel [r8] = r20
st2.rel [r18] = r22
zxt2 r15 = r21
.mii
- ld2.acq r19 = [r8]
+ ld2.acq r17 = [r8]
nop 0
;;
- zxt2 r17 = r19
+ zxt2 r19 = r19
.mib
nop 0
zxt2 r16 = r16
nop 0
;;
.mib
cmp4.ne p8, p9 = r11, r17
mov r19 = r17
(p8) br.cond.dpnt .L8
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